Forms of Corrosion and its classification

What is corrosion?

Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, sulfide, etc. It is the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reaction with their environment.

Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxide(s) or salt(s) of the original metal and results in a distinctive orange colouration. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term "degradation" is more common.

Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.

Why do Metals Corrode?

Metals corrode because we use them in environments where they are chemically unstable. All metals exhibit a tendency to be oxidized, some more easily than others. The driving force that causes metals to corrode is a natural consequence of their temporary existence in metallic form. To reach this metallic state from their occurrence in nature in the form of various ores, it is necessary for them to absorb energy by smelting, refining processes. These stored up energy is later returned by corrosion. Only the precious metals (gold, silver, platinum, etc.) are found in nature in their metallic state. All other metals are processed from minerals or ores into metals which are inherently unstable in their environments and has the tendency to return at its original state of ores.

 Most of the cases corrosion occurs through oxidation and reduction reactions.

 Oxidation describes the loss of electrons by a molecule, atom or ion

 Reduction describes the gain of electrons by a molecule, atom or ion

Pourbaix diagram for iron

A simplified Pourbaix diagram indicates regions of immunity/protection, corrosion and passivity.

Corrosion of iron (and other active metals such as Al) is indeed rapid in parts of the Pourbaix diagram where the element is oxidized to a soluble, ionic product such as Fe³⁺(aq) or Al³⁺(aq). However, solids such as Fe₂O₃, and especially Al₂O₃, form a protective coating on the metal that greatly impedes the corrosion reaction. This phenomenon is called passivation.

In the yellow part of the diagram, an active metal such as iron can be protected by a second mechanism, which is to bias it so that its potential is below the oxidation potential of the metal.

This cathodic protection strategy is most frequently carried out by connecting a more active metal such as Mg or Zn to the iron or steel object (e.g., the hull of a ship, or an underground gas pipeline) that is being protected.

The active metal (which must be higher than Fe in the activity series) is also in contact with the solution and slowly corrodes, so it must eventually be replaced.

In some cases, a battery or DC power supply - the anode of which oxidizes water to oxygen in the solution - is used instead to apply a negative bias.

Corrosive Environments:

1. Air and humidity
2. Fresh, distilled, salt and marine water
3. Natural, urban, marine and industrial atmospheres
4. Steam and gases, like chlorine
5. Ammonia
6. Hydrogen sulfide
7. Sulfur dioxide and oxides of nitrogen
8. Fuel Gases
9. Acids
10. Alkalies
11. Soils

Corrosion may severely affect the following functions of metals, plant and equipment:

(1) Impermeability: Environmental constituents must not be allowed to enter pipes, process equipment, food containers, tanks, etc. to minimize the possibility of corrosion.

(2) Mechanical strength: Corrosion should not affect the capability to withstand specified loads, and its strength should not be undermined by corrosion.

(3) Dimensional integrity: Maintaining dimensions is critical to engineering designs and they should not be affected by corrosion.

(4) Physical properties: For efficient operation, the physical properties of plants, equipment and materials, such as thermal conductivity and electrical properties, should not be allowed to be adversely affected by corrosion.

(5) Contamination: Corrosion, if allowed to build up, can contaminate processing equipment, food products, drugs and pharmaceutical products and endanger health and environmental safety.

(6) Damage to equipment: Equipment adjacent to one which has suffered corrosion failure, may be damaged.

Classification of Corrosion

Dry corrosion

• Corrosion occurs in the absence of moisture.

• It involves direct attack of dry chemicals/gases (Air and Oxygen) on the metal surface through chemical reactions.

•This type corrosion is not common and the process is slow.

• Corrosion products are produced at the site of corrosion.

• The process of corrosion is uniform.

 

Wet corrosion

• Corrosion occurs in presence of conducting/aqueous media (strong or dilute, acidic or alkaline) on metal through electrochemical reactions.

• It involves formation of electrochemical cells.

• This type of corrosion is quite common and it is a rapid process.

