Piping Corrosion and Corrosion Protection

1. Introduction

Corrosion is the degradation of a metal or alloy, as a result of chemical reaction with its environment. The degradation is caused by an oxidation reaction, In electrochemical sense, oxidation is metal atom (M) losing electrons. (Not necessarily to Oxygen).

M → M + e

By the corrosion attack of any type, component can weaken. The rate and nature of attack is influenced by mechanical stress, thermal stress, cyclic stress, temperature, type of environment, type and nature of corrosion product formed, etc. Since the parameters are at large and can vary, it is difficult to predict the corrosion behavior. The harm to the life of the component also depends on the type of corrosion attack Pitting type-localized corrosion may puncture the container earlier than a uniform corrosion. And the spalled corrosion product debris of uniform corrosion may choke the inside of heat exchanger. threatening its life.

Study of all parameters causing corrosion, selection of proper material, proper design and or corrosion protective system and the data of corrosion testing is very useful in reducing the premature failure of the components Preventive maintenance and monitoring during service is also important for obtaining reduced hazard level of corrosion.

Broadly corrosion process can be classified as dry corrosion and wet corrosion.

2. Dry Corrosion

In hot dry gas metal tends to oxidize to form an oxide scale on the surface If the gas is containing corrosive ingredients as sulfides or hydrogen the attack can he more severe. Thermodynamic considerations rule the oxidation tendency. Once the oxide film or any other corrosion product film is formed, farther corrosion is controlled by the properties of this film. If the film is not porous, thin, adherant to the base metal and chemically resistant, then the corrosion rates get drastically reduced once this film is formed e.g. stainless steel. In other case if the film is porous, loose, soft or chemically active, corrosion rates continue the pace. E.g. Magnesium, sodium. Usually with higher temperatures, the corrosion rate increases.

The attack of hydrogen, ammonia gas, and sulfide gas is observed to the specific metals. Materials like titanium or steels when exposed to the dissolved hydrogen, they tend to reduce toughness called hydrogen embrittlement. If the steel is exposed to hydrogen sulfide gas. it reacts to form iron sulfide and available hydrogen diffuses in to the steel. This diffused hydrogen reduces the toughness.

During electroplating, the hydrogen generated may diffuse inside the steel component to be plated. In acid pickling plant. hydrogen easily penetrates the container and makes it brittle. When this embrittlement is noticed, in some cases the baking to about 1500C is done to expel the hydrogen. Decarburization of steel components, exposed to higher temperatures is also a kind of dry corrosion.

3. Wet Corrosion

When a metal is exposed to an aqueous solution of acid or alkali, wet corrosion occurs. It is electrochemical in nature. The metal gets transformed to the metal ions and goes to the electrolyte. To continue the process, closed electrical circuit is necessary. Two electrodes as anode and cathode and the electrolyte are required- The electrodes can be two different components or the different ‘ areas of same component. At anodic site, corrosion takes place by oxidation reaction. At cathodic site, reduction reaction is observed. If electrolyte is acidic, hydrogen evolution is obser ved at cathode. The electrochemical reaction takes place by at least one oxidation and one reduction reaction. These individual reactions are called as half - reactions. e.g. if zinc rod is immersed in acid solution, Zinc will corrode by oxidation reaction. The hydrogen ions from solution are reduced by reaction 2 H+ + 2 e - -> H2 (gas).

4. Electrode Potential

All metals do not oxidize with same degree of ease. If two dissimilar metals are immersed in the electrolyte and electrically connected externally, then one metal shows oxidation while other is protected. The electrical current flows in the external circuit. This is called a galvanic couple. The potential difference across two electrodes in a galvanic couple is standard e.g. A Zinc- Iron couple shows 0.323 V. The magnitude of this voltage represents the driving force for the electrochemical reaction. For rating the tendency of the oxidation, the standard EMF series is introduced.

Hydrogen half-cell is used as a reference point. It is coupled with the standard half-cells of different metal. The emf generated is used for arranging the metals in a series called as EMF series. At the top of this series, noble or less corrosive metals (Gold, platinum) are placed Moving down metals become more active and more susceptible to oxidation.

