corrosion-selecting materials for sea water systems 001

29
BMCI MONOGRA / J ., ". ' MARINE ENGINEERING PRACTICE Volume I Part 10 SELECTING MATERIALS FOR SEA WATER SYSTEMS by B. TODD , M.Eng., C.Eng., F.LMar.E., F.I.M . ' . .. P. A. LOVETT, A. LM. . i IN ) Z-..;U 5 - Tfno. r.. "'..J 10 THE INSTITUTE OF MARINE ENGINEERS 1

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Page 1: Corrosion-selecting Materials for Sea Water Systems 001

BMCI MONOGRA

/

J ., • ". '

MARINE ENGINEERING PRACTICE

Volume I

Part 10

SELECTING MATERIALS

FOR

SEA WATER SYSTEMS

by

B. TODD, M.Eng. , C.Eng. , F.LMar.E., F.I.M .

' . ..

P. A. LOVETT, A.LM.

. i IN )

Z-..;U 5 -

Tfno. r.. "'..J 10

THE INSTITUTE OF MARINE ENGINEERS

1

Page 2: Corrosion-selecting Materials for Sea Water Systems 001

Published for The Institute of Marine Engineers

by

Marine Management (Holdings) Ltd.

76 Mark Lane, London EC3R 7JN

(England Reg. No. 1100(85)

Reprinted 1978, 1987, 1989.

Copyright © Reserved

This book is copyright under the Berne Convent ion. All rights reserved . Apart from any fair dealing for the purpose of private study, resea rch, criticism or review- as permilled under the Copyright Act 1956-no part of this publication may be reproduced, stored in a retrieval system or trans­mille~ in any form or oy any means, electronic, electrical, chemIcal, mechanical, optical photocopying, recording or otherwise, without the prior permission of the copyright owners. Enquiries should be addressed to Mari ne Manage ment (Holdings) Ltd., 76 Mark Lane, London EC3 R 7JN.

ISBN : 0900976 49 7

""I 1111 '1, " """"' hv I he Chameleon Press Ltd ., London

Non-ferrous sea-water systems using copper-nickel alloys and cast bronzes.

Information for marine engineers and deck officers, on design and operation of seawater systems for minimum maintenance.

CONTENTS

Part 1 . The insidious enemy - corrosion

Part 2 . Materials for sea-water circulation systems - where and how to use them

Part 3 . National materials standards

AUTHORS :

B. TODD, M .Eng .. C.Eng .. F.I. M ar.E .. F.I. M .

P. A . LOVETT, A.I. M .

9

27

52

3

Page 3: Corrosion-selecting Materials for Sea Water Systems 001

Combating the effects of corrosion has been a feature of seafaring life for centu ri es. Even 30 years ago a common complaint was 'condensentls' In steam ships but in time new materials were tried and tested to cu re this problem and others of simila r nature. It was often found that cu ring one problem produced another and so Improvements were brought about gradua lly and piecemeal. With the advent of large modern high-powered steam ships It became economically Justifiable to Incorporate more sophi sti ca ted materials in sa lt water ci rculating systems In an effort to obtain increased reliability . Unfor­tunately many of the problems which have subsequent ly developed appear to be associated with errors I n the design of minor details and the methods of Insta lling the pipes, fitt ings and components which comprise a 'system' . Because of thiS the in-serVice reliability of two similar systems may be very different. ThiS booklet IS Intended to explain how and why thi S can come about so that the reader can recognise potential difficulties before they become problems and to enable him to apply the correct material s and methods to the repa ir of an eXisting problem.

5

Page 4: Corrosion-selecting Materials for Sea Water Systems 001

6

8 PO INTS TO REMEMBER

1. In pipelines, do not do anything which unnecessarily increases the water speed .

2. Avoid anything which creates turbulence.

3. If you have to create 'crevice conditions' make certain that the metal in the crevice is either resistant to crevice corrosion or is sealed and protected f rom the sea water.

4. Avoid accumulation of debris in any tank, pipeline, va lve or pump.

5. Make repairs from matching materials. If you have to mix materials your repair may fail a year or so later and involve another engineer, so record your use of the wrong material before it is disguised by a coa t of paint and let the experts decide whether to replace it.

6. Regard stagnant water conditions in tanks, pumps, va lves and pipelines with deep suspicion and dra in down rather than risk pitting corrosion.

7. Maintain cathodic protection equipment carefully, check current settings.

8. Use the material specifications in Part 2 of this publication for guidance on materials for repair.

Part 1 . Contents

Electrochemical processes The driving force behind corrosion Two metals joined together in an electrolvte Some other sources of electrochemical potentials The effect of surface films Concentration corrosion cells

Types of corrosion associated with static sea water

General corrosion Pitting corrosion

Types of corrosion associated with flowing sea water

General corrosion Impingement attack Cavitation attack

Galvanic coupling and anode and cathode size effects

Selective corrosion Dezincification Graphitization

Stress corrosion

Cathodic protection

Summary

page 9 9

11 14 15 1 6

16 16 16

19 19 19 19

20

21 21 22

23

23

25

7

Page 5: Corrosion-selecting Materials for Sea Water Systems 001

1. The insidious enemy-corrosion.

Corrosion can be a baffling problem. even to the expert. and it IS therefore easy to dismiss corrosion prevention as being "too complicated and difficu lt". This has led. in the past. to the design of sea-water systems that are satisfactory from the engineering POint of view but diHlcu lt and cost ly to operate from the corrosion point of view. However. the expertise IS now readi ly avai lable to design a system that is not on ly good engineering but is high ly resistant to corrosion. Basically it IS the job of the designer to aVOid the worst errors but. even so. ci rcu lation systems can stili be vu lnerable to damage due to the use of unsuitable matenals as a resu lt of in-service repairs ca rned out In good faith by the operating personnel. The purpose of this publication IS to Introduce the reader to the mechanism of corrosion and show how corrosion damage c~n be aVOided when repairs are made. Whilst the subject matter IS slanted towards sea -water cooling systems. It w ill be of Interest to all sh ip -board personnel Involved In maintenance.

Electrochemical processes The driving force behind corrosion. All metals eXist in tne earth's crust In a form which is more or less chemical ly stable. that IS. their reaction with their surroundings is minimal. Gold and platinum are stable In the metallic form but most other metals are mined as OXide or su lphide minerals and energy has to be applied to them to convert them from minerals to the meta lliC form. see Fig 1 ThiS means that most of the metals used by engineers eXist In an unstable form and tend to react With their environment and revert to a more stable form. The reaClion IS knowl1 as corrOSion and frequent ly the corrosion pmau t IS chemically Simila r to the minerai from which thll motill was extracted. e.g. the rust formed on IrOll 1111<1 51001 111 <In oxygenated environment IS 5111111,11 tlJ tllO Iron OXide minerai used In steelmaklf1n The force whlLiI dllVl5 tir!. corrosion reaction IS the energy 'Iocked liP' III till ' Illetal known as free energy. (for an 11(11111'11111111 .1I1,lIo<JY consider the energy ' Iocked up' In iI Sprll1(J 11 1011 I iruld In compression). There are a numbor 01 WilY'. III will ir Ihe free energy can be released but In thiS puillicoitlOIl Wll are concerned with on ly one. electrocll mlc,11 (WI I) corrosion where the energy cause metal atoms to dl~solv Into a solution.

9

Page 6: Corrosion-selecting Materials for Sea Water Systems 001

1. In their natural, condition most metals exist as stable forms of

mineral. When they are changed to the

metallic form by refining, energy is

added. The metallic form always tends

to reduce its energy content by reverting

to a more stable (lower energy) form.

This is the driving force for corrosion.

Energy

J ®~ I~DJ Extraction and

refining

Mineral

t Metal

Energy

A meta l consists of atoms made up of an equal number of positive and negative charges ca lled protons and electrons. The protons and most of the electrons are tightly held in the core of the atom but some electrons are only loosely attached and there is a continuous interchange of these free electrons between neighbouring atoms. An atom which loses an electron is no longer electrically neutral and IS known as a metal ion and carries a positive charge sometimes denoted by symbols like Fe+ + or Cu +. When a metal is placed in a solution there is a tendency for the meta l atoms to change into the ionic sta te and enter Into solution. but a continuous flow of metal ions into solution IS checked by mutual repulsion from the boundary layer of ions already in solution and therefore an equilibrium condition is quickly established. Since the boundary layer consists of posi tive charges a potential difference wi ll exist between the layer and the metal itse lf which can be measured as a micro-voltage. The measurement cannot be done with a si ngle electrode (as a sing le metal in a solution is called) but it can be measured by placing a standard electrode in the soluti on and measuring the potenti al difference developed between them. If a series of different metals are compa red in this way with a standard electrode it will be found that they each have a cha racteristic potential and can be arranged in a list known as the Electromotive Force Series. see table 1.

Table 1. The electromotive

force series. The vol tage

developed when a metal is immersed in

a solution containing its own

ions is calleJ the Standard Electrode

Potential. Hydrogen is arbitrarily taken as zero potential.

These voltage change when the metal is immersed in other solutions

such as sea water.