• Corrosion occurs at anode but rust is deposited at cathode.

• It depends on the size of the anodic part of metal.

Fig: Main forms of corrosion attack regrouped by their ease of recognition

Uniform Corrosion

This is also called general corrosion. The surface effect produced by most direct chemical attacks is a uniform etching of the metal. Here, the corroded area is evenly distributed across the material being attacked. Uniform corrosion can render large amount of material useless quite rapidly because the attack occurs across the entirety of the exposed surface.

As corrosion occurs uniformly over the entire surface, it can be practically controlled by cathodic protection, use of coatings or paints, or simply by specifying a corrosion allowance.

In some cases uniform corrosion adds color and appeal to a surface. Two classics in this respect are the greenish patina created by naturally tarnishing copper and the rust hues produced on weathering steels.

Besides, the breakdown of protective coating systems on structures often leads to this form of corrosion. Dulling of a bright or polished surface, etching by acid cleaners, or oxidation (discoloration) of steel are examples of surface corrosion.

Cast irons and steels corrode uniformly when exposed to open atmospheres, soils and natural waters, leading to the rusty appearance.

The photos below are showing uniform corrosion (rusting)

Galvanic Corrosion

Galvanic corrosion may also be known as bimetallic corrosion or dissimilar metal corrosion.

It is an electrochemical action of two dissimilar metals in presence of an electrolyte and an electron conductive path. The driving force for corrosion is the potential difference between the different materials. In this corrosion process, one metal corrodes preferentially when it is acting as anode with respect to another which acts as cathode. A similar galvanic reaction is exploited in primary cells to generate a useful electrical voltage to power portable devices.

Therefore, for galvanic corrosion to occur, three conditions must all be present:

i. Electrochemically dissimilar metals must be present.

ii. Metals must be in electrical contact.

iii. Metals must be exposed to an electrolyte.

Main factors influencing galvanic corrosion rates are:

i. Potential difference between materials.

ii. Cathode efficiency.

iii. Surface areas of connected materials (area ratio).

iv. Electrical resistance of the connection between the materials and of the electrolyte.

Basically, metals and metal alloys possess different electrode potentials, a relative measure of a metal's tendency to become active in a given electrolyte. The more active or less noble a metal is, the more likely it will form an anode in an electrolytic environment. While the more noble a metal is, the more likely it will form a cathode when in the same environment.

The electrolyte acts as a conduit for ion migration, moving metal ions from the anode to the cathode. The galvanic series in seawater lists the common metals in order from the most anodic to most cathodic (noble). The further apart the metals are in this series, the greater the corrosion difference and speed between the two.

A tabulation of the relative electrochemical potential/ strength is called the galvanic series.

When galvanic cells are formed on different metals, the galvanic corrosion occurs.

Why does corrosion cell form?

Metallurgical factors: 

• Compositions.
• Microstructures.
• Inclusions.
• Precipitations.
• Heat treatment.
• Mechanical rolling and tempering.
• Welding.
• Work hardening.
• Fabrication, installation and external stress, strain factors.

Environmental factors:

• Concentration Cells.
• Environmental induced stress corrosion cracking (SCC), sulfide stress cracking (SSC), hydrogen-induced cracking (HIC), etc.
• Microbiologically Influenced Corrosion (MIC) - Microbial .
• Temperature induced corrosion.
• Mechanical environmental induced erosion, fretting, cavitation etc.
• Galvanic, CP and Impressed current anodic dissolution, stray current, cathodic embrittlement etc.

Concentration Cell Corrosion

This corrosion occurs when two or more areas of a metal surface are in contact with different concentrations of the same solution. It is the deterioration of parts of a metal surface at different rates, due to the parts of the surface coming into contact with different concentrations of the same electrolyte. The differing concentrations result in some parts of the metal acquiring different electric potentials. The extent of this corrosion reaction is proportionate to the difference in concentrations at contact points. It also varies with the type of electrolyte.

