faq - corrosion protection principle

A B Joe H

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Tugas 4 – Prinsip Proteksi Korosi

Classification of corrosion protection methods

Active corrosion protection

Passive corrosion protection

Permanent corrosion protection

Temporary corrosion protection

Active corrosion protection

The aim of active corrosion protection is to influence the reactions which proceed during corrosion, it being possible to control not only the package contents and the corrosive agent but also the reaction itself in such a manner that corrosion is avoided. Examples of such an approach are the development of corrosion-resistant alloys and the addition of inhibitors to the aggressive medium.

Passive corrosion protection

In passive corrosion protection, damage is prevented by mechanically isolating the package contents from the aggressive corrosive agents, for example by using protective layers, films or other coatings. However, this type of corrosion protection changes neither the general ability of the package contents to corrode, nor the aggressiveness of the corrosive agent and this is why this approach is known as passive corrosion protection. If the protective layer, film etc. is destroyed at any point, corrosion may occur within a very short time.

Permanent corrosion protection

The purpose of permanent corrosion protection methods is mainly to provide protection at the place of use. The stresses presented by climatic, biotic and chemical factors are relatively slight in this situation. Machines are located, for example, in factory sheds and are thus protected from extreme variations in temperature, which are frequently the cause of condensation.

Temporary corrosion protection

The stresses occurring during transport, handling and storage are much greater than those occurring at the place of use. Such stresses may be manifested, for example, as extreme variations in temperature, which result in a risk of condensation. Especially in maritime transport, the elevated salt content of the water and air in so-called salt aerosols (salt spray) may cause damage, as salts have a strongly corrosion-promoting action.

Pembagian Proteksi Korosi dibagi

menjadi 6 tipe:

(1) MATERIAL SELECTION

(selection of proper material for a particular

corrosive service)

Metallic [metal and alloy]

Nonmetallic [rubbers (natural and synthetic), plastics,

ceramics, carbon and graphite, and wood]

Metals and Alloys

No Environment Proper material

1 Nitric acid Stainless steels

2 Caustic Nickel and nickel

alloys

3 Hydrofluoric acid Monel (Ni-Cu)

4 Hot hydrochloric acid Hastelloys (Ni-Cr-

Mo) (Chlorimets)

5 Dilute sulfuric acid Lead

No Environment Proper material

6 Nonstaining atmospheric

exposure

Aluminium

7 Distilled water Tin

8 Hot strong oxidizing

solution

Titanium

9 Ultimate resistance Tantalum

10 Concentrated sulfuric

acid

Steel

[2]Protective Coatings / Wrapping

Provide barrier between metal and environment.

Coatings may act as sacrificial anode or release substance that

inhibit corrosive attack on substrate.

Metal coatings :

Noble – silver, copper, nickel, Cr, Sn, Pb on steel.

Should be free of pores/discontinuity coz creates

small anode-large cathode leading to rapid attack at

the damaged areas.

Sacrificial – Zn, Al, Cd on steel. Exposed substrate

will be cathodic & will be protected.

Application – hot dipping, flame spraying, cladding,

electroplating, vapor deposition, etc.

Surface modification – to structure or composition by use of directed energy or particle beams. E.g ion implantation and laser processing.

Inorganic coating : cement coatings, glass coatings, ceramic coatings, chemical conversion coatings.

Chemical conversion – anodizing, phosphatizing, oxide coating, chromate.

Organic coating : paints, lacquers, varnishes. Coating liquid generally consists of solvent, resin and pigment. The resin provides chemical and corrosion resistance, and pigments may also have corrosion inhibition functions.

[3]Alteration of Environment

Typical changes in medium are :

Lowering temperature – but there are cases where

increasing T decreases attack. E.g hot, fresh or salt water

is raised to boiling T and result in decreasing O2 solubility

with T.

Decreasing velocity – exception ; metals & alloys that

passivate (e.g stainless steel) generally have better

resistance to flowing mediums than stagnant. Avoid very

high velocity because of erosion-corrosion effects.

Removing oxygen or oxidizers – e.g boiler feedwater

was deaerated by passing it thru a large mass of scrap steel.

Modern practice – vacuum treatment, inert gas sparging, or

thru the use of oxygen scavengers. However, not

recommended for active-passive metals or alloys. These

materials require oxidizers to form protective oxide films.