Potassium - 2.922 Calcium - 2.87 Sodium - 2.712 Magnesium - 2.34 Berylium - 1.70 Aluminium - 1.67 Manganese - 1.05 Zinc - 0.762 Chromium - 0.71 Gallium - 0.52 Iron - 0.440 Cadmium - 0.402 Indium - 0.340 Telluri um - 0.336 Copper + 0.345 Cobalt - 0.277 Silver + 0.800 Nickel - 0.250 Palladium + 0.83 Tin 0.136 M ercury + 0.854 Lead 0.126 Platinum ca + 1.2 Hydrogen 0.000 Gold + 1.42

The metals In thiS list with high positive potentials are enerall reco nlzed as meta ls with ood corrosion

resistance and are 0 ten ca ed no e meta s. ey show litt le tendency to pass from the metal lic to the ionic state and they develop a sma ll potential between the meta l and the solution . On the other hand. those metals with negative potentia ls have a tendency to corrode rapidly but thiS will be modified If protective surface films can form. for example. sodium reacts with explosive violence in water but magnesium which is only a little less negative has a protective film and reacts very slowly In water. The Electromotive Force Series is measured under standard conditions and the main POint to be noted is that the electro potential may indicate a tendency to corrode but. whether It does or not will often depend on other factors some of which wil l be desCribed later. A more practica l series measured In sea water. called a Galvanic Series. is given in table 2 and In figure 12 on page 34 of Part 2.

Two metals joined together In an electrolvte If two meta ls of differing electro potentials are placed in an electroly te then each will soon reach Its equ ilibrium and tl10 passage of metal Into the electrolyte as Ions will cease. sec figure 2. However. If the metals are electrically conneCfi d the one with the lower potentia l (i.e. the greater tendency to dls~olve) will give up electrons which fI(Jw III the other meta l and. prOVided they can be absorllf'd 1111 "

11

. """I

Page 7: Corrosion-selecting Materials for Sea Water Systems 001

Table 2. The galvanic series of

metals and alloys in flowing sea water. The metals higher in this list wi ll be

corroded by the metals lower in the

list. The expressions active and passive

applied to stain less steels mean steel

w ithout and with its protective oxide film.

12

M agnesium Zinc Aluminium alloy N3 A luminium alloy H20 Aluminium alloy H9 Aluminium alloy N4 Low carbon steel Alloy steel Cast iron Stainless steel (active): 13Cr. Type 41 0 Stainless steel (active): 17Cr. Type 430 Stainless steel (active): 18Cr, 8Ni. Type 304 Stainless steel (active): 18Cr, 8Ni, 3 Mo. Type 316 Ni - Reoist iron Muntz metal 60-40 brass 70-30 brass Admiralty brass Aluminium brass 85- 15 brass Copper Aluminium bronze GunmetaI88-8-4 90/10 copper-nickel alloy 70/30 copper-nickel alloy - low iron 70/30 copper-nickel all oy - high iron Nickel Inconel* alloy 600 Silver Stainless steel (passive): Type 410 Stainless steel (passive): Type 430 Stainless steel (passive): Type 304 Stainless steel (passive): Type 316 Monel* alloy 400 Hastelloy* C alloy Titanium

*Trade mark by ions of that metal leaving solution and returning to the metallic form. the process will be continuous and the metal of the lower potential will dissolve continuously, see figure 3. In most cases of corrosion, in sea water for example, the reaction at the more noble metal-the cathode-does not involve the neutralization of metal ions as in figure 3 but the electrons are discharged by a reaction with water molecules and oxygen atoms as follows:- assuming for convenience that the anode is stee1. the anode reaction wi ll be denoted thus: 2Fe -+2Fe++ + 4e. In words this means two atoms of iron (2Fe) dissolve into the sea water to become two ions (2Fe++) and release four electrons (4e) .

2. If zinc and copper are placed in a

solution containing zinc and copper ions

both will react with th e solution and form a boundary layer of zinc and

copper ions respectively, but,

because zinc is more reactive (more

electro-chemica lly negative) th an

copper it will be surrounded by more of its ions. Once the

boundary layer is establ ished the

reaction ceases.

3. If the zinc and copper are connected

by a wire, electrons flow from the zinc

to the copper. Copper ions (i.e.

copper in solution) are neutralized by the electrons and

deposit as metallic copper on the copper.

Every time a copper ion is

neutralized a zinc atom enters solution

and gives up electrons which flow to the

copper. The zinc continues to dissolve

until the circuit is interrupted or all

the copper ions in solution have been

neutralized and the copper reaction

stops.

• • 0

D

• • 0

• D

r-- r-:-u c: '" N a.

a. 0

U

• c • 0

• \ D 'Zinc ions 0 Copper

• ions • • c

• a •

L....;--;; Q C

L--

Solution containing ZinC and copper 10l"lS

e-+

e..-A e .....

u ~

c: '" N a. a. 0

U

~

Anode Cathode

Page 8: Corrosion-selecting Materials for Sea Water Systems 001

14

The electrons pass through the external circuit to the cathode where they supply the four electrons needed to convert water and oxygen into four hydroxyl ions. 4(OH)­The cathode reaction is denoted:-

2H 20 + O2 t 4e --> 4(OH) -

The formation of these hydroxyl ions produces an alkaline solution in the region of the cathode and. theoretically. the reaction at both anode and cathode would then stop. but. provided oxygen is made avai lable. e.g. by moving sea water sweeping away the hydroxyl ions and thus providing a fresh oxygen supply. the cathode reaction will continue and. of course. the steel anode will continue to corrode. A constructive use of these electrochemical processes IS electro-plating. for example. nickel plating . Here an anode made of nickel IS caused to dissolve Into a nickel su lphate electrolyte and deposit ItS nickel on the cathode-the article to be plated . The electrolyte is a solution of nickel su lphate. This means that it contains a mixture of 'nickel' and ·su lphate·. not JOined together as molecules. but separated as positively charged nicke l atoms cal led ions and negatively charged sulphate ions. When the electrons from the anode arrive at the cathode they neutralize nickel Ions and turn them into metallic nickel which deposits on the cathode-hence plating. see figure 4.

Some other sources of electrochemical poremials So far only potential differences developed between different metals have been considered but potential differences can exist for other reasons. for example: a) Two or more areas of a sing le metal surrounded by an

electrolyte having different concentrations of oxygen. e.g. the metal surrounding a crevice and the metal at the bottom of the crevice.

b) Two or more areas of a single metal surrounded by different concentrations of electrolyte e.g. in areas not washed or mixed by fresh electrolyte such as a blanked-off spigot.

c) Adjoining or nearby areas of a sing le metal surface which have slightly different chemica l composition. e.g. an Impurity.

d) Adjoining or nearby areas of a Single metal component which have different levels of internal stress. e.g. between the head and the shank of a nail.

4. Nickel electrodeposition is a controlled form of corrosion where an

applied voltage induces the nickel anode to corrode

and the cathode to neutralize nickel

ions in the electrolyte causing them to

plate out as metallic nickel on the

cathode surface.

t e

e e

e ~

Article to be plated

Nickel su lphate electrolyte

e

Nickel e ~ anode Nickel entering

the electrolyte as a nickel ion

~ ~

Ni

A nickel ion is / neutralized at the cathode and becomes metallic nickel

These potentials can lead to corrosion in the same way as coupling of two metals of different electrochemica l potential.

Effect of surface films I f you exclude the precious metals like platinum and gold. all other metals and alloys form a thin metal oxide skin on their surface soon after the surface is exposed to the atmosphere. e.g. after machining or filing. The nature of the oxide coating varies from metal to metal and in the case of stainless steel and other highly corrosion-resistant al loys. it is a continuous non­permeable film. Structural steel. on the other hand. has a porous film which is easily displaced-hence the speed with which rusting occu rs. To maintain these films oxygen is needed and this IS normally available from the atmosphere and from sea water. Anything which destroys the oxide film wil l accelerate corrosion. In most metals it is the properties of the protectlv 111m which controls its corrosion behaviour rather than Ih electro potential of the metal itself. Thus alunlllllilm highly reactive metal. as can be seen from It', Irrw I I

Page 9: Corrosion-selecting Materials for Sea Water Systems 001

16

potentia l in Table 1. has good genera l corrosion resistance because It rapidly forms a protective film.

Concentration corrosion cells If one part of a metal su rface IS exposed to a higher concentration of oxygen than another part of the su rface t he higher oxygen area tends to become positive and the lower oxygen area negative (anodic) . Thus. a piece of me tal exposed to sea water wi th va ri ations in oxygen content wi ll tend to corrode in the low oxygen area. This is ca lled a concentration ce ll and the oxygen type is the one most common ly encountered. but. any va ri ation of ion concentrat ion against a metal su rface w il l give rise to simi lar corrosion .

Types of corrosion associated with static sea water

General corrosion Where a meta l IS unable to form a good protective fil m it is liable to corrode and the rate of corrosion w ill usua lly depend on the ava il ability of oxygen. Thus carbon steels. wh ich do not form an adherent protective film w hen immersed in sea water. corrode at a rate of about 0· 1 mm per year. and this is. for all practica l pu rposes. Independent of the steel composition. Thus. In marine en.v lronmen ts steels must be protected by pain t. zinc or some other coa ting. Metals which form pro tecti ve fi lms. for example copper­nickel all oys. corrode init ia lly at a rapid rate until the f ilm forms. therea'f ter the corrosion rate is very low i.e. less th an 0 ·01 mm per yea r and w ill remain so as long as the film is undamaged. Simi lar considerations app ly to stain less stee l and nickel base all oys.

Pitting corrosion Th is is the general name for corrosion which is so loca lised t hat it eats away at one spot and eventu ally perfo rates the metal. It can occur at a sing le point o f an otherwise uncorroded component but it may appear as a group of wel l-spaced PitS. For example. the fil m formed on ,tain less steel is no rmally very resistant to aqueous . envi ronments but ca n be penetrated locally by chlOride ions. Thu s in sea water. where such ions are present In large numbers. stainless steel IS prone to pi tti ng. The presence of impurities in the metal surface wi ll assist in causi ng f i lm breakdown because a galvanic couple IS set

5. Crevice corrosion An oxidation

concentra tion cell is set up by the

different level of oxygen under the

washer and outside th e washer.