If an area of the electrolyte close to the metal shows a lowered concentration of metal ions, the region has to turn anodic in comparison to different portions of metal surface. Thus, this part of the metal corrodes faster, so as to increase the local ion concentration in electrolyte.

Concentration cell corrosion is most prevalent in the presence of oxygen. When pure oxygen comes into contact with a wet metal surface, corrosion action is enabled. However, the corrosion is most severe in areas that have minimal oxygen contact. Parts of metal that are covered by scale will corrode faster because the contact with oxygen for these parts is restricted. Concentrated pitting can result due to this cumulative reaction.

If a piece of metal is immersed in an electrolyte and there is a difference in concentration of one or more dissolved compounds or gases in the electrolyte, two areas of metal in contact with solution differing in concentration will ordinarily differ in solution potential, forming a concentration cell. Two electrically connected pieces of a given metal could also form a concentration cell in the same manner.

 

There are three general types of concentration cell corrosion:

1. Metal ion concentration cells : In the presence of water, a high concentration of metal ions will exist under faying surfaces and a low concentration of metal ions will exist adjacent to the crevice created by the faying surfaces. The area of the metal in contact with the low concentration of ions will be cathodic and will be protected, and the area of metal in contact with the high ion concentration will be anodic and corroded. Proper protective coating application with inorganic zinc primers is also effective in reducing faying surface corrosion.

2. Oxygen concentration cells: An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse uniformly into the solution, thereby creating a difference in oxygen concentration between two points. Typical locations are under metallic or nonmetallic deposits (dirt) on the metal surface and under faying surfaces. Oxygen cells can also develop under gaskets, wood, rubber, plastic tape, and other materials in contact with metal surface. Corrosion will occur at the area of low-oxygen concentration (anode).

3. Active-passive cells: Metals that depend on a tightly adhering passive film (usually an oxide) for corrosion protection; e.g., austenitic corrosion-resistant steel, can be corroded by active-passive cells. The corrosive action usually starts as an oxygen concentration cell; e.g., salt deposits on the metal surface in the presence of water containing oxygen can create the oxygen cell. If the passive film is broken beneath the salt deposit, the active metal beneath the film will be exposed to corrosive attack. An electrical potential will develop between the large area of the cathode (passive film) and the small area of the anode (active metal). Rapid pitting of the active metal will result.

Pitting Corrosion

It is a localized corrosion that occurs on a metal surface where there are intermetallics or microscopic defects related to very localised thinning or rupture of the natural oxide film. These sites are anodic with respect to their vicinity, and corrosion pits can develop due to electrochemical mechanism. The pits are also found underneath surface deposits caused by corrosion product accumulation. It occurs on mainly passivated metals and alloys in environments containing chloride, bromide, iodide or perchlorate ions when the electrode potential exceeds a critical value, the pitting potential. This form of corrosion is characterized by narrow pits with a radius of the same order of magnitude as, or less than, the depth. The pits may be of different shape, but a common feature is the sharp boundary. Pitting is one of the most destructive forms of corrosion as it causes equipment failures due to perforation / penetration. Moreover, pitting is dangerous since the material may be penetrated without a clear warning (because the pits often are narrow and covered) and the pit growth is difficult to predict. Moreover, pitting corrosion is difficult to measure because the number and size of pits (diameter and depth) vary from region to region and within each region. Short-term testing in the laboratory for determination of pit growth is also problematic because, under realistic conditions, it may take long time, e.g. many months, before the pits become visible. Another problem is that the critical size, i.e. the maximum pit depth, increases with increasing surface area.

There are two types of pits –

i) stable pits (those that initiate immediately and then continue to grow in depth with time) and

ii) metastable pits (those that may initiate late or that may eventually stop growing (‘die’) in depth).

Finding the deepest actual pit requires a detailed inspection of the whole structure. As the area of the structure inspected decreases, so does the probability of finding the deepest actual pit. A number of statistical transformations are there to quantify the distributions in pitting variables. Gumbel has developed the extreme value statistics (EVS) for the characterization of pit depth distribution.