Changing concentration – higher concentration of acid

has higher amount of active species (H ions). However, for materials that exhibit passivity, effect is normally negligible.

Environment factors affecting

corrosion design :

Dust particles and man-made pollution – CO, NO,

methane, etc.

Temperature – high T & high humidity accelerates

corrosion.

Rainfall – excess washes corrosive materials and

debris but scarce may leave water droplets.

Proximity to sea

Air pollution – NaCl, SO2, sulfurous acid, etc.

Humidity – cause condensation.

[4] Cathodic Protection

Cathodic Protection: DC current injection towards pipeline,

result: pipeline is “shifted” as cathode.

Sacrificial anode

Impressed current

Galvanic Sacrificial Anode - Principle

Metal Driving voltage (Vd)

Negative Vd = active metal =tendency to corrode

Pipeline: connected with active metal

Anode: Mg, Zn Cathode: pipeline

Corrosion on Mg, Zn hence “sacrificial”

Galvanic Sacrificial Anode-Principle

Material Driving Voltage (V)

Silver (Ag/Ag+) +0.8 (cathodic)

Copper (Cu/Cu2+) +0.34

Water (O2+H20+4e- = 4OH-) +0.401

Hydrogen (H2) 0 (reference)

Iron (Fe/Fe2+) -0.44

Zinc (Zn/Zn2+) -0.76

Magnesium (Mg2+) -2.36 (anodic)

Galvanic Sacrificial Anode-Installation

(Source: Peabody’s Control of Pipeline Corrosion)

Galvanic Sacrificial Anode-Installation

(Source: Peabody’s Control of Pipeline Corrosion)

Impressed Current

Minimum potential: -0.85V Reference electrode: Copper Sulfate

(Source: Peabody’s Control of Pipeline Corrosion)

Galvanic Sacrificial Anode-Design

Calculate total area to be protected (Ap) Determine the current density (ρ) Calculate total protection current (Itot = ρ .Ap) Calculate R per anode : R = f(d,l,ρ) Calculate Ia per anode : (Vd-0.85/R) Calculate total anode needed : Initial : N=Itot/In Lifetime: N = f(mass,lifetime,A/poundyear)

Impressed Current-Principle

• Rectifier

• Variable voltage, variable current

• Higher current density (> 1 A)

• Higher soil resistivity (> 104 Ωcm)

Impressed Current-Installation

• Types of anode: Graphite : big CR (pound/A/year) High Silicon Cast Iron : medium CR Platinum & niobium : small CR

• Rectifier rating: Voltage, Amperes, Watt, Freq. Cooling System : Air, Oil Efficiency

CR: Consumption Rate

Impressed Current - Installation

(Source: Peabody’s Control of Pipeline Corrosion)

Impressed Current - Design

Calculate total area to be protected (Ap) Determine curent density (ρ) Calculate total protection current (Ip) Calculate total anode needed (N) Initial Lifetime

Calculate total anode resistance (Rtot = f(N,ρ,d,L,spacing)) Calculate rectifier specification (Vdc, Idc, Pdc = f(Rtot,Ip)

Different Corrosion protection methods Anodizing

Advantages

A tough surface layer which has very good corrosion protection

properties and very good adhesion to the surface.

Disadvantages

Must be applied after welding or brazing if the joining areas are to be

protected. This can be complicated for large structures. Can not be done

on site.

When to use

For protection against weathering and scratching/abrasion.

Conversion coating

Advantages

A non costly and simple protection of the aluminum surface, which in addition

increases the adhesion of lacquers and adhesives.

Disadvantages

Has limited resistance to mechanical and thermal influence.

When to use

Primarily used as a pre-treatment before lacquering or adhesive bonding.

Lacquering

Advantages

A high quality lacquering system has very good corrosion protection

properties.

Disadvantages

The performance of the lacquering system is very dependent on the quality

of the pre-treatment and application work.

Relatively expensive.

When to use lacquering

Where appearance and/or corrosion performance is very important

Inhibitors

Advantages

Can be tailored to give excellent protection in specific

environments.

Disadvantages

Expensive to use with large amounts of liquid.

May cause increased corrosion if incorrectly used.

When to use inhibitors

For protection against internal corrosion in closed systems,

circulating or non-circulating

Protective adhesive tapes

Advantages

Prevents galvanic contact. Grease filled tapes will seal crevices.

Disadvantages

Costly to apply. May need to be supported in place.

When to use

Buried pipelines