For more realistic and practical ranking of metals the relative reactivities of metals and commercial alloys in seawater are considered. This introduced to a series called galvanic series, (Refer table I). Alloys near top are cathodic and unreactive, while those at bottom are anodic.

Table 1 Typical galvanic series of metals and alloys

* Stainless steels are shown as occupying two positions because they exhibit erratic potential depending upon the incidence of pitting. The two positions are intended to represent possible extremes of behaviour.

5. Corrosion Rate

Corrosion penetration rate (CPR) or thickness loss of material per unit time is the unit for corrosion rate measurement. It is measured in mils per year (mpy) or millimeters per year (mm/yr.). In corrosion testing of metals, usually accelerated tests are used for prediction of life of component, or quality of material received.

6. Passivity

Under particular environmental conditions, some normally active metals and alloys lose their chemical reactivity and become extremely inert. This is termed passivity. Sometimes, a highly adherent and very thin oxide film on the metal surface serves a s a protective barrier to farther corrosion. This causes the passivation, (e.g. stainless steels). Sometimes hydrogen over potential or higher metal ion concentration in the electrolyte, near electrode regions also introduces the passivity. If the oxide film breaks or the electrolyte is circulated the metals goes back to the active state and further corrosion occurs.

7. Forms of Corrosion

For convenience, the corrosion attack is classified as following,

Uniform,        Intergranular,
Galvanic,       Selective,
Crevice,        erosion-corrosion,
Pitting,          Stress-Corrosion.

8. Uniform Corrosion

Occurs with similar intensity over the entire exposed surface, Rusting of steel and iron, tarnishing of silver ware are examples.

When two metals or alloys of different compositions are electrically coupled and exposed to the electrolyte, GALVANIC CORROSION takes place. Less Noble metal becomes anode and will experience corrosion. The other more inert will be cathode and is protected e.g. steel tube connected to brass tube will corrode in the vicinity of brass tube end. If the anode area exposed is smaller, then the corrosion attack is very severe. Proper selection of adjacent materials and/or electrical insulation of components are used to reduce galvanic corrosion.

9. Crevice Corrosion

When the ion concentration in the electrolyte is different at different locations, a concentration cell is established. The area of electrode in contact with the low concentration electrolyte becomes anodic and corrosion is observed. For example under the washers in an assembly, usually the corrosion is observed. In the stagnant solutions, localized depletion of dissolved oxygen causes this type of crevice corrosion, To avoid the crevice corrosion; sometimes rivetting is replaced by welding.

10. Pitting

This is a localized attack in which small pits or holes form. Pitting is very dangerous form of corrosion A pit may be initiated by focalized defect at surface such as scratch, inclusion, etc. The electrolyte in the tip of p-it becomes dense and concentrated as pit grows. A better surface -finish reduces the danger of pitting.

11. Intergranular Corrosion

This is a corrosion, which preferentially attacks the grain boundary area It causes failure of alloy along the grain boundaries A specific example can be given of austeritic stainless steel (AISI 304) If this alloy is slow cooled in the range of 800/⁰C to 500/⁰C , a chromium carbide precipitation takes place along the grain boundries. This depletes the chromium content in solution in the region near grain boundary If chromium content goes below I 2% then the steel becomes susceptible to corrosion called as sensitization In the hostile
atmosphere corrosion takes place in the grain boundary region and may lead to failure In the welded stainless steel component, the problem of sensitization and corrosion is faced by material near the weldment. It is called as weld decay. It can be avoided by use of low carbon grade (e.g. AISI 304L) steels or stabilized (e.g.AISI 316) steels or giving heat temperament called as solutionising after welding.

12. Selective Leaching

Sometimes in corrosive atmospheres, one element of alloy is removed as corrosion proceeds called as selective leaching. Dezincitication of brass is a very common example. Mechanical properties are drastically reduced by this form of corrosion.