The EVS procedure is to measure maximum pit depths on several replicate specimens that have been pitted, then arrange the pit depth values in order of increasing rank. The Gumbel or extreme value cumulative probability function F(x), is shown in Eq. 1, where λ and α are the location and scale parameters, respectively. This probability function can be used to characterize the data set and estimate the extreme pit depth that possibly can affect the system from which the data was initially produced.

Crevice Corrosion

It occurs at the region of contact of metals with metals or metals with nonmetals. This is localized corrosion concentrated in crevices in which the gap is sufficiently wide for liquid to penetrate into the crevice and sufficiently narrow for the liquid in the crevice to be stagnant. It may occur at washers, under barnacles, at sand grains, under applied protective films, and at pockets formed by threaded joints, beneath flange gaskets, nail and screw heads, in overlap joints, between tubes and tube plates in heat exchangers etc.

The most typical crevice corrosion occurs on materials that are passive beforehand, or materials that can easily be passivated (stainless steels, aluminium, unalloyed or low alloy steels in more or less alkaline environments etc.), when these materials are exposed to aggressive species (e.g. chlorides) that can lead to local breakdown of the surface oxide layer. Materials like conventional stainless steels can be heavily attacked by deposit corrosion in stagnant or slowly flowing seawater. A critical velocity of about 2 m/s has often been assumed, but more recent studies have indicated that crevice corrosion can occur at higher velocities too.

Crevice corrosion is affected by several factors, of a metallurgical, environmental, electrochemical, surface physical, and last but not least, a geometrical nature. One of the most important factors is the crevice gap. A special form of crevice corrosion that can develop beneath a protecting film of lacquer, enamel, phosphate or metal is the so–called filiform corrosion, which leads to a characteristic stripe pattern. It has been observed most frequently in cans exposed to the moisturized atmosphere.

Intergranular Corrosion

It is a localized attack on or adjacent to the grain boundaries of a metal or alloy with insignificant corrosion on other parts of the surface. This is a dangerous form of corrosion because the cohesive forces between the grains may be too small to withstand tensile stresses; the toughness of the material is seriously reduced at a relatively early stage, and fracture can occur without warning. Grains may fall out, leaving pits or grooves, but this may not be particularly important.

The general cause of intergranular corrosion is the presence of galvanic elements due to differences in concentration of impurities or alloying elements between the material in or at the grain boundaries and the interior of the grains:

a) Impurities segregated to the grain boundaries (e.g. the AlFe secondary phase in aluminium).

b) Larger amount of a dissolved alloying element at the grain boundaries (e.g. Zn in brass).

c) Smaller amount of a dissolved alloying element at the grain boundaries (e.g. Cr in stainless steel).

In most cases there is a zone of less noble material in/at the grain boundaries, which acts as an anode, while the other parts of the surface form the cathode. The area ratio between the cathode and the anode is very large, and the corrosion intensity can therefore be high. In some cases, precipitates at the grain boundaries may be more noble than the bulk material; these precipitates will stimulate grain boundary attacks by acting as efficient local cathodes (e.g. CuAl₂ in aluminium alloys). Intergranular corrosion occurs in stainless steels and alloys based on nickel, aluminium, magnesium, copper and cast zinc. In the following sections we shall look at the three former groups in some detail.

Stress Corrosion Cracking (SCC)

SCC is the crack formation caused by simultaneous effects of tensile stress and a specific corrosive environment. SCC is highly chemical specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments.

Metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure.

The required stresses may be due to applied load or in the form of residual stresses from the manufacturing process, or a combination of both. Cold deformation and forming, welding, heat treatment, machining and grinding can introduce residual stresses. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material.