Sea water elec trolyte ~ ~ Pit tlng in low oxygen area

up Wi th the impurity acting as the anode and the remaining meta l surface as the cathode. As corrosion proceeds the environmen t around the anode changes by becoming both exhausted m oxygen and acid ic. This tends to accelerate the attack because of the oxygen concen tration ce ll effects and because the acidi ty in the pit prevents the me tal from reforming a p rotec tive f ilm. A Simi lar problem wi th stain less steel in sea water is corros ion in crevices. i.e. where the surface of the steel. although wetted by the sea water. is covered by another piece of meta l or another materia l such as sand or barnac les. see fi gures 5 & 6. The metal in the crevice is deprived of oxygen and t he protective f ilm is pa rti cularly vu lnerab le to breakdown followed by pitt ing. In time. because of this oxygen concentration ce ll . metal w ill be dissolved from the metal in the crevice and deep pilling wi ll occur. Figure 7 shows an examp le of crevice corrosion inside the socket of a wi re strop. Simi lar effoe t may be found under marine growth and indeed in OilY ur where the water is not regu larly replen ished and <Ill

become dep leted in oxygen. The lesson to learn IS th static sea water is liable to be ke t in i elmes. um tanks. e .. in ort or durin re ai rs. it is better III ( r

own. ertaln a oys i e ea - tm al oys an 11111. if ch romium-molybdenum alloys are high ly fIIS I .1 111 '

Page 10: Corrosion-selecting Materials for Sea Water Systems 001

6. The crevice corrosion of a stainless steel

spacer ring in a bow thruster propeller unit is due to th e absence

of oxygen to maintain the protective surface film on the metal. An oxygen concen tration

cell has been set up between the metal

exposed to fresh sea w ater and the meta l

exposed to the oxygen deficient sea wate r in

the crevice. Th is oxygen deficiency also occurs behi nd fairings

and covers. The schemat ic diagram

shows the position of the damage and the

photograph is an end-on view of surface A and a

foreshortened view of surface B.

7 . Crevice corrosion of a w ire strop due

to oxygen starvation.

18

pitting and most of the bronzes. gun metals, copper-n icke l all oys and brasses are sat isfactory but stain less steels. apa rt from the high -nickel-chromium-molybdenum grades, sometimes pit at an alarmingly high speed in static sea water.

Types of corrosion associated with flowing sea water

General corrosion In the case of steel, increasing the flow of sea water across the su rface Increases the oxygen supp ly and thus stimu lates corroSion . The static corrosion rate of about 0 ·1 mm per yea r may Increase to 1 mm per year at 3 metres per second velocity. Thus, the use of steel in ci rcu lating pipes leads to a high corrosion rate. a short life and high maintenance costs. For stain less steels the greater supply of oxygen Improves the protective effect of the surface film and above abou t 1 metre/second (3 It/s) the danger of pitting decreases. At stili higher velOCities, even up to 40 m/s (130 It/s), the film remains protective and the 'genera l corrosion' rate remains very low. Note however, that as crevices on the stain less steel surfaces do not benefit from thiS supply of oxygen any corrOSion which has already started In PitS and crevices wi ll continue. Initially, copper-base all oys are not affected by Increases in velocity but eventua lly a critical velOCity IS exceeded and an intense localized corrOSion ca lled impingement attack occurs.

Impingement attack Some metals and alloys have a well -established su rface water speed above which their protective film is removed and corrosion IS accelerated because a galvaniC coup le IS set up between the exposed and unexposed meta l. For example, 90-10 copper-nlckel- Iron alloy IS hardly affected at ve locities up to 3 m/s (10 ft/s). Copper on the other hand will be adversely affected by water speeds abou t 0·6 m/s (2 It/s) and deep pitting occurs. All copper -base alloys behave in a Similar way to copper but higher velocities are needed 10 produce Impingement attack. see figure 11 on page 31 of Part 2. Straightforward water velOCities In pipelines and va lves are easy to ca lculate, the pitfall IS where an unplanned constriction of the pipe occurs. e.g . a partly -open va lve or a partial blockage by debriS from corrosion occurring in some other pa rt of the CirCUit . Locally. on the downstream side of such constri ctions, a very high water speed can occur and the pipe Wil l be corroded.

Cavitation attack This attack not only removes the oxide skin but phY~1I ,Illy

1

Page 11: Corrosion-selecting Materials for Sea Water Systems 001

8. The steel rivets in a copper sheet (top) are heavily corroded

because the differ­ence in potential is exaggerated by the large current drawn

from the steel by the large area of copper

surrou nding the rivets. Conversely, copper rivets in a

steel sheet (bottom) are unaffected and

the steel is only mildly corroded by

the very small area of copper drawing current from the

large area o f steel.

20

removes the metal as well until a ho le is worn In the meta l. The Intense nature of the attack is due partly to the high water speed but particularly to the 'water hammenng' effect of streams of vapour bubbles caused by turbulence which fo rm and burst over one smal l area of metal surface. Exact ly the same damage IS caused to propeller blades by the vapour bubbles which form and collapse on the propeller blades. A typical component that IS prone to caVitation IS a pa rtially throttled valve.

Galvanic coupling and anode and cathode size effects The rate of corrosive attack depends on how much curren t per square centimetre (cu rrent density) is being drawn out of the anodiC area. As already ment ioned the corrosive current IS less If the two metals coupled together are close together in the galvanic tab le. e.g . brass and copper. because the galvaniC voltage is low. The rate of corrosion

wi ll be high if steel (or Zinc) and copper are coupled because the ga lvanic vo ltage IS qUite high. So, do not fasten down copper sheeting Wi th ga lvanised iron nails. Another problem IS the effect of a large area of the cathode on a smal l area of anode. In the example given above the ga lvanlsed Iron nail IS the anode and the copper sheet is the cathode. Therefore. on top of the ordinary ga lvanic potentia l difference. the attack IS Intensi fied because the area of anode IS smal l compared With the cathode and the current density therefore high, thus the nai l IS rapid ly corroded. Conversely, If a ga lvanlsed stee l sheet IS fastened by a copper nail, the area of anode (the ga lvanlsed sheet) IS so large that the amount of cu rrent leaving each square cen timetre of sheet (the cu rrent density) IS very smal l and corrosion will be negligible. When diSS imi lar metal s have to be used In construction It IS considerati ons of relative anode-cathode size which can lead to a decIsion to coat the calhode (the more noble and corrOSion - resistant meta l In the coup le) rather than the anode because If the cathode is left uncoated and a sma ll break occurs In a protective coa ting of the anode, thiS sma ll area of anode IS left at the mercy, so to speak. of the large cathode area and Inlense loca lized corrosion wil l occur. Figure 8 illustrates clearly a demonstration expenment cond ucted to show th e effects of smal l anode area/la rge ca thode area and the converse effect of large anode areal sma ll cathode area on the rate of corrosive attack.

Selective corrosion

Dezincificalion of brass ThiS IS one of several forms of selective corrosion which affect some al loys. In the case of brass it IS the removal of the ZinC content o f brass (a copper-zinc alloy) by the sea water or hot fresh water, leaving behind a porous and weak sponge of copper. The charactenstlc appearance of a dezinclfled brass IS the coppery colour of the affected area. Single phase brass al loys such as aluminium brass can be inh ibited against dezlnclflcatlon by the addition of a s:-na ll amount of arsenic but dup lex al loys such as 60-40 brass cannot be Inhibited. although the addition of 1 % tin will retard the corrOSion 60-40 brass shou ld not be used In any part of a sea water system where it IS In con tact With sea water and Ihe 60-39- 1 copper-zlnc-tln all oy. naval brass, should only be used in heavy sections w here

21

Page 12: Corrosion-selecting Materials for Sea Water Systems 001

9. Cast iron sea valves are vulnerable

to corrosion by graphitization. The

damage can easily be overlooked because

the hole is usually disguised by the

graphite infilling. The right -hand sketch shows a possible

repair using a length of cast iron pipe.

22

-Cast Iron pipe. Stress relieve

Look for corrosion here.lt may look

£=~~~ like graphite. £=~~~ after welding.

the corrosion damage can be tolerated. or where it is cathodica lly protected . Where these conditions do not apply other more resistant al loys shou ld be used.

Graphitization or graphitic corrosion Under a microscope cast iron can be seen to consist of a steel-like matrix and flakes or spheroids of graphite. In sea water the Iron in the matrix can be selectively corroded away leaving behind a fragile shell largely consisting of graphite which fractures as soon as it is stressed. Frequently the external appearance of a graphitized component is unchanged and the damage is easily overlooked until a leak or fracture occurs. Important components which are prone to graphitization are ship's sea valves. see Figure 9. The galvanic effect of graphitization on nearby metals. e.g., valve seatings can be serious since the layer of graphite formed on the surface of the casting IS more noble than copper-base alloys, see figure 12 on page 34, and It will cause them to corrode. Should graphite surface coatings be found during routine maintenance of cast Iron valves and pumps, check the degree of damage and also examine nearby non-ferrous metals for corrosion damage induced by the graphite.

