Corrosion

For more than one and half centuries, the topic of ‘Corrosion’ has been curiously and vividly scrutinized in numerous scientific studies conducted all over the world. Corrosion can be simply as described as a natural phenomenon which is marked by deterioration or degradation of materials (generally metals) and its general properties resulting from their reactions with the environment around them. Just like every other naturally occurring hazard, e.g. tidal waves, severe snowfalls, flash floods, earthquakes etc., corrosion can result in hefty damages, often causing death-blows to bridges, buildings, pipelines, vehicles. In the very same manner, simple appliances used at homes are also subject to corrosion. However, unlike the disasters caused by the weather, there are various ways of controlling and even preventing corrosion. These methods are time-proven and can bring down the magnitude and severity of this phenomenon. As a result, its various ill effects on the society and the environment can be lessened down. Though metals are more vulnerable to corrosion, all types of materials fall prey to corrosion e.g. Aircraft wiring insulations (polymeric), especially in ageing aircrafts are prone to corrosion, also, Corrosion has been noticed in ceramics.

It can be said that the metal actually gets consumed by Corrosion and this results directly in a considerable and significant loss in its load-carrying capabilities. Corrosion also causes stress concentration in these objects. Human beings have tried to comprehend corrosion and have strived to find means to control it since the time they have started using metals. Since that time, corrosion has resulted in monetary losses for individuals and the society. As mentioned earlier, Corrosion, currently, has a phenomenally large impact, on both the finances and the environment, globally. The costs incurred via the infrastructure damages are augmented by the costs of replacing them. As a result, corrosion takes a large bite on the gross incomes of the countries. It costs the nations around the globe close to $2 trillion annually for fighting corrosion. This, in statistical terms, comes down to almost 5% of the Gross Domestic Products of the industrial nations.

The science behind preventing and controlling corrosion is a complex one, especially due to the fact that corrosion can be witnessed in varied forms. Also, there are various external factors that cause corrosion, and this adds-up to the complexity. The science of preventing corrosion is being alleviated by the scientists around the world working relentlessly to find better and easier ways to bring down the intensity of this devastation. There are also numerous professionals, highly trained and experienced, whose job is to keep a leash on the effects of corrosion, using state-of-the art technological innovations. Unfortunately, most of the decision making authorities, of the industries or the governments, fail to comprehend the results of extensive corrosion or the criticality of controlling corrosion. A nonchalant and laid back attitude of the impending situation and a serious lack of appreciation of the research and development work are also seen. Amongst the members of the society, there exists a general ignorance about corruption and the ways it is negatively affecting their own finances. Various studies that have been conducted by the experts at World Corrosion Organization (WCO) show that both the government officials and the public need to be educated on factors like controlling corrosion, development of materials and their selection, contemporary R & D, monitoring (condition-based), innovative methods of surface protection etc. As a move in this direction, the WCO has globally employed close to 50,000 scientists, technicians & educators and have also tied up with 30 separate organizations for the promotion of training, exchange of technologies and awareness-raising.

 

How Corrosion is caused

It has been noticed that most commonly the corrosion process results from the involvement of at least two separate reactions, electrochemical in nature. One of them is anodic and the other cathodic. The surface that is undergoing corrosion can be imagined as a battery which has been short-circuited (it is the conductor – created electrical connection between two physical surfaces, which have been segregated by a very little distance). The process of metal-corrosion (anodic reaction) dissolving in the form of ions, produce electrons and these are consumed by the cathodic reaction process. Now, these two ongoing processes need to balance their charges and thus the corroding process carries on.

Therefore, it can be understood from here that studying the process of corrosion takes the involvement of the same techniques and tools that are employed by the electrochemists for the purpose of conducting studies on fuel-cells etc. With iron and quite a few other metals, the resultant oxide fails to conform to the metal-surface and scales peel off readily. This process is called ‘’pitting’’ and “pits” and are created by concentrated corrosion. An even more extended form causes “cracks” and “cracking” and results in a complete wastage of the surface of the whole metal body.    Corrosion occurs only and only in the presence of moisture. The other general ingredients are water, acids, salts, oils etc. Even exposure to materials which are gaseous, e.g. ammonia, formaldehyde etc. have been noticed to cause corrosion.

