Engineering Failure Analysis 39 (2014) 65–71

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Engineering Failure Analysis

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Corrosion in water supply pipe stainless steel 304 and a supplyline of helium in stainless steel 316

http://dx.doi.org/10.1016/j.engfailanal.2014.01.0171350-6307/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +351 22 508 1643; fax: +351 22 508 1449.E-mail address: jipm@fe.up.pt (J.I. Martins).

C.M.B. Martins a,b, J.L. Moreira a, J.I. Martins a,⇑a Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Roberto Frias, 4200-465 Porto, Portugalb Universidade do Minho, CITCEM, Post-Doctoral scholarship granted by FCT, Portugal

a r t i c l e i n f o

Article history:Received 11 October 2013Received in revised form 22 December 2013Accepted 25 January 2014Available online 2 February 2014

Keywords:Stainless steel 304Stainless steel 316Pitting corrosionHypochloriteWelding

a b s t r a c t

A corrosion spreading throughout the 304 stainless steel tubing of a water system tosupply various buildings was observed, and also leaks were detected in welding zones.The same place is also crossed by a gas distribution network, with a helium pipe 316 ofstainless steel that required periodic repairs more or less every two years, due to thepresence of leaks. The tests showed that both types of stainless steel have sufferedlocalized corrosion induced by hypochlorite ion, in a mechanism of dry/wet, and that thewelding procedure performed on the 304 stainless steel is unacceptable. Some immediateprocedures were undertaken to keep systems running, but recommendations were givenfor a final resolution of the problems.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Stainless steels (SS) contain in their composition essentially iron, carbon and at least 11% of chromium, the elementresponsible for the corrosion resistance and that also improves the material performance to scaling, strength and wear.The high corrosion resistance of these alloys is attributed to the presence of a thin invisible film and passive [1] characterizedby stability, durability, adhesion and self-repairs. Austenitic stainless steels are a special family of alloys, with exceptionalcorrosion resistance and outstanding mechanical properties, which make them suitable for being used as a constructionmaterial in aggressive environments, such as heat exchangers, drilling rigs for oil extraction, dairy industry, beverageindustry, and wastewater treatment.

The alloy 304 SS is austenitic steel with a minimum of 18% chromium, 8% nickel and up to 0.08% carbon. It is non-magneticsteel, not susceptible of hardening by heat treatment, but alternatively can be cold deformed with the increasing tensilestrength. The alloy 316 SS is also austenitic steel, with molybdenum up to 3% and higher nickel content (10–14%) of type304 SS, which gives the possibility to be cold-hardened with a lower deformation rate. In the annealed state, exhibits excellentductility and can be easily stretched and folded. The more deformed parts should be annealed to remove strains.

Despite the extraordinary corrosion resistance in various media, these austenitic stainless steels in chloride environments[2] may undergo pitting corrosion. The aggressiveness and ability of Cl� ions to initiate this type of corrosion are well known[3–5]. There are several theories trying to explain the pitting initiation, such as local acidification [6,7], depassivation–repassivation behavior [8,9], point defects models [10,11], and anion penetration/migration [12,13], but for all them it isnecessary to assume the adsorption of chloride anion on the metal surface.

66 C.M.B. Martins et al. / Engineering Failure Analysis 39 (2014) 65–71

What essentially distinguishes the type 316 SS from the type 304 SS is the addition of molybdenum up to about 3%. Themolybdenum increases the corrosion resistance of this alloy in various environments (brines, bleaching, biofluids, etc.), and, inparticular, reduces or inhibits the pitting corrosion induced by the chlorides. The pitting resistant equivalent (PRE) iscommonly used as an indicator of stainless steels resistance to pitting. PRE is calculated from the following expression, whereone can see the importance of molybdenum, given the usual content of nitrogen in the stainless steels 300 series (60.10%):

Table 1Chemic

Stee

304316

PRE ¼ %Crþ 3:3�%Moþ 16�%N ð1Þ

The rate of pitting corrosion can be quite serious once initiated; there is a strong tendency to continue growing, eventhough most adjacent steel remains intact. Temperature is also a key factor in the early pitting [14,15] and, based on thepotentiostatic determinations at +300 mV vs. SCE at pH = 6.0 and 0.1% Cl�, the critical pitting temperature (CPT) for the304 and 316 SS is around 30 �C and 55 �C, respectively.

The hypochlorite ion (ClO�) is aggressive towards stainless steels, acting similarly to moist chlorine gas, and the chlorideion (Cl�) is the initiator of pitting corrosion. The pitting or crevice corrosion can occur in various types of stainless steels in15% hypochlorite solutions at room temperature. There is also an additional risk of stress corrosion at elevated temperatures.Some cases of localized corrosion on 304 SS have been reported in domestic environments associated with leachate. If thisoccurs, an immediate dilution by washing prevents corrosion; otherwise, a pitting corrosion can occur overnight.