Stress corrosion This type of attack is less obviously associated with electro-chemical currents-particu larly stress.corrosion of brass which is sometimes referred to as 'season cracking'. If a brass sheet IS bent and shaped. co ld work is app lied to the bent and shaped areas and the metal will be unequally stressed as between unworked and highly worked areas. ThiS IS enough to set up a galvanic couple between the adjacent areas of unequal stress but the corrosive attack is not caused directly by sea water. The troublemaker is often considered to be ammonia which can se t up a concentrated attack along the grain boundaries in the areas of unequal stress which quickly embnttles the brass. ThiS type of attack affects all the brasses. including aluminium brass. but gunmetal. tin bronzes. aluminium bronze and copper-nickel alloys are not affected. Due recognition of thiS fact has been made in selecting materials on page 36 and by specifying stress -relief heat treatment where appropriate. It shou ld be noted that hydrazine added to boiler water decomposes to produce ammonia and under certain conditions this endangers brass condenser tubes. Another POint to remember is that most non - ferrous metals and alloys can be stress cracked by molten brazing alloy and should be stress relieved before braZing . . Austenitic stain less steels are also susceptib le to stress corrosion under certain conditions which are rarely met In marine service. The attacking agent most li ke ly to be encountered is ch loride (e.g. from the sea water) but failures at ambient temperatures are rare. The danger areas In ships are heat exchanger tubes and it should be noted ·that on the evaporating su rfaces it is possible for even a dilute salt solution to build up high concentrations of chlOride and thus increase the danger of attack.

Cathodic protection Corrosion in an electrolyte requires the formation of anodes and cathodes either on the su rface of one metal or the surfaces of different mota Is. In such sys tems a corrosion current flows from the anodes through the electrolyte to the cathodes. It IS possible to arrange an aUXIliary anode such that the current flows from it to the corroding system, thus turning the system Into a cathode. For such an anode to be effective it must be of lower potential (more anodic) than any of the anodes In the corroding system. For

23

Page 13: Corrosion-selecting Materials for Sea Water Systems 001

10. Current distribut ion paths

between anode and cathode in a

cathod ic protection system. The current

density at point B will be insufficient

to give enough protection due to the voltage drop

caused by the electrical resistance

of th e electrolyte and the length of

path A. Other points between B- B

have proportionally

24

higher current densities w ith a maximum at the

points where the path to the anode

is shortest.

Sacr i f icial anode

Protec ted metal made cathodic

example. If iron and copper are connected in sea water corrosion of the iron wi ll be stimu lated by the presence of the copper cathode. If a piece of zinc (see Table 2) is con nected to thi S system then the current wi ll fl ow from the zinc to the iron and copper. tu rn ing the Iron Into a cathode. Th is is the basis of the system of protection known as Cathodic Protect ion so ca lled because the parts to be pro tected are made the cathode of the system. Cathodic protecti on by means of sacrifi cial anodes IS well known and examples are the zinc anodes around the stern (i n this case to nullify the galvanic cou ple between the steel and the bronze propeller) and the Iron anodes In lined wa ter boxes to protect the tube ends and the tube pl ate. Other areas of usage are in sea inlet chests and on the hu ll around sea connections w here it provides loca l protecti on to the stee l hull fro'Tl the effects of the non- ferrous metal. An alterna t ive method for large installations is to use an Inert materi al l ike graph ite or pl atinized titanium for the anode and use a DC supply for the vo ltage. The anode can on ly service a certain area of cathode because the electrica l resistance to the 'current' fl ow through the elec trolyte increases With the distance between anode and cathode until the voltage drop is such that

protection ceases. see figu re 10. path A. Increasing the supp ly vo ltage w ill force more cu rren t through to the remoter areas but there are limits to the increases because the curren t density for the nearer cathode su rfaces is proportiona lly greater and eventua lly undeSirable effects associated with gassing occu r. This dimenSiona l effect IS no t disSim ilar to th at cathode size effects described on page 20 and It explains why It IS not possib le to protect the InSide of condenser tubes. with Iron anodes placed near the tube plate. for more than a distance of about 4x tube diameter. Conversely. It also explains why only a su rface area equa l to 4x diameter of a non - ferrous sea Inlet or out let tube takes part In the corrosion of the nearby steel hu ll. rather than the whole Interna l su rface of the PIPing system.

POints 10 remember are:

1. There must be enough anodes to produce the fi ght cu rrent density over the entire surface to be protected.

2. Sacfl flclal anodes must co rrode to do their Job and therefore mus t also be examined regu larly and replaced before they corrode away comp letely.

3. The polamv of the electflcal conneClions of the external DC supplV in a non-sacflflclal system IS vilal. Reversed connections have been known and the resu lt IS rapi d and disastrous corroSion of the componen t to be protected .

Summary A t the start of the explanation of corrosion we sa id i t IS easy to dismiSS corros ion prevention as 'too complica ted and di ff icu lt' . BaSica lly It IS the Job of the deSigner to avoid the worse errors bu t the marine engineer needs to t hink abou t possible corrosion damage whenever a repai r IS ca rned out and 'the on ly ava il able materia ls' are used. For example a cas t Iron va lve In a copper-n icke l or aluminium brass pipe line would have a very limited life. A piece of copper pipe Inserted In a copper- nicke l pipe line wi ll not last long because the deSigned water speed for the copper-nickel ploellne IS much too high fo r the copper. Fastenings are a cons tant problem. A steel bolt In a copper or brass fl ange w ill corrode more qUickly than a steel bolt In a steel flange. Brass bolts will stress-corrode when tig htened.

25

Page 14: Corrosion-selecting Materials for Sea Water Systems 001

Part 2. Contents page

Design and cost considerations 29 The choice of materia ls for sea-wa ter sys tems 30 Galvanic effeces 33 Seress corrosion and selective corrosIOn 35

Sea -water systems specification 36 Sea-water Inlets and ou tlets 36 Grids 36 Sea mlee boxes 36 Air release venr 36 Water oUllels 36

PIPing 37 Plpmg and fillings 37 Pipe jomls and fl1lmgs 38 Flanges 39 Pipe bendmg 39 M mor fill mgs 40 Valves 40 General noees 40 Globe valves 41 Gate valves 41 BUllerflv valves 42 Diaphragm valves 42

Heat exchangers 42 Tubmg 42 Tube plates 43 Water boxes and end covers 43

Pumps 44 Strainers 44

Initial treatment for a new system 45

Procedures for minimizing maintenance in existing systems 46 Genera l operation 46 Ferrous su lphate treatment 46 Chlorine treatment 47

Repair and improvement of existing systems 4B Pipes 411 Va lves 411 H eat exchangers 4'1 Pumps 41

27 26

Page 15: Corrosion-selecting Materials for Sea Water Systems 001

28

1 I

2. Materials for sea-water circulation systems-where and how to use them.

Part 2 of this publication specifies materials and provides gUidance on operating procedures which will give a sea-water system with minimum maintenance for the life of a ship (this assumed to be 20 years) . Minimum maintenance IS defined as no major renewals for items such as pump casings. Impel lers. valve bodies. piping. heat exchanger tubing and tube plates. but does include such Items as wear rings. bearings. seals and valve trim. where periodiC rep lacement may be required .

Design and cost considerations Increasing size and number of sea-water systems on board modern ships. together With the reduction In engine room staff available for repair work on board. has resulted In a need for high reliability. low maintenance sea-water systems. The normal guarantee period for a ship IS one year and It IS relativel y easy for a shipyard to meet this requirement With materials which corrode In sea water and which cannot be expected to last the life of the ship. If It IS deCided to up -grade any part of the system the effect on the whole system must be considered. For example. In some ships. parts of the sea-water systems have been up-graded dUring design. by flltlng non-ferrous instead of galvanized steel circulating pipes. but this resulted In a shorter rather than longer life for the system as corrosion of other components. such as cast iron valves was stimulated by the galvaniC effect produced by the copper-base alloy PIPing . Another example shows that Improved water box coatings together With non- ferrous piping. have produced conditions detrimental to the 'aluminium brass heat-exchanger tubing . For many years thiS materia l has been almost standard In shlp's heat exchangers but it is now realised that ItS performance has been greatly improved by the presence of iron Ions in the sea water (derived from corrosion of the hul l and ferrous components in the system) and by the cathodic protec ti on of the tube ends provided by ferrous water boxes. The remova l of these beneficial factors has resu lted in increaslli I inCidence of tube inlet-end corrosion and to improve reliability either an Improved heat exchanger tube m, Iprl .1 must be used or the system must be dosed wi th ferrelll ions. tube in let end Inserts fitted and cathodic proll r. llun provided in the water boxes. In recent years scoop circulation has become cnrnrnr I main condensers. In some cases fai lures of cOlld. II I have occurred due to high water ve loci ti es !I. I II I

Page 16: Corrosion-selecting Materials for Sea Water Systems 001

30

these systems to ach ieve a given ve locity distribution across the tube plate seems difficult-and ingestion of large quantities of air with the sea water and the difficulties of removing this before the water reaches the heat-exchanger tubing . Scoop intakes fitted to the ship's bottom seem to be worse in this respect than those situated on the ship's Side where there IS more likelihood of ai r release before the sea water enters the scoop. The use of automatic contro l and data logging on sea -water systems has also emphasized the need for reliabi lity of the basic system. Although It IS illogica l to app ly expensive control equipment to a sea-water system which IS fundamentally unreliable I t IS common practice. and shipowners often spend large sums of money on su ch contro ls wh ilst refUSing to pay smal l extras to up -grade. for example. the va lve material. Data on the cost of sea -water systems both as regards Initial and maintenance costs are not readily available and It IS significant that those which have been published have not come from shipya rds who presumably have exact figures for Initia l costs. It is clear. however. that as a percentage of total ship cost. the cost of the sea -water system IS fairly sma ll and the di fference In cost between a cheap. low-reliability system and a more expensive. high-reliability system must be very smal l. For examp le. a 10.000 ton dry cargo li ner With all the engine room sea-water piping In 90-10 cupro-nlckel uses approximately 10 tons of this alloy at a cost of approximately £15.000. For a VLCC the figure wou ld be approximately double. Labour costs associated With pipe repairs are frequently 6-8 times the cost of the baSIC rep lacement invo lved . ThiS IS due to the work associated With the repair. such as lifting floor plates. removing other pipes etc. to reach the one to be repai red. fabricating the new pipe. galvaniSing It. refitting the pipe etc. The cost of the repair therefore bears little relationship to the cost of the pipe material so that even on a DCF cost basis the use of more expensive. improved piping materials can be justified .