Various kinds of Corrosion

The classification of corrosion, carried out based upon attack morphologies and environmental exposure of the surface, indicates that it can be of many types. These are:

Uniform or general corrosion – This kind of corrosion can be seen to take a uniform spread over the affected surface. This is the most commonly noticed and the widespread of the corrosion kind. Also, this nature of corrosion has been seen to proceed at a rate which is uniform rate/pace on the surface. The surface carries on getting thinner till it breaks down. From the viewpoint of wastage, this probably causes the biggest and the most amount of damage of all the existing types of corrosions. Steel and cast iron experiences uniform corrosion with constant exposure to atmosphere, water and soil. Oxygen is generally held to be responsible as the major cause behind causing corrosion in steels and every other alloy or metal. Examples of this can be seen in the form of steel bridge rusting, underground & surface pipeline rusting, silver tarnishing, formation of patina on the bronze surfaces & copper roofs. also, surface etches created by acid cleaners, steel losing its shiny look are some other examples of surface corrosion. Any piece of unprotected metal that has been left outside would catch this kind of corrosion promptly. Fortunately enough, the nature of this type of corrosion is predictable and thus preventive and controlling measures can be easily adopted. Generally, disastrous failures in controlling uniform corrosion are not noticed as the applicable prevention methods are simple and can be carried out by any given individual. Pretty often, this type of corrosion stands to carry objections only from the fact that it messes up the appearance of the surface. This can be done through painting any given surface which might be prone to corrosion. Alternatively, layers of any kind of ‘sacrificial metal’ like zinc etc. can be applied on the surface. In this case, a galvanic or bimetallic corrosion takes place on the surface layer of zinc. In a battery, general corrosion can be used to the advantage. In some of the other metals, like aluminum, titanium and stainless steel, protection can be created from corrosion by a super thin film of oxide. This film is a natural formation and the practical applications of various materials would be impossible without this naturally formed protective surface. One can take the example of airplanes, or any other given surface/structure built out of aluminum and how corrosion would stand in the way of their practical applications. Unfortunately enough, local breakdown of this protective layer is very much a possibility and this again exposes the surface to corrosion infestation, like crevice corrosion in case of fasters in stainless steel / pitting problems in plates made of aluminum / cracking of pipes resulting from stress corrosion, seen in nuclear reactors. Nonetheless, creating a protection for these structures, as far as these kinds of corrosion are concerned, is very much possible. Along with the prevention methods that have been mentioned earlier, corrosion inhibitors can also be used for fulfilling the purpose. A modification in the environment to which the surface is exposed, can also be very helpful to suit the purpose. Materials of thicker nature might be used for corrosion allowance.

Galvanic Corrosion: Galvanic corrosion, also known as “Bimetallic Corrosion” or “Dissimilar Metal Corrosion” is a process in which two metals preferentially corrodes when they are in contact with each other with the medium of electricity and are immersed in an electrolyte. Each type of metal and alloy has different electrode potentials and a galvanic setup gets established when different metals come in contact in an electrolyte. In this setup, one metal is considered as anode because it has negative potential due to which it corrodes faster and the other one as cathode because it is less reactive as compared to the other one and corrodes at a slower rate. The one metal which has more electrical potential acts as anode and is dissolved into the electrolyte which drives the ion deposition on cathode. The electrolyte material provides a medium for ion migration between the two metals.

Galvanic Series: since galvanic corrosion depends on the reactivity of the metals, a list of metals known as Galvanic series is created so that while placing two types of metals in seawater the chances of corrosion gets reduces. Platinum is the most noble metal i.e. the least reactive has been placed at the end of the list and Magnesium is the highly reactive metal which has been placed at the top of the series. While immersing two metals in seawater, it is likely to choose a combination of the metals which are closest in the series.