13. Erosion Corrosion

When mechanical abrasion or wear because of fluid motion is associated with corrosion, effective corrosion rates are very high. The oxide film formed on the surface usually reduce the corrosion rate sometimes passivate the material. By erosion action, the film is removed, exposing bare metal to the farther corrosion.

It is most commonly found in piping, at bends, elbows, propellers turbine blades, valves, pumps, etc. soft metals are more susceptible to this kind of attack.

14. Stress Corrosion

Cracking results from combined action of tensile stress and corrosion. In the presence of corrosive atmosphere, the crack is initiated in the stressed component at stress lower than the strength of material. Crack propagates perpendicular to the stress direction. Ultimately it leads to the final failure of the component. The stress may be either the effect of direct loading or resuidual. Residual stresses are observed in welded or cold worked components.

Usually ductile material also fails in a brittle manner because of stress corrosion. Stainless steel and single-phase brass are susceptible to intergranular stress corrosion cracking. Appropriate heat treatment for reducing internal stress level or use of higher strength! higher thickness material reduces the susceptibility of stress corrosion cracking. Corrosion can be also classified in forms like corrosion fatigue, fretting corrosion, concentration corrosion, cavitation corrosion, etc.

15. Prevention of Corrosion

Corrosion process can be reduced or eliminated by various procedures as listed below.
Proper selection of material.

• Proper designing.
• Separation of corrosive environment from component surface.
• Altering the corrosive environment.
• Use of electrical current.
• Use of galvanic couple.
• Altering the characteristic of material by heat treatment.

16. Material Selection

For proper selection of materials, knowledge of exact working conditions and properties and behavior of all available materials is very necessary.

Use of high purity metals tends to reduce pitting corrosion e.g. Aluminum. Alloy addition improves corrosion resistance e.g. Titanium is added to austenitic stainless steel for stabilization. This reduces the tendency of sensitization. Use of I -% tin, in single-phase brass provides higher corrosion resistance in massive application.

For acidic environment, stainless steels are used. For caustic environments Nickel base alloys are used Distilled water is handled with Tin container Finegrained metals and alloys have less corrosion resistance than coarse grained.

In assembly, components connected should be close as per the galvanic Series. Single-phase alloys exhibit better resistance to corrosion than polyphase alloys.

17. Design for Corrosion Resistances

Following points are considered for reducing corrosion attack while designing a component.

• Welding is preferred over riveted joint.
• In assembly dissimilar alloy parts are separated electrically by using rubber gasket.
• Excessive turbulence should be avoided by providing round elbow in piping.
• Proper drains should be provided in water storage.
• If galvanic corrosion cannot be avoided, anodic area should be large than cathodic area.
• Vibration level should be reduced.
• Retention of air in the electrolyte should be avoided.

18. Application of Heat Treatment

1) The alloys susceptible to stress corrosion, stress relief treatment should be used to improve their corrosion resistance e.g. cartridge brass.

2) Cast alloys having tendency to coring should be heat-treated to remove coring Coarse gained structures -show- less corrosion resistance. E.g. Tin bronzes.

3) Solution quenching treatment is recommended for alloy showing precipitation of a phase which lowers its corrosion resistance E g Precipitate Cr x Cy in a sensitized austenitic at 10500C (solutionising) If this is followed by quenching (rapid cooling) re-precipitation of Cr x Cy is avoided. This treatment is offered to homogenize the metals and alloys.

19. Alteration of Environment

The environment should be modified so that the corrosion problems are minimized. The working conditions can be altered considering the following points:

1) Usually corrosion rate decreases with lowering temperature.

2) Very high velocities should be avoided to reduce errossion-corrossion effects. Passivating metals generally have better resistance to flowing liquid than stagnant solutions.

3) De-aeration of liquid helps prevention of corrosion. De-aeration removes oxygen from liquid. It can be done by using vacuums or purging a neutral gas or adding certain chemicals, which reacts with dissolved oxygen and reduce its percentage.

4) Higher moisture content of the atmosphere leads to corrossion. So the relative humidity should be controlled.

5) Alkaline neutralizers should be used to reduce the effect of electrolytes if possible.