Under the combined effect of stress and corrosion, stainless steels will be subject to a more rapid and severe corrosion,and also stress corrosion cracking (SCC). Consequently, the austenitic stainless steels are limited in their application toaqueous media to about 50 �C with small quantities of chlorides (parts per million). It is risky to establish the use limits, sinceconditions wet/dry concentrate chlorides may increase the probability of SCC.

This paper analyzes the problems of corrosion observed in metallic materials in a sector that supplies water to severalbuildings, and where also passes the piping system of gas distribution to one building.

2. Experimental details

The chemical composition of the stainless steels is presented in Table 1.The scanning electron microscopy (SEM) images and the analysis by energy dispersive spectroscopy X-ray (EDS) were

made with a FEI Quanta 400FEG equipment, fitted with a probe for micro analysis EDAX Genesis X4 M. The pressure insidethe chamber was about 6 � 10�2 Pa. The distance between the objective lens and the sample, ranged between 6 mm and15 mm. The SEM filament was operated at variable current and at 15 kV voltage using magnifications 100�, 250� and1000�. The SEM/EDS analysis has been performed in CEMUP (Materials Centre of the Oporto University).

The electrochemical experiments were performed in a one-compartment cell with three electrodes connected to Autolabmodel PGSTAT20 potentiostat/galvanostat with pilot integration controlled by GPES 4.4 software. Platinum has been used asan auxiliary electrode and Ag/AgCl (1 M in KCl) as a reference electrode. The section annulus samples of 316 SS weremounted in an Epofix resin, mechanically polished with abrasive papers SiC of decreasing sizes (250, 500, 1000, 2500 mesh)and finally with paste of diamond of 6 lm. Before the trials, the samples were cleaned in acetone and ethanol baths providedwith ultra-sounds; the open circuit potential stabilized in a period of 15 min, and finally it was polarized by applying a scanrate of 10 mV s�1.

Radiographs were performed in situ using the equipment XRS-3 with a pulsed X-ray source and the Vidisco imagingsystem.

The total chlorine (aqueous molecular chlorine, hypochlorous acid and hypochlorite ion) of the water in the tank resultingfrom the hypochlorite hydrolysis was determinate by DPD method, and shows a value of 1.08 mg as Cl2/L. The watercondensate on a tube near the place of the tank where the recirculated water enters presents values of the order of0.08–0.10 mg as Cl2/L.

The temperature and humidity in the room fluctuates between 15 �C and 30 �C and 45% and 75%, respectively, dependingon the season.

The methodology used to determine the eventual leak in piping was the following: (1) a general cut in gas supplies to allservers; (2) let pass about 15 min to stabilize the pressure across the network; and (3) finally, the pressure values wereregistered at various points. A variation of 0.25 bars is very noticeable and was taken as an indication of a leak.

3. Characterization of the water supply system

Fig. 1 shows the system of water supply to a set of several buildings. The public water supplies a reservoir with a capacityof 300 m3, which finality is to regulate the average daily consumption of 80 m3. The water of the tank comprising four

al composition of stainless steels.

l type C P Mn Si S Ni Cr Mo

0.08 0.04 1.65 0.02 0.03 9.45 18.25 –0.08 0.04 1.95 0.50 0.03 10.30 17.65 1.68

(a)

11 11 11 116

7

8

9

10

12

14

5

4

13

4

(b)

2

18

(c)

610

94

43

Fig. 1. a) View of the storage tank; b) View of the water inlet and distribution; c) Detail of the pumping system. Legend: 1) Piping of the public water supply(76.10 mm OD � 2.00 mm wall, 304 SS); 2) Filter of sand and carbon for solids retention; 3) Supply of water to each cell of the tank (76.10 mm OD �2.00 mm wall, 304 SS); 4) Water outlet of each compartment (219.08 mm OD � 3.759 mm wall, normal painted steel); 5) Piping union of the tank outputs;6) Group pumping CRN32; 7) Manifold piping the output of the pumps (168.30 mm OD � 2.77 mm wall, 316 SS); 8) Water supply pipe to the buildings(204.00 mm OD � 2.00 mm wall, 304 SS); 9) Recirculating water pump; 10) Water recirculation piping to the tank compartments (50 mm OD � 9 mm wall,PVC); 11) Overflow tube (104.00 mm OD � 2.00 mm wall, 304 SS); 12) Water discharge channel of filter system and cleaning of compartments tank; 13)Frame tank with metal mesh for ventilation; 14) Stairway to access the tank compartments.

C.M.B. Martins et al. / Engineering Failure Analysis 39 (2014) 65–71 67

separate sections is partially recirculated to assure a permanent aeration. Its pH and total residual chlorine are controlled byperiodic additions of sodium hypochlorite to a pH around 7–8, which means that the available chlorine activity is HOCl andClO�.