Pitting Corrosion: pitting corrosion leads to the creation of deep holes in the metal just like cavities. This corrosion is considered as one of the extremely localized corrosion which is confined to a point or a small area. Pitting corrosion is found mostly on the passive metals and alloys viz. Aluminum alloys, stainless steel and nickel alloys. It starts oxidation of some ultra thin passive films that does not immediately lead to re-passivity.  As per studies, the main reason of Pitting corrosion is Environment. The presence of chlorides in seawater significantly starts the creation of pits in the metals that can cause a great deal of damage. The other reason that is observed is the Material Factors (metallurgy). Pitting due to metallurgy is caused by containing inclusions or defects (MnS is the major culprit for the initiation of pitting in steels). Pitting can be initiated by a small surface defect, being a scratch or a local change in composition, or damage to protective coating. Polished surfaces display higher resistance to pitting. Thereby determining whether an existing pit can be repassivated or not is dependent on The ENVIRONMENT (chemistry) and the MATERIAL (metallurgy) factors. Formation of pitting corrosion can be initiated and can take different shapes such as wide and shallow or narrow and deep which can rapidly perforate the wall thickness of a metal. Analysis also showed that penetration may be 10 to 100 times that by general corrosion, pitting corrosion has been considered to be more dangerous than the uniform corrosion damage because it is very difficult to detect, predict and design against. Moreover the most severe example of pitting erosion could be the case the explosion in Guadalajara, Mexico on April 22, 1992, when gasoline fumes accumulated in sewers destroyed kilometers of streets. A single hole was created that produced vapors from a leak of gasoline between a steel gasoline pipe and a zinc-plated water pipe.

To prevent pitting corrosion, the following methods can be adopted:

• Proper material selection e.g. SS316 with molydenum having higher pitting resistance compare to SS304
• Use higher alloys (ASTM G48) for increased resistance to pitting corrosion
• Control oxygen level by injecting oxygen scavenger in boiler water system
• Control pH, chloride concentration and temperature
• Cathodic protection and/or Anodic Protection
• Proper monitoring of oxygen & chloride contents by routine sampling
• Agitation of stagnant fluid

With the above said methods the chances of pitting corrosion can be reduced.

Stress corrosion cracking (SCC): This type of corrosion is considered as a complex type of erosion that is a result of the combined effects of stress and corrosion. Mostly it arises during some special circumstances for a given alloy namely specific alloy condition plus specific corrosive media and sufficient local tensile stress to some extent of elevated temperature depending on the nature of different metals. The alloys mostly undergo SCC when they are exposed to a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal otherwise. Stress corrosion cracking presents an especially difficult problem, since not only is it highly localized but it can occur in environments that are merely mildly corrosive to the material. However it is possible that damaging concentration of the harmful ions in that environment may be quite small and difficult to detect and, even in the absence of applied stress, residual stresses in a structure can often be of a sufficiently high level to cause SCC and failure in service. The growth rate of SCC cracks for a specific alloy environment combination can be determined by the concentration of stress at the tip of a sharp crack or flaw can be quantified in terms of the Stress Intensity Factor. Moreover ther is no unified concept available to understand the mechanism for stress corrosion cracking but still there are some models which have been proposed previously that are named as “Absorption model”, “Film rupture model”, “Pre-existing active path model”, “Pre-existing active path model”. The major example of this corrosion can be stated as of the 32 inch diameter gas transmission pipeline, north of Natchitoches, Louisiana, belonging to the Tennessee Gas Pipeline exploded and burned from SCC on March 4, 1965, killing 17 people. In this case, SCC caused the catastrophic collapse of the Silver Bridge that caused a failure in an eye bar suspension bridge. Another example that one can giveaway is of the seasonal cracking of brass cartridge cases, a problem experienced by the British army in India in the early 19th century.