6) The concentration of corrosive environment should be reduced. e.g., many times, tap water is used for cooling purpose in Industry. Tap water contains chloride ions, which may start corrosion of stainless steel.

20. Cathodic and Anodic Protection

a) Cathodic protection is achieved by supplying electrons to the metal structure to be protected i.e. making cathode to the component to be protected. Addition of electrons to the component tends to suppress its dissolution. Two methods are used for cathodic protection.

1) Impressed current method and
2) Use of sacrificial Anode.

Usually, underground tanks and pipes are protected by impressed current method(Fig. 1).

Figure 1 Impressed current method for cathodic protection of an underground tank.

In this method, an external d. c. power supply is used. The. negative terminal of power supply is connected to underground component and positive to an inert anode. e. g. graphite. Therefore, current passes to component and corrosion is suppressed.

Cathodic protection is also achieved by coupling a component to metal which is more anodic. This metal is called as sacrificial anode, e.g. Magnesium is anodic with respect to steel and corrodes when galvanically coupled. Magnesium gets consumed to protect the steel, hence it is called as sacrificial anode. These anodes are used in the following application (Fig. 2).

Figure 2 Sacrificial anode for cathode protection of
(a) Underground pipe and (b) Domestic water heater.

• Protection of hull of ship.
• Domestic and industrial water heater.
• Elevated water storage tank
• Underground pipes
• Boilers

b) Anodic protection: In this method, the metal to be protected is made more anodic so that it forms a passive film. This is carried out by using a potentiostat which maintains a metal at a constant potential with respect to reference electrode. (fig 3).

Figure 3 Anodic protection of a steel storage tank for H2S04

Formation of such passive film tends to decrease the corrosion rate. However, not all metals tend to passivate. Metals like chromium, nickel, titanium etc. show active passive transition. The anodic protection is advantageous in extremely corrosive environment.

21. Use of Inhibitors

Inhibitors are chemicals. They are added to corrosive solutions to reduce the corrosive effect of solution. Usually, the inhibitor forms a protective film on the metal surface. Various types of inhibitors with different compositions are used. Following types of inhibitors are used.

(1) Adsorption type inhibitors: these are the organic compounds. They are absorbed on the metal surface. This reduces metal dissolution.

(2) Hydrogen evolution retarding inhibitors: They are added to retard hydrogen evolution, process. They are effective in acidic electrolytes and control the cathodic reactions.

(3) Scavengers: They are used to remove the corrosive reagents from electrolyte, e.g. sodium sulphate. They remove dissolved oxygen from electrolyte. However, they are not recommended for highly acidic solutions.

(4) Oxidizers: Oxidizers are-used for metals which show active passive transition. For example, Chromate, Nitrate etc. are commonly used oxidizers.

(5) Vapor phase inhibitors : they are used mostly to reduce atmospheric corrosion effect They are placed in the cavities or similar areas of a metal component. They posses a very high vapor pressure Therefore they undergo sublimation and get condensed on metal surface thus protecting metal surface from corrosion. Vapor phase inhibitors are
used in closed spaces such as during packing or storage of metal components.

The inhibitors are also coated on a paper. Components wrapped with such a paper are better protected from corrosion.

Now—a-days, sonic of the automotive components are packed and sold in plastic bags coated with vapor phase inhibitors internally.

Table 2 gives a list of inhibitors used for typical metals in various environments

Though the inhibitors can suppress the corrosion effectively, they have following limitations:

1) Inhibitors may contaminate the environment.
2) Most of the inhibitors are toxic
3) Some of the Inhibitors can not be used in the industry dealing with food products
4) Most of the inhibitors loose their effectiveness with changes in environment intensity and temperature.
5) Inhibitors may become ineffective if stored for a long period.
6) Inhibitors cannot be recycled or reused. They should therefore, be selected keeping in view their effectiveness as well as their economy in use.

22. Use of Surface Coatings

Surface coatings are externally applied substances or compounds, which resist corrossion.

Coatings have two principal aims They are as follows:

1) To improve corrosion resistance i.e. the coated material will have better corrosion resistance than the metal component.