A set of four pumps Grundfos CRN32 (multistage centrifugal pumps with the head and all components in contact with theliquid in 316 SS) bombards the water from the reservoir through the carbon steel pipe and distributes it through a pipe 304SS. As further noted, the continuity of the pipe water supply is processed mainly using cross-connections by welding ratherthan connections phalanges.

One piping of helium (99.9999%) in 316 SS passes in the same place of the water supply system, where tubes and elbowsare connected through threaded connections in the same material.

4. Assessment of the situation

On the basis of the gathered information and visual observations of the pipes of the water supply, Fig. 2, it is deduced thefollowing:

– A state of localized corrosion throughout the metal pipe system for water supply with the 304 SS after twelve years ofoperation, Fig. 2(a)–(d).

– The presence of water leakage in the areas of the transversal welding supply line, Fig. 2(b).

In the case of 316 SS for transporting helium was also observed a localized attack, Fig. 3(a), similar to the one observed inthe 304 SS, and, in recent times, helium leaks in the network have appeared, more or less, at two years intervals.

(a)

(c) (d)

(b)

Fig. 2. Aspect of corrosion on metal parts within the water supply system. (a) Elbow: weld bead transverse and flange bolts in 304 SS; (b) elbow: weld beadtransverse in 304 SS; (c) zone of the longitudinal welding of tubes in 304 SS; and (d) flange connection area in 304 SS.

(a)

(b)

Fig. 3. Analysis of the morphology of 316 SS tube surface: (a) optical photograph; and (b) backscattering electrons SEM image.

68 C.M.B. Martins et al. / Engineering Failure Analysis 39 (2014) 65–71

Fig. 4. Linear potential dynamic curves of 316 SS in hypochlorite solutions. Scan rate 10 mV s�1 and temperature 20 �C.

Table 2SEM/EDS analysis of several points on the surface of the SS 316 pipe.

Area Elementary composition (%)

C O Al Na Si P S Cl K Mn Cr Fe Ni

Z1 8.30 35.34 0.42 – 0.42 0.19 1.62 4.77 0.50 – 8.45 36.70 3.38Z2 10.02 35.73 – 0.28 0.33 0.17 1.11 5.00 0.44 0.47 4.53 38.11 3.82Z3 – 38.41 – 0.50 0.51 0.23 2.53 5.59 0.63 – 12.51 34.93 4.16Z4 23.42 10.43 – – 0.13 0.16 0.18 0.79 0.19 1.02 3.80 56.68 3.21

C.M.B. Martins et al. / Engineering Failure Analysis 39 (2014) 65–71 69

5. Results

5.1. Scanning electron microscopy

Fig. 3(b) shows the morphology of the helium perforated pipe from backscattering electrons SEM image, where it is pos-sible to observe pits with diameters of the order of 0.06–0.12 mm. The analysis by SEM/EDS of some areas in the surfaceshow clear the presence of chlorine, Table 2, which come to confirm that the surface of the 316 SS tube is submitted to pittingcorrosion.

5.2. Electrochemical behavior of 316 SS in hypochlorite medium

The voltammograms performed in several hypochlorite solutions are synthesised in Fig. 4. The results show the follow-ing: (1) the passivation current density is practically the same, 4.5–5.4 lA cm�2, in the range of 5–100 ppm of hypochloriteconcentration; (2) the pitting potential is more active with the hypochlorite concentration, +185 to �130 V vs. SCE, whichmeans a reduction of the passivation plateau; and (3) a repassivation of the material occurs at more noble potentials, butwith a higher current density, 20–30 lA cm�2.

The action of hypochlorite in stainless steel is attributed to the following cathodic reaction:

OCl� þH2Oþ 2e! Cl� þ 2OH� ð2Þ

This reaction is an alternative to the oxygen reduction. Thus, the adsorption of chloride ion on the steel surface promotesthe formation of an intermediate complex [16], which leads to the dissolution of the passive film, i.e., the nucleation for pit-ting corrosion. The following reactions may elucidate this behavior:

FeðOHÞ2 þ Cl� ! FeðOHClÞ þ OH� ð3Þ

FeðOHClÞ þ nCl� ! ½FeðOHÞClðnþ1Þ��n ð4Þ

FeðOHÞ3 þ Cl� ! FeðOHÞ2Clþ OH� ð5Þ

FeðOHÞ2Clþ nCl� ! ½FeðOHÞ2Clðnþ1Þ��n ð6Þ

½FeðOHÞClðnþ1Þ��n þHþ ! H2Oþ Fe2þ þ ðnþ 1ÞCl� ð7Þ

½FeðOHÞ2Clðnþ1Þ��n þ 2Hþ ! 2H2Oþ Fe3þ þ ðnþ 1ÞCl� ð8Þ

(a) (b)

(c) (d)

Fig. 5. Radiographs in the welding zones of the 314 SS piping.