To prevent Stress corrosion cracking, following measures can be followed:

• Avoid chemical that causes SCC
• Stress level and hardness should be controlled and managed
• Temperature or the electrochemical potential of the alloy needs to be controlled.
• Avoid the exposure of material to a corrosive environment

Further some new alloys that are more resistant to SCC were developed. However this was a costly affair and requires a massive time investment to achieve only marginal success.

Corrosion Fatigue: Corrosion-fatigue is the result of the combined action of an alternating or cycling stresses and a corrosive environment. The fatigue process is thought to cause rupture of the protective passive film, upon which corrosion is accelerated. However if the metal is exposed to a corrosive environment then it is likely that the failure can take place at even lower loads and after shorter time. You can consider it as the mechanical degradation of a material under the joint action of corrosion and cyclic loading. It is likely that the reader may get confused between Corrosion Fatigue and Stress corrosion cracking. The only difference or it should be said that the only requirement for corrosion fatigue is that the sample be under tensile stress. The fatigue fracture is brittle and the cracks are most often transgranular, as in stress-corrosion cracking, but not branched. The corrosive environment can cause a faster crack growth and/or crack growth at a lower tension level than in dry air. Even relatively mild corrosive atmospheres can reduce the fatigue strength of aluminum structures considerably, down to 75 to 25% of the fatigue strength in dry air.

• Preventive steps that can be adopted:
• Minimize or eliminate cyclic stresses
• Reduce stress concentration or redistribute stress (balance strength and stress throughout the component)
• Select the correct shape of critical sections
• Provide against rapid changes of loading, temperature or pressure
• Avoid internal stress
• Avoid fluttering and vibration-producing or vibration-transmitting design
• Increase natural frequency for reduction of resonance corrosion fatigue
• Limit corrosion factor in the corrosion-fatigue process (more resistant material / less corrosive environment).

Intergranular Corrosion: “Intergranular” or ‘intercrystalline” means between grains or crystals. During the tensile stress cracking may occur along grain boundaries and this type of corrosion is frequently called “intergranular stress corrosion cracking (IGSCC)” or simply “intergranular corrosion cracking”. In most cases of corrosion, including uniform corrosion, the grain boundaries behave in essentially the same way as the grains themselves. However, in certain conditions, the grain boundaries can undergo marked localized attack while the rest of the material remains unaffected. This form of corrosion is usually associated with chemical segregation effects (impurities have a tendency to be enriched at grain boundaries) or specific phases precipitated on the grain boundaries. Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity. The most common reason of such corrosion is the impurities present in the boundaries, or to local enrichment or depletion of one or more alloying elements. This form of corrosion results in a loss of strength in metal parts where the grains have fallen out. Certain precipitate phases (e.g. Mg₅Al₈, Mg₂Si, MgZn₂, MnAl₆, etc.) are also known to cause or enhance intergranular attack of high strength aluminum alloys, particularly in chloride-rich media. A classic example is the sensitization of stainless steels or weld decay. Chromium-rich grain boundary precipitates lead to a local depletion of Cr immediately adjacent to these precipitates, leaving these areas vulnerable to corrosive attack in certain electrolytes. Reheating a welded component during multi-pass welding is a common cause of this problem.

Intergranular corrosion can be prevented through:

• Using low carbon grade of Stainless Steel
• Use stabilized grades alloyed with titanium (for example type 321) or niobium (for example type 347). Titanium and niobium are strong carbide- formers. They react with the carbon to form the corresponding carbides thereby preventing chromium depletion.
• Use post-weld heat treatment