The choice of materials for sea-water systems Carbon steel and cast Iron corrode In sea water at a rate "npendent upon the availability of oxygen . In quiet sea Wolt r the general corrosion rate of both these materials IS ,lppruxll11i1tely 0 ·1 mm per year With pItting at several times

11 . The increasing probability of

impingement attack on various tube

alloys with increase in sea-water velocity.

After T. P. Gtlbert and R Mav

,,;.

" .~ ro

l!' c: :J " co E E " "

c: C.

c: '0.

'0 . ~

2 3 Nominal water ve locity. m /$

4 5 7

thiS rate. The corrosion rate Increases With the rate of sea -water flow so that at 3 metres per second (10 ft/s) It is approximately 1 mm per year. (It should be noted that In areas of tu rbul ence velOCIties Will be much higher than the normal deSign fl ow rate and the rate of corrosion Wi ll also be higher) . Steel pipes. valves etc In systems subject to sea -water fl ow Wi ll thus begin to fail In those areas where the water velOCity IS high. Thinner sections Wi ll fall frrst and the rate of failure Will steadily increase with the age of the ship as the thicker pipes begin to fall also. ThiS co rrosion cannot be con trolled by cathodic protection as the sp read of cu rrent inside the piping IS poor and a large number of anodes wou ld be needed. Coatings are generally unrel iable and can only be used In certain appl ications such as rubber linings on butterfly-valve bodies where therr Integrity can be guaranteed and physica l damage is unlikely Copper-base alloys corrode at a low rate In qUiet sea water-generally less than 0 02 mm per year-and With litt le tendency to pit. ThiS remains farrly constant With velocity until a certain critical velOCity IS exceeded when intense local corrosion ca lled Impingement attack occu rs. It is essential. therefore. when uSing copper -base all oys. to deSign at a flow rate lower th an that which Will cause impingement attack. see figure 11 . It IS also necessary to minimize turbulence ,n the system by avoiding deSign and fabrication detai ls which cause turbu lence . Such details

31

Page 17: Corrosion-selecting Materials for Sea Water Systems 001

Table 3.

32

are small radius bends. mi sa ligned pipes. partly protruding jointing. shallow water boxes. partly throttled va lves etc. It is particularly bad to have two turbulence raisers in close proximity. e.g .. a tight bend followed by a throttled valve as this will have an Increased effect. A straight length of piping should be provided between the two components. Provided the system is designed at a flow velocity acceptable to the alloy. and provided it is fabri cated correctly. high reliability for the life of the ship can be obtained. The materials speci fied In this pub lication are therefore predominant ly copper base and the design conditions are chosen to suit these matenals. Most requirements can be met by copper-base alloys but some components requ ire properties which are not available in these alloys. for example. resistance to very high water velocities IS needed for valve seats and pump

. impellers. These applications requi re materials which retain their protective oxide coating in high velocity sea water and do not suffer from impingement attack. Provided cavitation conditlons .are avoided. nickel-base alloys and stainless steels wi ll suffer neg ligible corrosion even at the highest flow rates encountered in sea-water systems. The most common ly used nickel-base alloys are nickel-copper alloyst. such as Monel' al loy 400 and K500.

t Details of materials compositions are given in Part 3 Trade mark

Material Corrosion rate

M edium Low velocity velocity

General 5 mls corrosion Pitting (16 It/s)

mm/y mm/ y mm / y

Carbon steel and cast irons 0.1 0.5 1 Copper base alloys 0.02 0.02 Varies with

alloy. some suffer

impingement attack

Stainless steel 0.D1 0.5 0.D1 Nickel-copper alloys 0.D1 0.2 0.01

'-

High velocity

30 mls (tOO It/s)

mm/v

10 1

0.01 0.01

Many stainless steels are available. most of which are unsuitable for use In sea water due to their tendency to pit under quiet conditions. For general usage in sea water only the molybdenum -containing austenitiC stainless steels corresponding to BS 1449 -316 S16. together with its cast eqUivalent BS 1632 . need be considered . Ferritic stainless steels such as 13% chromium steel shou ld not be used In contact with sea water. The general corrosion behaViour of matenals In sea water can thus be summanzed as shown In Table 3. The above figures are only given as a general guide and are not to be taken as actua l or design figures. Apart from the necessity to have matena ls which are resistant to corrosion. It IS also necessary that they shou ld readdy be made Into the components used In the system. Welding. casting and fabncatlon properties are all Important in certain components and have been taken Into account in the se lecti on of matenals on page 36.

Galvanic effects I t IS necessary to use different materials for different components In the system and It IS Important that these be compatib le galvanica lly In sea water. Figure 12 gives a ga lvaniC series measured In sea wa ter from whichjt can be seen that the copper-base alloys have simi lar electrode potentials and can usually be JOined together Without severe ga lvaniC corrosion resu lting . Where it is necessary to use matenals widely separated In electrode potentia l the foll owing guidelines should be followed .

Ensure that the material of less noble' potential IS present in much greater area than the more noble material. The spread of increased corrosion on the less noble material will reduce its Intensity to a level which can be toleraced.

Ensure that the key component is made from the more noble material. for example. In a valve a small amount of corrosion on the valve seat can result in failure of the valve. whereas a similar amount of corrosIOn on the body is of no consequence. Hence. valve seats should be made of more noble material than the body.

I n this context noble means the material with the higher or more positive electro· potential. In Figure 12 noble materials are those to the left.

33

Page 18: Corrosion-selecting Materials for Sea Water Systems 001

12. The Galvanic Series of metals and

alloys in flowing sea water. The metals at

the bottom are the most noble metals

(note also the posi ­tion of the non-metal graphite) . In flowing sea water the metals

higher in the series will be anodic to

those lower and thus suffer corrosion if coupled together.

Under varying local conditions the

electrochemical potential can vary

over th e range indicated by the

width of the black bar. In the case of stain ­

less steel any damage to the protective

oxide film results in a very marked change

in the electro potential and the steel will

become anodic to any material situated

to the left of the cross-hatched bars;

this of course in ­cludes any stainless steels with an intact

protective film.

34

Volts, Saturated calomel half·cell reference electrode

+0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6

J I Ma~nes'um ..

. Be;~~~U~ I .. ___ Aluminum Alloys

_ Cadmluml I .. M,id Steed:ast Iron

Ijl low Alloy Steel I _ Austenitic Nickel Cast Iron

::: Aluminum Bronze I I .:. T~~:;:~ Ts.vellO

I

W Brass

l

, Red Brass

_ Lead -Tin Solder _ Admiralty Brass, Aluminum Brass

_ Mangane~e Bronze I • Slll~on Bronze I _ Gunmetals

~_ I ll'i!I"IJ Stainless Steel-Types 410.416 _ Nickel Silver I I I

_ 90:10 Copper-Nickel _ 80-20 Copper Nickell

lead I IIIIIIllIl[ rta lnle"l Steel-TI ype 430

70-30 Copper Nickel

~ Nlckel·A!umlnum Brcnze

• mjmm Nickel Chromium Alloy 600 Silver B:razlng Alloye I I Nickel 200 I

_ Silver I • I IIII1IIlIJ Stainless Steel-Types 302.304.321. 34 7 _ NlCkel-Copper Alloys 400. K500 I I ~ I _Stainless Steel·Types316.317

-t- Alloy '20' Stamless Steels,cast and wr6ught .~ NlckeHron·Chr.omlum Alloy 825 • N,· Cr·Mo·Cu·SiAlloy B _~Tltanipm I _~ NI ·Cr .M IOAIIOY1C

• Platinum

~ Graphite

Consider carefully the galvanic effects before paintmg or otherwise coating a component. It is often better (0 paint the more noble components and leave the less noble anodic one bare. This ensures that any breaks in the coating will not result in rapid galvanic corrosion as would be the case if the anode were painted and the cathode left unpainted.

Stress corrosion and selective corrosion. These have been described on pages 21-2 3 and due attenti on shou ld be paid to them In se lecting materials for sea-water service.

35

Page 19: Corrosion-selecting Materials for Sea Water Systems 001

36

Sea-water systems specifications.

Sea-water inlets and outlets

Grids These are preferably made from gunmeta l BS 1400 LG4. Where a ga lvanized steel p late grid IS fitted this should be painted uSing a paint system compatib le wi th the galvanizing. for example. prrmlng with ca lci um plumbate prrmer. Steel grrds shou ld also be electrrca lly bonded to the ships hull with a phosphor -bronze strrp and zinc anodes placed near the grid so that It IS ca th odica lly protected .

Sea tnlet boxes These are to be adequately sized to achieve low turbulence and water ve locities In the box. They shou ld not be posit ioned In areas where air release IS likely. for example. at the terml natron of the bilge keel or where wate r from overboa rd discharges may b.e entrained. The box IS to be coated Internally with an approved paint system and zinc or alumin ium anodes provided to preven t loca l galvanic corrosion from the copper-a ll oy piping . Inlets to the pIping sys tem shou ld be provided on the lower side of the box to ensure good ai r release and at least 330 mm (13 Inches) clea rance between the highest pOint of the Intake pipe and the ai r release vent must be provided .

Air release vent ThiS should be placed at the highest pOint of the sea Inlet box and run to a pOint above the deepest load li ne and preferably onto an open deck. The pipe IS to be made of 90-10 cupro-n lckel. as speci f ied below under Piping and Fittings. and should run vertical ly or at a sharply inclined ang le to ensure good air release. The pipe should be 57 mm (23;: Inches) mi nimum nomina l diameter.