2) Coating will cut direct contact of metal component with environment.

Coatings are classified as:

1) Metallic coatings;
2) Inorganic coatings and
3) Organic coatings.

These coatings are applied by the following ways:

1) Electrodeposition or Electroplating: In this method metal to be protected is made cathode, while metal to be plated in made either anode or its aqueous solution as electrolyte. By using appropriate plating conditions such as current density, time, temperature and concentration of electrolyte, effective plating can be achieved. Sometimes, more than one metal is plated. For example, before silver plating, copper plating is essential for better adhesion.

Hard plantings are used to resist erosion corrosion.

2) Metallising: This is a method of surface coating used to improve the wear resistance of metal. Before coating, the surface is made rough to obtain better mechanical bonding. in this method, a suitable metal is melted by using gas flame (e.g. Oxy-acetyiene).The melted particles are sprayed over the surface to be protected Zinc or Aluminum is used
for metalising of steel. Metals with high melting points can be deposited with plasma jet spraying. Various tanks, vessels, bridge shuffles and fabricated iron and steel products are metallised.

3) Cladding : In this method, a thin sheet of corrosion resistance materiel mechanically bonded with another material. For e.g. Nickel sheet is hot rolled with a steel sheet. This is called as cladding of steel with nickel. For protection of steel, nickel, aluminum, copper etc. are used as cladding materials.

4) Hot Dipping: In this method, the metal to be protected is dipped in a molten metal bath. Hot dipping is done in bath of the metals having low melting point e.g. lead, tin, zinc or aluminum.

5) Vapour Deposition: In this method, the metal to be deposited is vaporized and condensed on the part to be protected. This is done in high vacuum. Vapor deposited is recommended for space crafts, rockets, missile parts etc.

6) Chemical Conversion Coating: In this method, the surface to be protected is corroded intentionally so that corrosion products form a protective film Anodizing is anodic oxidation of metal in an acid bath Aluminum is anodized to prevent corrosion. This forms a protective film of AL203 on the surface.

Anodizing is also carried out by using electrolytic process, The component to be protected is made anode and using proper cathode and electrolyte anodizing can be done Similarly phosphanzing in phosphoric acid and chromatizing in chromic acid is carried out.

7) Use of Organic Coatings: - These coatings are used to separate metal surface from corrosive environment. Paints, Vamishees, lacqers are from this group. Before coating, surface preparation is a must. It is then followed by priming coat and then final coat- Multiple coats are needed to cover total surface. Various polymer mixed paints are also
familiar. Zinc rich paints (i.e. Zinc powder is added to paints) are also suitable in marine applications.

Table 3 Selection of typical coating method for a particular application

23. Corrosion Testing and Monitoring

It is suggested by the industrial experts and consultants that in the engineering application at large, the corrosion of the materials can not be totally restricted. However corrosion can be controlled to certain extent and the occurring corrosion can be monitored for the safe operation of the component. Corrosion testing help rank the materials according to the corrosion resistance for a particular environment and condition. It can also help understand performance of the corrosion protection methods implemented.

Various corrosion-monitoring methods are

• Visual Testing
• Ultrasonic Testing
• Eddy Current Testing
• Radiography
• Coupon Testing
• Electrical Resistance Measurement
• Galvanic Probes
• Chemical Analysis
• Electrochemical Potential Measurements
Various corrosion-testing methods are
• lntergranular Corrosion Resistance tests for Stainless Steels
• Pitting corrosion Resistance test
• Salt Spray test
• AC Impedence test
• Linear Polarisation Resistance test
• Stress Corrosion Cracking resistance test
• Atmospheric corrosion test
• Immersion corrosion test

24. Corrosion-monitoring Methods

• Visual Testing

To understand the type of corrosion occurring is very first and important step in the corrosion control and monitoring. Visual inspection is the tool that can help identifying the form of corrosion. Visual inspection of corroded sample involves observations such as type, location and extent of corrosion.

• Ultrasonic Testing

The loss in thickness of the liquid carrying containers and pipelines, as an outcome of the corrosion is always a concern. Without opening the assemblies, by using Ultrasonic probes, thickness can be measured from outside and the extent of corrosion is detected.