Crevice Corrosion: This type of corrosion occurs when there is small volumes of stagnant solution is present in the occluded interstices, beneath deposits and seals, or in crevices, e.g. at nuts and rivet heads. In other words, it occurs in the confined spaces to which the access of the working fluid from the environment is limited. These spaces are known as crevices. Crevice corrosion is encountered particularly in metals and alloys which owe their resistance to the stability of a passive film because these films become unstable when they come in contact with solutions in which there is presence of high concentrations of Cl^– and H⁺ ions. However the damage caused by crevice corrosion is normally confined to one metal at localized area within or close to the joining surfaces. If we talk about the mechanism of crevice corrosion in detail then actually it occurs because of the difference in concentration of some chemical constituents, usually oxygen, which set up an electrochemical concentration cell (differential aeration cell in the case of oxygen). Outside of the crevice (the cathode), the oxygen content and the pH are higher – but chlorides are lower. As soon as the crevice gets formed, the propagation mechanism for crevice corrosion is the same as for pitting corrosion. Factors that influences the occurring of crevice corrosion are as follows:

• crevice type: metal-to-metal, metal-to-non-metal
• crevice geometry: gap size, depth, surface roughness
• material: alloy composition (e.g. Cr, Mo), structure
• environment: pH, temperature, halide ions, oxygen

It can also be said that a common form of crevice failure occurs due to stress corrosion cracking, where a crack or cracks develop from the base of the crevice where the stress concentration is greatest. For Example we can take the instance of the fall of the Silver Bridge in 1967 in West Virginia, where a single critical crack only about 3 mm long suddenly grew and fractured a tie bar joint. The rest of the bridge fell in less than a minute.

Filiform corrosion: Filiform corrosion also known as “under film Corrosion” or “filamentary corrosion” is often associated with aluminum and magnesium alloys that have an organic coating which is typically 0.1 mm thick. This type of corrosion has occurs under some thin coatings in the form of randomly distributed threadlike filaments. The characteristics of this erosion have number of elements which are much similar to the crevice corrosion. Viz.

• The coating allows oxygen and water to migrate through it.
• The concentration of dissolved oxygen becomes highest at the back of the head near the region of the tail. This region becomes the cathode.
• Oxygen becomes depleted at the head. This region becomes the anode.
• Corrosion is driven by the potential difference between these regions, a potential difference which can rise to several tenths of a volt.
• Metal ion formation and dissolution proceeds at the head while oxygen is reduced closer to the tail.

During the corrosion a number of threads or worms can appear under the coating. When either of the head of the worm meets another one, at the same instance the propagation tends to stop. Anyhow if it approaches the inactive tail, it tends to be deflected.

To reduce filiform corrosion, following approaches needs to be followed:

Erosion corrosion: It occurs when there is degradation in the surface of the material due to the relative motion of a corrosive fluid and a metal surface. Erosion corrosion is mostly observed in Soft Alloys (i.e. copper, aluminum and lead alloys). At the initial stage of this corrosion, mechanical removal of a metal’s protective film and then corrosion of bare metal by a flowing corrosive can be observed. Erosion corrosion is especially located near high flow rates around tube blockages, tube inlet ends, or in pump impellers. It is also considered that Erosion corrosion is the second most common cause of copper tube failure.

• Follow the approaches given below to reduce the chances of such corrosion:
• streamline the piping to reduce turbulence
• control fluid velocity
• using more resistant materials
• using corrosion inhibitors or cathodic protection to minimize erosion corrosion

Fretting Corrosion: It refers to the damage caused by corrosion at the asperities of contact surfaces. In other words you can say that fretting corrosion is an attack that is accelerated by the relative motion of contacting surfaces. Fretting corrosion is majorly found in many tight fitting parts like machinery, bolted assemblies and ball or roller bearings that are designed to slip against each other.  The surfaces which are in contact with each other and are exposed to vibration when are transported from one place to another, chances of fretting Corrosion increases within those parts. This type of corrosion is generally a combination of corrosion and abrasive wear. Abrasive wear is a material, often a mineral that is used to shape or finish a work piece through rubbing which leads to part of the work piece being worn away. Whenever the two surfaces which are tightly joined that slip against each other, the protective films present on the surfaces of the metals gets removed that results in accelerated attack. Also, most corrosion products are abrasive and their presence increases the removal of protective films and in direct abrasion of the metal. Fretting corrosion is different from ordinary wear in that it occurs rapidly with little movement. The most common examples of Fretting corrosion can be seen in riveted joints on ships and other riveted structures where cyclic loads were experienced, but some welded construction is done on the surface so that this can be largely eliminated. The most easiest way to prevent fretting corrosion is to lubricate the surfaces so that while the surfaces slips against each other, there will not be any removal of the protective layer as the lubrication will provide easy movement of the surfaces. Further you need to check the lubrication again and again otherwise it will result in removal of the protective layer.