Water outlets Zinc or al,m,n,um anodes shou ld be provided adjacent ttl!' sea-water outlets to protect the hull from galvanic to uffm ts from the copper-alloy piping.

Table 4. Pipe sizes.

BS 2871 Part 2.

Tube size 0 .0 . 0 .0. Preferred max. min. standard

thickness mm mm mm mm

3 3.045 2.965 0 .8 4 4.045 3.965 0.8 6 6.045 5.965 0.8 8 8.045 7.965 0.8

10 10.045 9.965 0 .8 12 12.045 11.965 0 .8 16 16.045 15.965 1.0 20 20.055 19.975 1.0 25 25.055 24.975 1.5 30 30.055 29.99 1.5 38 38.07 37.99 1.5 44 .5 44 .57 44.49 1.5 57 57.20 57.12 1.5 76 .1 76.30 76.15 2.0 88.9 89.15 89.00 2.5

108 108.25 108.00 2.5 133 133.50 133.25 2.5 159 159.50 159.25 2.5 193.7 194.50 194.25 3.0 219 .1 219 .90 218.30 3.0 267 268 .00 266.40 3.0 323 324.90 323.30 4.0 368 369.00 367.40 4.0 410 420.00 418.40 4.0 457 .2 458.20 455.20 4.0 508 509.00 506.00 4.5

Piping

Piping and fi{{ings All piping hand li ng fl OWing sea water IS to be madeof 90-10 cupro- ni cke l" supplied to BS 287 1-CN 102 or recognised equivalent. Dimensions and thicknesses shou ld correspond to those given in BS 2871 Part 2. The maximum deSign veloc ity for pIping of 108 mm (4 inch) nomi na l diameter and above shall be 3 metres per second (10 ft/s) . For sizes below thiS the maximum veloci ty is to be 2·5 metres per second (8 ft/s ) . For large diameter pipes. I.e. above 267 mm (10~ Inch) nominal diameter. the deSign velocity may be Increased by 10%. All ca lculations should be based on the pump discharge flow and the PIi1 fl

Where cupro nickel IS referred to In th .s specification the alillY should have an li on addit ion appropriate to the alloy

II

Page 20: Corrosion-selecting Materials for Sea Water Systems 001

38

size for the pump suction shou ld be one size larger than that ca lcu lated for the discharge. Pipe sizes are given in Table 4 and correspond to BS 2871 Part 2. Pipe bends should be of as large a radiu s as possible to ensure minimum turbulence. Where bends of radiUS less than 3 x pipe diameter are made care should be taken to ensure that no other turbulence raisers. such as valves or branch pieces. are situated near the bend . A minimum length of straight pipe of 5 times the diameter should be interposed. Where flow from the pump discharge is spilt into two or more branches. the pressure loss through each branch should be the same so that the flow quanti ties and corresponding velOCities con form to design requirements. If .. on ca lculation. these losses are not the same. then Orifice plates made from Monel alloy 400 must be fitted to balance the systems. A straight length of pipe 5 x pipe diameter IS to be fitted downstream of all Orifi ce plates.

Pipe joints and fi {{ings All jOints in pipes 108 mm (4 Inch) nominal diameter and above shall. preferably. be made by inert -gas welding. uSing either the tungsten Inert gas (TIG) or metal Inert gas (MIG) process. Manual welding with approved electrodes will also be accepted. Joints involving brazing should. wherever possible. be avoided In pipes of these dimensions. The use of backing gas IS essentia l where the weld root is inaccessible. All filler metals for welding sha ll contain titanium deoxidants. Where the weld IS in contact with flowing sea water the weld metal shou ld be of 70 -30 cupro-nlckel with appropriate iron additions. Welds exposed to static sea water or free from corrosion hazard can be made irom 90-10 cupro -nlckel (plus iron) . For pipe sizes below 108 mm (4 inch) nominal diameter brazing Isacceptable. In such cases the brazing alloy is to comp ly with BS 1845' AGI silver brazing. Allovs of silver content less than 50% may corrode preferentially in sea water and shou ld not be used. Under no ci rcumstances will a copper-zinc brazing alloy be accepted . Where welds are made between 90 -10 cupro-nlckel and steel the Initial run. where high base-metal dilution can be expected. should be made In 70-30 nickel-copper alloy to avoid hot cracking . Subsequent runs can be made with 90 -10 cupro -nickel . Copper-nickel/steel welds should not be exposed to sea water.

• Other alternatives are given In Part 3. page 56 .

Pipe fitting s manufactured by a recognized suppli er should . where possible. be used. Where connecti ons such as branch pieces are requtred to be fabri ca ted. they should be made so as to give a smooth streamlined flow of sea w ater. For pipes 25 mm (1 Inch) nomina l bore and under compression filling s may be used Hangers for pipes shou ld be placed at In tervals o f approximately 3 metres (10 It) but where the con f igu rat ion of plpework IS com pl ex. addi ti ona l hangers should be fitted The hangers are to be lined With a solt packing. free from ammoniaca l compounds. to prevent chafing and to permit free expansion and contracti on between anchor pipes and fltlt ngs w hich JOin mach inery. expanSion pieces etc and shou ld be adequately supported to prevent excessive loads being transferred to them Ri gid piping shou ld be connec ted to resilient -mounted machinery by fl exib le pipes or expansion pieces

Flanges Wherever pOSSi bl e loose fl anges are to be used to faCi litate pipe fitting . These ca n be of the Van Stone ' type. Yorca lloy ' type With brazed -on gunmetal co ll ar or stub -en d flanged pieces for butt welding to pipes. JOints should be made as speCi fi ed on page 38. pipe JOints and fitti ngs The flanges can be made of stee l. mal leable tron or ducttle cas t tron .

Pipe bendmg Bends may be formed by bending straight lengths of piping . The minimum radiUS of such bends should be three times the pipe diameter. Where bends of lesser radiU S than this are requtred a pre -formed bend supplied by a speCialist manufactu rer shGu ld be used. Wrinkle bending and mitrebends will not be accepted. A smooth bend IS preferred but a 'Iobster-back' bend IS acceptab le. Cold bending IS to be used. If for any reason hot bending IS destred the pipe supplier should be consulted and hiS recommendattons foll owed. Where the pipes have to be ftll ed for bending thi S shou ld preferab ly be With sodium thlosulphate. Where carbonaceous f i llers are used c'are must be taken to ensure that no ca rbonaceous films are left on the pipe surface. ThiS can be done by heattng the pipes to 600-650'C whilst passing a stream of atr through the bore. 90-10 cupro -nlckel has high resis tance to stress -corrosion

'Trade mark 39

Page 21: Corrosion-selecting Materials for Sea Water Systems 001

40

cracking and stress relief after fabnca tl on is not essent ial. It shou ld be no ted however. th at molten metal. for example molten brazing alloy. may cause crack ing of the stressed all oy and It IS necessary to ensu re that pipes are stress relieved before bnnglng them Into con tact w ith molten braZing all oy. ThiS can be achieved by heating to 350 -450·C for one to two hours.

Mmor flClmgs ProvIsion should be made for injec ti on of ferro us sulphate to all the sea-wa ter sys tems on the sucti on Side of the pump. Bosses of 90 -10 cupro-nlckel shou ld be brazed to the pipe for temperatu re and pressu re measuring pOints. The sensing elemen ts of these Instruments shou ld be sheathed With matenals resis tan t to sea water. such as Monel all oy 400.90 -10 or 70 -30 cupro-nlckel or nickel alu minium bronze . The use of brasses or stain less steels should be aVOided for these shea ths

Valves

General notes Va lves even when fu lly open produce some turbulence. the amount va rYing w ith the va lve type and whether It IS full y open or pa rt ia lly thrott led . Globe va lves can produce severe turbulence even when fully open and thiS IS refl ec ted In the pressu re loss across the va lve. Va lve ma teria ls must be ei ther resistant to sea water and ga lvanica lly compat ible With the pip ing or protected by a coa ting w hich w ill prevent access of sea water to the metal parts o f the valve. Expenence has shown that va lve coa ti ngs are on ly reli able w hen they are thick and appl ied to smooth surfaces of Simple shape. For example. rubber li nings on butterfl y va lves. Coatings on complex shapes. such as globe va lves. are unreli able and unacceptable and such va lves must be made from a corrosion- resistant matenal. The key components In a va l vr~ are the seat and stem and these must be made from corrosion- resis tant matenal or. as In diaphragm va lves. access to sea water must be aVOided. In general. coa ted componen ts w ill not be accepted for va lve Interna l tnm. For shlp's Side connec tions globe valves are to be used w herever pOSSi ble. elsewhere diaphragm valves are pro forred

Table 5.

Table 6.

Globe valves

Component Preferred Acceptable Unacceptable

Body Ni -AI-bronze Leaded gun- Coated cast BS 1400AB2C metal BS iron or steel

1400 LG4

Seats· and Monel alloy Stainless Copper-base Disc (to be 400 BS 3076 steel type alloys. such pressed in 1449316S16 as gun metal and secured) aluminium

bronze or brasses

Stem Monel alloy Ni -AI -bronze Brasses 400 or K500 BS 2874 BS 3076 CA104

Securing pins Monel alloy (eg for valve 400 BS 3076 seat to body or disc to steam connections)

• All valves to be of such a design that sp inning Will not occur

Gate valves

Component Preferred Acceptable Unacceptable

Body Ni -AI -bronze Leaded Coated cast BS 1400 gunmetal BS iron or steel AB2C 1400 LG4

Seats in Monel alloy Stainless Copper-base body and 400 BS 3076 steel type alloys. such disc 1449316S16 as gunmetal.

aluminium bronze or brasses

Stem Monel alloy Ni -AI-bronze Brasses 400 or K500 BS 2874 BS 3076 CA104

41

Page 22: Corrosion-selecting Materials for Sea Water Systems 001

Table 7.