• Eddy Current Testing

For plated and coated samples, eddy current testing is used to find thickness and continuity of the coat.

• Radiography

To detect the internal cracking and corrosion activity inside the assemblies, radiography is used effectively. Cracks developed as an effect of stress corrosion cracking, hydrogen blistering and loss in thickness can be detected by radiography.

• Coupon Testing

Small coupons of the material are inserted in the operating environment and these coupons are monitored at regular intervals. E.g. in the pipelines, a port is provided for insertion of coupons. Coupons are inserted through these ports such that minimum turbulence in the inside fluid flow occurs. The corrosiovity of the environment and behavior of the material can be measured by studying the coupons. Weight loss method applied to the coupons can directly predict the corrosion rates in the mils/year or mm/year and the life prediction of the component is possible.

• Electrical Resistance Measurement

Corrosion causes the loss in thickness, which alters the electrical resistance of the coupon. The electrical resistance variation of the coupon can be directly used as an online data used for corrosion monitoring. A Wheatston bridge arrangement is used for measurement of electrical resistance of the coupons.

• Galvanic Probes

Galvanic probes are inserted in the environment and the corrosivity of the environment is monitored through measurement of the potentials.

• Chemical Analysis

The analysis of the corrosion product received as a sludge in the fluids handled by the system, can be used for identifying which component in assembly is corroding and to what extent.

• Electrochemical Potential Measurements

The potential of the component is measured, across the half-cell like Cu/CuSO4 reference electrode. This can give information on the corrosion activity in the component E.g. in the long distance pipelines, cathodic protection is provided by using impressed current method. The efficacy of this can be done by surveying potential measurements
at different locations of the pipeline.

25. Corrosion-testing Methods

• lntergranular Corrosion Resistance (IGCR)tests for Stainless Steels

ASTM a 262 practice A through F specifies various corrosion tests. The accelerated test involves boiling the samples in different environments and observe the response of the coupon samples. The sensitization of stainless steel due to chromium carbide precipitation can be detected by these techniques.

• Pitting corrosion Resistance test

Cyclic Potentiodynamic test, performed on the sample of the passivating material like stainless steel, can reveal the resistance to the pitting corrosion. The pitting potential at which the passivity is lost, is the measure of the pitting corrosion resistance.

• Salt Spray test

In this test, a fog of humidified air is introduced in a specially designed chamber. The components are kept inside the chamber. The surface of the components is observed at regular interval. Plated or coated samples are tested in this test and the time required to start the corrosion of the base is the life of the coating. This is an accelerated test and can be further intensified by temperature and pH control of the atmosphere.

• AC Impedance test

When inhibitor is used to control the corrosion rate, or the passive film developed on the surface controls the corrosion resistance of the material, then AC Impedance test is useful in predicting and testing the behavior of the material in a particular corrosive environment. In this test, the inhibitor film or the passivating film on the surface of the component is tested for its capacitance. The Nyquest diagram thus obtained is used for analyzing the material and environment.

• Linear Polarization Resistance test

The electrochemical test in which the environment and material behavior can be analyzed.

• Stress Corrosion Cracking resistance test

The sample is stressed by bending in a fixture, and then an accelerated corrosion test is performed. The behavior of the material in stressed condition can be evaluated by this technique.

• Atmospheric corrosion test

To observe the realistic effect of various atmospheres like marine, dry, saline, moist, this test is performed. E.g. to test the corrosion resistance of the stainless steel to the saline atmospheres, the sheets samples are mounted on a rack. The different racks are kept on seashore, 500 meters, 1Km, 2Km away from seashore. At regular time intervals, the surface condition is observed and reported.

• Immersion corrosion test

The test samples are immersed in the corrosive fluids, and the corrosion activity is monitored. Weight loss, potential measurement and visual observation at regular intervals is correlated to the corrosion rate of the material. Crevice corrosion, pitting corrosion and uniform corrosion tendencies can be revealed in this technique.

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