Usually, most of the surface remains unattacked, but with fine cracks penetrating into the material. In the microstructure, these cracks can have an intergranular or a transgranular morphology. Macroscopically, SCC fractures have a brittle appearance. SCC is classified as a catastrophic form of corrosion, as the detection of such fine cracks can be very difficult and the damage not easily predicted. Experimental SCC data is notorious for a wide range of scatter. A disastrous failure may occur unexpectedly, with minimal overall material loss.

Corrosion Fatigue

Corrosion fatigue is a special case of stress corrosion caused by the combined effects of cyclic stress and corrosion. When metals are exposed to the simultaneous actions of corrosive environment and repeated stress, the fatigue behavior becomes quite different from that in air and there is a significant decrease in fatigue strength. Thus this phenomenon is called corrosion fatigue (CF). It is a fatigue in corrosive environment and should not be confused with SCC.

No metal is immune from some reduction of its resistance to cyclic stressing if the metal is in a corrosive environment. Nearly all engineering structures experience some form of alternating stress, and are exposed to harmful environments during their service life. The environment plays a significant role in the fatigue of high-strength structural materials like steel, aluminum alloys and titanium alloys. Materials with high specific strength are being developed to meet the requirements of advancing technology. However, their usefulness depends to a large extent on the degree to which they resist corrosion fatigue.

In corrosion fatigue, the fatigue-crack-growth rate is enhanced by corrosion. The threshold is lower at all stress intensity factors. Specimen fracture occurs when the stress-intensity-factor range is equal to the applicable threshold stress- intensity factor for stress-corrosion cracking.

Common types of corrosion include filiform, pitting, exfoliation, intergranular; each will affect crack growth in a particular material in a distinct way. The degree to which corrosion affects crack-growth rates also depends on fatigue load levels; for instance, corrosion can cause a greater increase in crack-growth rates at a low load than it does at a high load.

Corrosion-fatigue process is thought to cause rupture of the protective passive film, upon which corrosion is accelerated. If the metal is simultaneously exposed to a corrosive environment, the failure can take place at even lower loads and after shorter time.

In a corrosive environment the stress level at which it could be assumed a material has infinite life is lowered or removed completely. Moreover, contrary to a pure mechanical fatigue, there is no fatigue limit load in corrosion assisted fatigue.

 

Protection Possibilities Checklist for CF:

i. Minimize or eliminate cyclic stresses

ii. Reduce stress concentration or redistribute stress (balance strength and stress throughout the component)

iii. Select the correct shape of critical sections

iv. Provide against rapid changes of loading, temperature or pressure

v. Avoid internal stress

vi. Avoid fluttering and vibration-producing or vibration-transmitting design

vii. Increase natural frequency for reduction of resonance corrosion fatigue

viii. Limit corrosion factor in the corrosion-fatigue process (more resistant material / less corrosive environment).

Fretting Corrosion

It refers to corrosion damage at the asperities of contact surfaces. Fretting corrosion results from the combined effects of wear and corrosion and takes place when vibration contact is made at the interface. In other words, the rapid corrosion that occurs at the interface between contacting, highly loaded metal surfaces when subjected to slight vibratory motions is known as fretting corrosion. For fretting corrosion to occur, the following conditions to be satisfied:

i. Interface must be under load

ii. Relative motion must occur and should be sufficient enough to produce deformation on the surface

Pits or grooves and oxide debris characterize this corrosion damage, typically found in machinery, bolted assemblies and ball or roller bearings. Damage can occur at the interface of two highly loaded surfaces which are not designed to move against each other. The most common type of fretting is caused by vibration. The protective film on the metal surfaces is removed by the rubbing action and exposes fresh, active metal to the corrosive action of the atmosphere.

Factors affecting fretting corrosion include contact load, amplitude, frequency, temperature, and corrosivity of the environment.

Fretting corrosion can be prevented by:

i. Reducing relative movement between materials

ii. Using materials that are not susceptible to fretting corrosion

iii. Increasing the hardness of one or both materials

iv. Using contact lubricants

v. Using seals to absorb vibrations

Microbial Corrosion

Microbial corrosion (also called microbiologically - influenced corrosion or MIC) is caused by the presence and activities of micro-biological organisms or microbes. MIC deteriorates the metal surface through the metabolic activity of micro-organisms. This process of chiefly acts on metalloids, metals and rock-based matter.