Table 8.

Table 9.

42

Butlerflv valves

Component Preferred Acceptable Unacceptable

Body Rubber- lined Rubber- lined Unlined cast Ni - Resist grey cast iron or steel cast iron BS iron" 3468 AUS 202A

Seat Rubber Rubber Disc Ni-AI-bronze Stain less Cast iron or

BS 1400 steel BS 1632 brass AB2C

Stem Monel alloy Ni -AI-bronze 13% Cr 400 or K500 BS 2874 stainless steel BS 3076 CA104 and brass

Securing M onel alloy - -pins (eg 400 BS 3076 disc to stem)

• Where a grey cast Ifon or similar non ·corroslon·resisran l matenal IS used a separate sea l must be provided to prevent access of sea water to the body w here the stems penelrale the rubber lining

Diaphragm valves

Component Preferred Acceptable Unacceptable

Body Gunmetal BS Rubber-lined Unlined cast 1400 lG4 cast iron iron or steel

Heat Exchangers

T-ubing

General purposes '

For condensers where scoop intake is used

90 -10 cupro -nickel BS 2871 CN 102

70-30 cupro -nickel 2% Fe 2% Mn BS 2871 ·CN 108

' Where the manufacturers standard specificat ion uses alloys of improved corrosion resistance to 90 -10 cupro-nickel for example 70 -30 cupro-n lckel (BS 2871 CN 107) or 70 -30 cupro-nlckel 2% Fe 2% Mn. the improved matenal should be used as this choice may be related to the design of the particular component. Design water ve locity in main con densers and other tubular heat exchangers must not exceed 2.3 metreslsecond (75 hIs) or be less than 1 metre/second (3.3 h/s) .

Table 10.

Table 11.

Tube places

Preferred Acceptable Unacceptable

90-10 cupro - Aluminium bronze BS 2875 60 -40 brass nickel BS CA 105 and also 60 -39-1 2875 CN 102 Cu -Zn -Sn Naval brass BS

2875 CN 112 (where adequate cathodic protection is provided)

Water boxes and end covers Water boxes should be generou sly sized and designed so as to minimize turbulence at the tube in lets. Means should be provided to attach soft iron anodes - the area of such anodes being 10 per cent of the area of the tube plate. The water box and the tube plate should be electri ca lly bonded to ensure tha t the protect ive current fl ows from the anodes to the tube plate and tube ends. Air vent pipes should be fitted at the highest point of all water boxes and return ends .

Component Preferred Acceptable Unacceptable

large water 90-10 cupro- Cast Ni -AI - Unlined cast ·

boxes nickel l steel bronze BS iron or steel

fabrications 1400 AB2C, Ni - Resist cast iron BS 3648 AUS 202A, or gunmetal BS 1400 lG2 or lG4, steel lined with 90 -10 cupro-nickel steel

Small w ater Ni -Resist cast - Unlined cast

boxes iron BS 3468 iron or steel

AUS 202A, cast aluminium bronze BS 1400 AB2C or gunmetal BS 1400 lG2 or lG4

43

Page 23: Corrosion-selecting Materials for Sea Water Systems 001

Pumps

Table 12. Component Preferred Acceptable Unacceptable

Casing Ni -AI -bronze Gunmetal BS Cast iron BS 1400 1400 LG4 AB2C

Impeller Monel alloy Stainless Gunmetal BS 3071 steel B S 1 632 NA13 grade B or

ACI CD4-MCu

Shaft Monel alloy Stainless Brasses K500 steel E!:; 1 449

316 S16

Shaft Monel alloy Stainless Brasses sleeves' K500 steel B S 1449

316 S16

Wear rings Monel alloy ACI CN7N -505 (casi ng) (alloy 20) Monel all oy ACI CD4 -400 (impeller) MCu

' Where mechanica l sea ls are used these w ill nol normally be li lted

Strainers

Table 13. Component Preferred Acceptable Unacceptable

Body Gunmetal BS Ni-Resist Unlined cast 1409 LG4. cast iron BS iron or steel fabricated 3468 AUS 90 -10 202A cupro-nickel

Wire mesh or Monel alloy Stainless Brass. sheet perforated 400 BS 3075 steel BS 1449 or wire plate NA13 (wire) 316 S16

BS 3072 NA13 (plate)

Fasteners Monel alloy Ni -AI-bronze Brass or 13% and internal 400 BS 3076 8S2874 Cr stainless fittings NA13 CA104 steel

44

Initial treatment for a new system.

If the sea water available to the new system is clean and unpolluted its cI rcu lation through the system wi ll enable the matertals to form protectIve fIlms whIch wi ll reduce corrosion rates to very low levels. However. In most ftttlng-out basins. the sea water IS polluted and it is desirable and often essential to take the following precautions to enable the matertals to form protective films.

The system should be filled tn/llally wllh fresh water contatntng 5 ppm ferrous sulphate and thiS should be left tn the system for one day.

Following thiS. the system can be used for normal purposes during filting -out but ferrous sulphate should be added to the system and circulated for one hour per day at a concentration of 5 ppm throughout the fltling -out period.

The best crtterton for correct treatment of a system IS the appearance of the Internal surface of the piping and tubes. 1 hiS should' be red brown In colour. I he treatments described above can be repeated as often as required to produce such a film and. as ferrous sulphate IS a safe Inhibitor. no problems are likely to ensue from over-dosage. The BSRA note on ferrous sulphate treatment gives good practical adVice on the use of ferrous sulphate treatment. The above procedure should also be followed where units are re-tubed or renewed .

45

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46

Procedures for minimizing maintainance in existing systems.

General operation High water velocities are the cause of most corrosion failures In piping systems. A system designed in accordance with this document and using the specified materral is unlikely to experience serious trouble dUring normal operation . It IS common practice In pIping systems to run several components In parallel from one pump and where Independent components are shut down the flow through the remaining parts of the system will increase and failures may occur. Short (a few days) occasional periods of exposure to water speeds 10-15 per cent above design are unlikely to give problems In components which have formed good protective frlms. However. long and prolonged exposure to such conditions can cause problems and by-passes should then be arranged or water should be allowed to flow through the heat exchanger not In use. When units are to be left out of service for more than one week. for example during dry-docking or dUring long voyages (a good example IS a Butterworth Heater). they should be drained and flushed with fresh water. If left full of sea water this may become deoxygenated giving rise to pitting .

Ferrous sulphate treatment If the materrals and design conditions specified in this document are followed regular treatment of the system with ferrous sulphate should not be required. If. however. exposure to polluted water is likely to occur due to enterrng a port known to be polluted. or where farlures have occurred. then either of the following treatments can be applied:

5 ppm ferrous sulphate shoulcLbe added 10 the svstem for one hour per dav dUring the three davs prior 10 entering and after leaving POri and whilst in POri. DUring a prolonged vovage one treatment per week can be applied throughout the vovage.

I t should be noted that there are many ships In service which do not use ferrous sulphate treatment and yet have no serious corrosion problems In coolrng systems.

Ferrous sulphate shou ld not. therefore. be regarded as essentia l to successful performance but rather as a remedy where trouble has occurred or is likely. Provision shou ld. therefore . be made on new vessels for injection points to the system as this is inexpensive at the building stage. If ferrous su lphate injection is later decided upon it can then be fitted at minimum cost. Another method of adding iron is by means of a driven Iron anode. This method uses a piece of Iron which is made the anode in an impressed current system similar to a cathodic protection system. The current flow IS contro lled to give the correct concentration of Iron In the sea water. Similar precautions to those taken for cathodic protection systems must be observed.

Chlorine treatment Ch lorine. or more correct ly hypochlorite. IS sometimes added to sea-water systems to prevent marine fouling . The problems associated with fou ling are-

Pieces of shell etc. become dislodged and parliallv block tubes downstream causing local impingement attack and tube perforation.

In bad cases almost complete blockage of piping svstems can occur. thus restricting the water flow.

• C> Fouling can only settle on tubes when the water velOCity IS lower than about 1 metre/second (3 ft/s) so that the rrsk is greatest when the ship is in harbour and the system IS shut down. Copper-base alloys have good resistance to marine fouling (copper is used for anti­fouling paints) and even where foul ing attaches itse lf to a' copper-alloy surface. its growth and adhesion are restricted and on exposure to flowing sea water It IS usually swept away. There have been cases. however. on steel and alu minium brass pipes where severe fou ling and blockage have occurred and in such cases the fllting of anti-fouling devices is advisable .

47

Page 25: Corrosion-selecting Materials for Sea Water Systems 001

48

Repair and improvement of existing systems.

The general aim of repair and renewal of existing systems should be to make it conform to the materials and design criteria given In this document. Specific advice on some common ly experienced problems is as follows:

Pipes (90-10 cupro-nickel) Problems in the pipes themselves are rare provided they are made from non -ferrous materials and any failures which may occur are generally con fined to the pipe jOints. ie. brazed and welded con nections. If the failure is In a brazed joint. a repair by welding should be considered . particularly if the pipe is of large diameter. as experience shows that brazed jOin ts in such pipes are difficult to manufacture sa tisfactori ly. The braZing materials shou ld be of the type specified on page 38 and the alloy originally used should be checked as the use of an inferior alloy wou ld be an indi ca tion of more genera l failures in other joints. Failures of a welded JOint may indicate a loca l vibration and the need for extra pipe supports or a fl exible connection. Where such factors are absent the joints should be re -welded with the materia ls indicated in page 38 .