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In brief, the chlorine acts on the stainless steel in a state of wet/dry as follows: (1) it is transported to the steel surface bycondensing water vapor from the storage tank; (2) the continuity of this phenomenon promotes increased concentration ofhypochlorite on the steel surface, as well as of chloride ion; and (3) when it is reached the critical concentration of chlorideon the metal surface occurs the pitting corrosion.

5.3. Test gas tightness

The tubing network that supplies the gas has an internal diameter of 8.8 mm, which means that it has an interior volumeof 61 ml per meter length. So, estimating the distances between the various sectors of the network, allowed the calculation ofthe gas volume inside the pipes.

Defining a hypothetical leakage flow of 10, 20, 50 and 100 ml/min, the necessary time to attain different values of pres-sure drop in the sectors, was determined, assuming that the leak flow remains constant and that the pressure in the systemdoes not practically change. The results of tightness test showed a clear depressurization of helium which means that a leakin the respective piping was found. On the other hand the pressure drop of 0.25 bars is the smallest value of the scale intervalfor gauge pressure reading, which may correspond to a reading error or even a thermal oscillation of the entire network dur-ing a long period of time. So, we may assume that the pressure remained unchanged for the other gases.

Regarding the tightness test for the helium network the leaks locations were investigated. Thus, the network was discon-nected by sectors and subsequently pressurized for pressure monitoring along time. There were detected two sites of leakagein the piping: at the elbow when gas network enters inside the technical shaft and at a distance of about 2 m from this area.

5.4. Radiographs of welding

The observation of the radiographs made in welding zones of the water supply pipe, Fig. 5, allows the following remarks:(1) the presence of pits in the welding zone, Fig. 5(a) and (b); (2) welding leaks, irregular cord, lack of fusion and porosity,Fig. 5(c); and (3) corroded thickness from the outside to the inside (1.04–1.77 mm), and the irregularity of the welding,Fig. 5(d).

These results state that the procedure used is a TIG welding without the forming gas use inside the pipe. Because of thisthe root of the weld is made of individual hills and the surface is rough and flourished, as a result of the oxidized surface ofthe weld pool which could not be spilled. This welding procedure for a water supply system leads the formation of Fe2O3 andcorrosion of weld from the inside of the pipe.

It is important to refer that the 316 SS tubing shows no welding problems, because it has been realized through threadedconnections.

6. Discussion

The material used to supply water to several buildings, 304 SS, is not the most convenient, as can be deduced from theobservation of corrosion on its outer surface.

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In fact, the aeration of water tank requires a water recirculation and, therefore, increases the relative humidity in theplace where they are located. Water vapor brings the hypochlorite ion that condenses on the metal surface. Due to themechanical ventilation of the site (running or stopped) and season (high or low temperatures), it is developed a wet/dryprocess on metallic surfaces that leads to a high concentration of hypochlorite ion in the condensate. This ion inducesthe formation of chloride ion and promotes pitting corrosion. The corrosion on outside surface will be yet aggravated bythe differential concentration cells developed in the crusts formed along the piping, as the result of the action of moistureon the plaster of walls or ceilings.

The welding procedure used for joining pipe elements instead of flanges for the conditions of humidity in the place tran-scribed is another misconception. Indeed, the radiographs showed a wrong welding technique allowing corrosion from theinside to outside of pipe, which was an aggravating factor for the behavior of 304 SS under the wet/dry external conditions.

The design of the network of gas should never have passed through this space. The 316 SS of the helium network has beenreplaced more or less within two years periodicity. The phenomenon of pitting occurs similarly to that observed for the 304SS tubing.

The problems of water leakage in the 304 SS pipe were immediately solved as follows: (1) in the elbows by performingin situ a new welding; and (2) in rectilinear duct by placing a tight rubber seal with a clamp.

Regarding the 316 SS tubing, where leakage was detected, inside the technical shaft, the pipe was replaced and,additionally, protected with a rubber sheath, i.e., eliminating the contact with the corrosive environmental. Also, to avoidthe piping corrosion, inside the technical shaft, has also been eliminated the opening that allowed the air circulation throughthe duct.

7. Conclusions and recommendations

According to the observations and experimental results it was concluded the following:

1. Stainless steel 304 and 316 are not suitable materials for being in contact with vapor containing the hypochlorite ion, dueto the potential development of dry/wet conditions that induce pitting corrosion.

2. The welding of stainless steel pipe sections must be performed by skilled technicians, and in accordance with the stan-dards prescribed. The radiographs performed have showed technical deficiencies in its execution, a difficult problem tosolve later.

In order to solve the problem still open on the piping water supply should be taken into consideration the following rec-ommendation: replacing all 304 SS tubing by another polymeric material, polypropylene for example.

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