Biological organisms influence this type of corrosion. Microbial corrosion is not caused by one microbe, but can be attributed to several microbes. The common bacteria associated with MIC are sulfate-reducing bacteria, acid producing bacteria, and iron-reducing bacteria. Apart from bacteria, microbial corrosion can also be influenced by micro algae, inorganic and organic chemicals. This influence usually results in a substantially faster corrosion rate. It affects almost all types of alloys like stainless, ductile iron and copper, but not titanium. The effect differs among alloys—steel corrodes faster than ductile iron.

In general, the microbes responsible for microbial corrosion can be categorized in two groups according to oxygen requirements:

i. Aerobic (needing oxygen): like bacteria capable of sulfur oxidizing

ii. Anaerobic (needing no or little oxygen): like bacteria that are sulfate reducing

Almost all microbial corrosion takes the appearance of pits forming underneath living matter colonies, minerals, and bio deposits. This results in a biofilm that results in a confined environment where the conditions can be corrosive.

This, in turn, hastens the corrosion process.

The development of microbial corrosion happens in three stages:

i. Microbe attachment (creation of biofilm)

ii. Growth of initial pit and nodule (change of environment at the metal surface)

iii. Maturation of nodule and pit (deterioration of the metal)

Any area collecting stagnant water or polluted water is very susceptible to microbial corrosion.

Furthermore, micro-organisms that are capable of utilizing hydrocarbons like pseudomonas aeruginosa can be found in aviation fuel. This forms dark brown or green mats similar to a gel, and leads to microbial corrosion on the rubber and plastic parts of the fuel system of turbine or turbo jet engine.

This corrosion can take many forms and can be controlled by utilizing mechanical cleaning techniques and biocides or by conventional corrosion control methods.

Erosion Corrosion (E-C)

It arises from the combined effect of chemical/electro-chemical attack and physical abrasion as a consequence of the rapid flow of any turbulent fluid on a metal surface. Pitting often found on the inner surfaces of pipes is the cause of turbulence. The rate of erosion increases in turbulent conditions and can result in leakages in tubes and pipes.

Erosion corrosion can also result from poor workmanship. When burrs in the tubes are not removed during installation, these inner burrs cause localized turbulence and hinder the smooth flow of the fluid. This leads to high rates of pitting in the tubes.

The metal usually has a protective film, which is the first part to be eroded by the fluid. Once the film is gone, the bare metal is exposed to corrosion. This type of corrosion is common in constriction areas. These are areas where there are blockages, inlet ends, pump impellers as well as other places where there are high rates of flow.

One form of erosion corrosion is the cultivation corrosion. This is a special type caused by water bubbles produced by high-speed impellers. This causes the formation of pits on the surface of the metal.

Erosion corrosion is more severe for (i) sour water or seawater on metals at velocities higher than the design values, (ii) impingement attack by entrained gas bubbles, and/or (iii) abrasion by water loaded with suspended sand or other solid particulate matter. Such corrosion is anticipated to be more common during offshore operations. The metal surface assumes a rough touch and acquires a shiny silver or golden luster due to the loss of the natural protective film.

Erosion corrosion can be prevented or reduced through any of the following methods:

i. Reduce the turbulence of the fluid in the tube by streamlining the piping.

ii. Control the velocity of the fluid to reduce high-flow velocities.

iii. Use corrosion-resistant materials.

iv. Use corrosion inhibitors and cathodic protection.

v. Ensure that the entire piping system has been de-burred.

vi. Replace sharp angles in the piping system with gentler angles to avoid constrictions.

vii. Reduce the amount of oxygen dissolved in the fluid.

viii. Adjust the pH value of the fluid.

ix. Change the metal alloy.