Valves These are the most common sou rce of failure in piping systems and often resu lt from unsatisfactory choice of material for the valve and particularly for details such as valve trim . Renewals should be made With the va lve types and materials indicated In pages 40-42. Where pipes have to be cropped to fit other types of valve this can easily be achieved by removing a section behind the flange and butt welding the flange section to the remainder of the pipe (assuming thiS IS 90-10 cupro- nickel) . Where the problem is due to loca l erosion to a va lve seat. for example. In a globe valve. it may be pOSSible to mach;ne out the defective area and fit a new seat as indica ted In Table 5. Another common source of trouble In valves IS the use of brasses and 13% Cr steel for stems. These corrode

------- ~

rapidly and shou ld be replaced With materials given In pages 40-42 . The life of such materi als as brass and 13% Cr steel IS likely to be short (two years. even under optimum condi tions) and If a repa ir has to be made With such materials Immed iate steps should be taken to prOVide the correct material as an early replacement.

Heat exchangers Failures of hea t-exchanger tubes In the alloys selected on page 42 are usually rare when the system IS properly deSigned. manufactured and carefully operated. Where failures occur. the reason may be obscu re and the tube supplier should be consulted With samples of tubes sen t to him for metallurgica l examination and adVice for further action . Where failures are occurnng reg ularly the introduction of ferrous su lphate treatmen t ca n be helpful. If tube Inlet -end corrosion IS the problem then the use of nylon tube -Inserts w ill often be benefiCial. In genera l If retublng IS conSidered necessary then upgrading from 90-10 cupro -n lckel to 70-30 cupro -nlckel will norma lly give Improved results. particularly If the 2% Fe 2% Mn 70-30 cupro -n lckells chosen. There should be no galvaniC problems from the use of say 70-30 cupro-nlckel In a unit otherWise tubed With 90- 10 cupro -n lcke l or aluminium brass Tube plate failures are rare and are usually due to dezlnclfl cat lon where brasses are used. The dezlnclflcatlon layer IS easily eroded by turbulent sea water so th at deeply eroded or pitted areas may form. CathodiC protec tion will reduce or elimi na te thi S problem and If It occurs ca thodiC protection by means of Iron anodes should be proVided as Indi ca ted on page 47 ·

Pumps One cause of maintenance In pumps IS erosion of the pump Impeller. Copper-base all oys are often used for Impellers and If the water velOCi ty IS high they w ill corrode rapid ly near the tip. Where thiS occu rs the replacemen t Impeller should be In the materials given on page 44. Wear rings will require occaSional renewa l but prOVided materi als such as Indicated on page 44 are used. thiS shou ld be Infrequent. Severe co rrosion/erosion of pump casings IS unusual but has been experienced In some larger tankers In recent years. Where thiS occu rs replacement of the casing may

49

Page 26: Corrosion-selecting Materials for Sea Water Systems 001

50

be necessary, In such cases Improved materials should be used, for examp le, If the casing IS made from gunmeta l then NI-AI -bronze should be fitted, Slight erosion of a casing can be repa ired by welding but for thiS the casi ng must be taken ashore to a specia li st repai r shop, Welding materi als are available for the materials common ly used for pump casings. If damage to the pump (usually the Impeller) has been caused by cavitation then a change of material IS unlikely to give much Irl)provement and the remedy wou ld be best sought by Improving the Inlet piping to the pump or a change of Impeller design (where thiS IS possil::> le). Sleeve wea r may occu r If the packing IS tightened excessively. ThiS ca n sometimes be repaired by welding and remachlnlng but replacement IS normally easier. Shaft failures by corrosion fatigue occasiona lly occu r and If experienced the presence of vibration crltlca ls and a dynamiC balance of the rotating parts shou ld be checked. Replacemen t of the shaft with a material of higher corrosion fatigue strength (for example, Monel alloy K500 Instead of stain less steel) and attention to deSign detail (ie, proviSion of generous fill et rad II) may solve the problem.

Recommended further reading

W . H. Falconer and L. K. Wong 'Sea water systems' Ins!. Mar. Eng. Materials section symposium, March 1968.

S. A. Fielding . 'Design study of condenser and ci rcu lation system' Marine Tech . 1971, April. Paper presented to the Chesapeake Section of the Soc. of Naval Arch. and Mar. Eng. 1970, January.

P. T. Gilbert and W. North . 'Copper alloys In marine engineering' Trans. Ins!. Mar. Eng . 1972 . vol. 84.

S. H. Frederick 'Wastage of non-ferrous salt water systems' BSRA Report No NS375, July. 1973.

51

Page 27: Corrosion-selecting Materials for Sea Water Systems 001

Table 14.

Table 15.

52

3. National materials standards.

Copper-nickel heat exchanger tube and piping alloys

Alloy British USA Japan Standard ASTM JIS

90 -10 2871 Part B111 or H3632 Copper- 2 or 3 B466. CNTF1 or nickel-iron CN 102 Alloy 706 H3251

Class 1

70 -30 2871 Part B 111 or H3632 Copper- 2 or 3 B466. CNTF3 ni ckel-iron CN 107 Alloy 715

66 -30 -2-2 287 1 Part - -Copper - 3 CN 108 nickel- iron -manganese

Germany DIN

1755 or 1785 2.0872

1785 2.0082

-

BnlJsh Siandard composilions (main alloymg elemems). weigh I per cem

Alloy Copper Nickel Iron Manganese

90 -10 remainder 10.0 -11 .0 1.00-2.00 0.50-1.00 Copper-nickel-iro n

70-30 remainder 30.0 -32.0 0.40-1.00 0.50-1.50 Copper -nickel- iron

70-30 remainder 29.0-32.0 1.7 -2.3 1.5 -2.5 Copper-nickel-iron-manganese

Table 16.

Table 17.

Nickel -copper alloys

Form British USA Germany Standard

Rods and 3076 NA 13 ASTM B164 DIN 17743 secti ons Class A 2.4360

QQ- N-286(a) (AM554676-M onel alloy K500)

Sheet and 3072 ASTM B127 DIN 17743 plate 2.4360

Castings 3071 (NA 13) - -

Bmish Siandard composilions (main allovmg elemems). weigh I per cem

Form Copper Silicon Iron Nickel

Wrought 28.0 -34.0 - 2.5 max not less than 63%

Cast 28.0-32.0 0.5 -1.5 3.0 max remainder (3071 NAI )

53

Page 28: Corrosion-selecting Materials for Sea Water Systems 001

Table 18.

Table 19.

Nickel aluminium bronze

Form British USA Germany Standard

Castings 1400 AB2-C B148 DIN 1714 Alloy 90 G-Cu -AI -

10 Ni

Plate 2875 B 171 DIN 17670 (CA 105) Alloy 628 AI Bz 10 Ni

Rod. bar 2872.2874 B124 B150 DIN 17672 and forgings (CA 104) Alloy 630 AI Bz 10 Ni

Bmish Standard compositions (main allOYing elements). w eight per cent

Form Copper Aluminium Nickel Iron

Castings remainder 8.5 -10.5 4.5 -6.5 3.5 -5.5

Plate remainder 8.5-10.5 4.0 -7.0 1.5 -3.5

Rod. bar remainder 8.5-11 .0 4.0-6.0 4.0-6.0 and forg ings

Table 20.

Table 21 .

Table 22.

Table 23.

Copper-tin alloys (Gunmetals)

Alloy British USA Germany Standard

Gunmetals 1400G.l B143 DIN 1705 (a ll cast) Alloy lA Alloy G-Cu

10 Zn

1400 LG .2 B145 1705 Alloy Alloy 4A G Cu 5 Zn Pb

1400 LG.4 - -

Briush Standard compositions (main alloying elements). weight per cene

Alloy Copper Tin Zinc Lead Nickel

BS1400Gl remainder 9.7 -10.5 1.75-2.75 1.5 max 1.0 max BS 1400 LG2 remainder 4.0-6.0 4.0-6.0 4.0-6.0 2.0max BS 1400 LG4 remainder 6.0 -8.0 1.5-3.0 2.5 -3.5 2.0'

' Tin I ! nickel contenl 7.0 -8.0

Stainless steels

Form British USA Germany Japan Standard

Wrought BS 1449 AISI 316 DIN 17007 SUS32 stainless Part 4 1.4401 steel 316S16' 1.4436

Cast BS 1632 ACI 1.4408 -stainless Grade B' CF-8M steel

• Low carbon or stabil ized grades should be used w here welding IS

10 be carried oul Grades 316S 12 (wroughl) and grade C or F (casl) .

Bnush Standard composiuons (main alloying elements). weight per cent

Alloy Carbon Chromium Nickel Molybdenum

B1449316S16 0.07 max 16.5-18.15 10.13 2.25-3.0 B 1632 Grade B 0.08 max 17.0-20.0 10.0 min 2.0 -3.0

55

Page 29: Corrosion-selecting Materials for Sea Water Systems 001

Table 24.

Table 25.

Tabl e 26.

Tabl e 27

56

Austenitic cast iron

Alloy British USA Germany Standard

Ni-Resist BS 3468 ASTM DIN 1694 Iron type 02 AUS 202A A439 -70 GGG -NI CR 20.2

Brtush Siandard composlllons (mam alloymg e/emenls). welglll per cem

Alloy Copper Sili con Nickel Chromium

AUS 202A 3.00 max 1.75 ·3.00 18.0 -22.0 1.75 -2.50

Silver solders

Alloy USA Germany

BS 1845 AG I ASTM B260-B -AG IA L-AG50C D

B-AG3 WN 25143 DI N 8513

Brtush Siandard composlllons (main alloymg elemems) . w eigh I per cem

Alloy Silver Copper Zinc Cadmium

BS 1845 AGI 49.0-51 .0 14.0 -16.0 15.0 -17.0 18.0 -20.0

Suitable proprietary alloys of slight ly different composi tion are also avai lable for example EASYFLO' No 3

• Trade mark