Stray current Corrosion: Corrosion that is caused by the stray current -current flowing through paths other than the intended circuit. In other words, Stray currents which cause corrosion may originate from direct-current distribution lines, substations, or street railway systems, etc., and flow into a pipe system or other steel structure. Stray current corrosion is a phenomenon of natural electrolysis. Moreover the damage or loss of metal that is caused by this type of erosion depends on the extent of the magnitude of stray current passing through the system. However the stray current corrosion is somewhat different from the natural corrosion because this kind damage is caused by the external forces namely the externally induced electrical current and is basically independent of such environmental factors as oxygen concentration or pH. Also it is likely that the environmental factors may accelerate other types of the corrosion process but the portion which has been corroded by the passing of stray current will remain unaffected. The main sources of Stray current could be Electric Railways, Cathodic protection systems, Electrical welding machines, Grounded DC electric sources. Thus if you want to prevent such corrosion, you may need to:

• Identify the source of stray current
• Stop the leakage from the intended circuit by maintaining good electrical connections and insulation.
• Install impressed cathodic protection system to offset the effect of stray current.

Corrosion in Concrete: To strengthen each type of construction it is necessary to include some metallic material, example carbon steel reinforcing rods, cable, and wires inside the structure. Thus all the metallic material that has been included in the construction, there are chances of developing some corrosion within the metal. The reasons that can be traced for developing corrosion in the structure could be poor construction and an unfavorable environment, such as environments that favor impact, abrasion, chemical attack, and freeze-thaw cycles. As per after reading the above types of corrosion the reader would have understood that the corrosion in steel is caused in the presence of an anode, a cathode, a metallic path and an electrolyte. Oxidation occurs at the anode and reduction occurs at the cathode. In a concrete construction, there is an alkaline environment present in the concrete material that provides some protection against corrosion. The reaction between the alkaline environment of concrete and steel forms a film that passivates and protects the steel. Also the reader would be wondering that if there is some presence of chloride ions then it may weaken the film but concrete can absorb chloride through admixtures, deicing salts, chemicals, seawater or contamination of the concrete mix. This means that if there is presence of oxygen and humidity, it will begin the process of corrosion. However the process would be gradual but it is possible that environmental conditions can accelerate the process. As soon as the corrosion expands the steel it will start taking up more space in the construction which may cause cracks in the concrete construction which will allow water, oxygen, and chlorides to enter inside the construction. This will weaken the construction and may cause a fall in the construction. Thus climatic conditions determine the rate of corrosion. This will bring a point in front of us that states that if a concrete structure is located in an air conditioned building, the controlled temperature and humidity will delay the corrosion process or may prevent it completely. The reason behind this could be that if the concrete is saturated with water (that is, exposed to 100% relative humidity constantly) will corrode at a slower rate or could possibly not corrode. The concrete corrosion majorly affects the:

• Bridges: the bridges that are constructed starts to get weaken within a period of 5-10 years of construction.
• Parking Garages: The prime vulnerability in parking structures is vehicles carrying deicing salts that are used to melt ice on the roads leading to the garages.
• Structures in rivers and marine environment
• Building: As explained above.

The preventive steps that one can follow are:

• There should be adequate coverage of concrete is necessary. It is advised to cover the metal with a minimum of 2 inches of concrete.
• Use epoxy to make a coating on steel
• Cathodic protection system needs to be employed
• Try to maintain dryness on the concrete structure.