Materials Corrosion - Basic Types Of Corrosion That Designers And Engineers Must Be Able To Competently Address.

MATERIALS CORROSION - BASIC TYPES OF CORROSION THAT DESIGNERS AND ENGINEEERS MUST BE ABLE TO COMPETENTLY ADDRESS   - by Desi J. Kiss, M.S., P.E. 

An estimated $ 1.5 trillion are lost annually to corrosion worldwide.   The cost of corrosion loss each year in the United States has reached an estimated $ 150 billion according to the Battle Memorial Institute and the Specialty Steel Industry of North America.   The answer to the question why corrosion occurs involves thermodynamics, calculations involving free energy, and reaction kinetics.

Corrosion of metals is typically an electrochemical reaction.   Corrosion of metals is driven by the basic thermodynamic force of a metal reverting to its chemically stable oxide or sulfide form - the electrochemistry of the reaction of a metal in an electrolytic solution, ie water and steel.    

In the case of plastics, composites and ceramics corrosion occurs when bonds between the organic molecules making up the structure are affected by various corrosive environments.  These materials do not rust, however deterioration is in the form of loss of chemical properties.  Plastics and ceramics as well as composite materials are poor conductors and are not susceptible to the same electrochemical corrosion as metals.   These materials do corrode but not as readily - the process is environmentally selective.   

EIGHT BASIC FORMS OF CORROSION

As suggested above, all materials that we can form into usable implements, structures and conveyances are subject to corrosion.  The various forms discussed below can impact any material.  Most of the materials that are used to carry or withstand heavy loads are either metal or metal containing, i.e. steel reinforced concrete.  The discussion of the eight forms of corrosion below focuses mainly onthe impact of corrosion on metal under physical stress.

1. Stress-corrosion cracking.    It is different from other corrosion processes in that the material deterioration is due not to significant material removal but to cracking.  Cracking caused by the simultaneous action of a tensile stress and a specific corrosive medium is called stress-corrosion.   The stress may be a result of applied loads or "locked-in" residual stress.    This type of corrosion is time dependent, sometimes taking minutes or several months, even years to occur.  Andit is strongly influenced by heat and applied stresses in the corroding element.    Stress-corrosion cracks have been apprehended by engineers since at least 1895, when developed cracks were noted on iron tires of wagon wheels exposed to humid climates.    The mechanics of stress-corrosion cracking is a very complex phenomenon not yet fully understood.   Stress-corrosion cracking has been observed in some stainless steels that are completely corrosion resistant when unstressed.   It also occurs in many plastics, aluminum, copper, magnesium and other metal alloys as well as titanium and carbon steels.   A special kind of corrosion cracking is exfoliation, or scaling of a surface in flakes/layers as the result of corrosion.     Rolled or extruded steel and some aluminum alloys with the elongated or flattened grains as well as many plastic materials can manifest exfoliation. Caustic embrittlement is another form of corrosion cracking.   It is the embrittlement of a metal by an alkaline environment.

 2. Erosion-corrosion.    Deterioration at an accelerated rate that is caused by relative movement between a corrosive fluid and a metal surface is called erosion-corrosion.   Generally the fluid velocity is high.   Mechanical wear may include abrasion when the fluid contains suspended solids.  Erosion destroys protective surface films and enhances chemical attack. A special kind of erosion-corrosion is cavitation, which arises from the formation and collapse of vapor bubbles near the metal surface.  Rapid bubble collapse can produce shock waves that cause local deformation of the metal surface.    Materials that have a good resistance to cavitation have tenacious passive films, high strength and hardness.   Titanium and cobalt-base alloys render the best cavitation resistance in a vide range of environments.  Another special form of erosion-corrosion is fretting corrosion, sometimes referred to as wear or rubbing corrosion, chafing or friction oxidation.   It occurs between two surfaces under load that are subjected to cycles of relative motion of small amplitude typically of the order of 0.01 to 0.20 mm.   Fretting produces a breakdown of the surface into oxide debris and results in surface pits and cracks that usually lead to fatigue cracks.   Examples include press fits, bolted and riveted connections, and machine components that experience vibration.

Other forms of erosion-corrosion are:

Liquid impingement.   This is material removal due to the action of an impinging stream of fluid.    The mechanism of attack is removal of protective films, which leads to an accelerated corrosion.

Liquid erosion.    This type is similar to impingement however the exception is that the fluid flow is parallel to the surface.  The mechanism is the same: removal of metal or films by mechanical action plus corrosion of the "active" metal.

Slurry erosion.   This is metal removal due to the combined action of wear and corrosion.   The source of wear and an accelerator of corrosion are abrasive particles dispersed in the slurry.   The best systems to resist

slurry erosion are ceramics and elastomers.  Both ceramic and elastomeric piping and pumps are commercially available.

3. Crevice corrosion.   An intense localized corrosion frequently occurs within crevices and other shielded areas on metal surfaces exposed to corrosive attack.   This type of attack usually is associated with small volumes of stagnant liquid at design details such as holes, gasket surfaces, lap joints, and crevices under bolt and rivet heads.   Crevice corrosion like pitting could be very damaging because the destructive action is extremely localized.    Crevice corrosion or concentration cell occurs in poorly gasketed flanges and under bolt heads and attachment components immersed in liquids.    Welded joints in preference to bolted or riveted joints should be used when practical. Incomplete penetration welds must be avoided.   The unused part of the joint becomes a concentration cell.   Inert gas backing or pickling after welding must be used to prevent weld-contamination corrosion of steel and stainless steel.  Designers must avoid details that put design elements in contact with materials known to contain corrosive elements or which are hygroscopic because they may accelerate the cell effect. A special form of crevice corrosion is theunderdeposit attack, which is the corrosion under/around a localized deposit on a metal or alloy surface. Stainless steels and aluminum are susceptible to stagnant areas and crevices.

4. Galvanic corrosion.    The potential difference that exists when to dissimilar metals are immersed in a corrosive or conductive solution is responsible for galvanic corrosion.   The less-resistant (anodic) metal is corroded relative to the cathodic metal.   Magnesium to steel is one example, with magnesium being the metal that will take all the attack.   If a metal mix is unavoidable, the two metals should be electrically insulated so that there is no path for current flow between the two.  As a frequent example, an aluminum bolt in a steel plate is a typical case of galvanic corrosion, the aluminum bolt being the small anode and the steel plate the large cathode.  Designers should avoid the use of combinations of metals which have potentials widely separated in galvanic series as well as combinations where the area of anodic metal is small relative to that of the cathode.   Use of chemical inhibitors reduce galvanic effects when metals are exposed to fluids.

5. Intergranular  attack corrosion.

Annealing the alloy at proper temperature followed by rapid quenching, use of stainless steel alloys having low carbon content, as well as the use of stainless steel alloys that contain stabilizing elements such as Ta, Ti, Cb are just a few methods for preventing the intergranular attack corrosion.  

6. Uniform attack.   The most common and simplest form of corrosion is uniform attack.   It is characterized by a chemical or electrochemical reaction that proceeds uniformly over the entire exposed surface area.  The metal becomes thinner and eventually fails.   Plastics and ceramics being poor conductors of electrons also corrode by direct attack.   Electrochemical cell action does not play a role.  As an example, nylon in strong oxidizing solutions turns soft and gooey.   The most typical situation of uniform attack involves electrochemical reactions.    Ordinary atmospheric rusting can be uniform in nature.  

7. Pitting.  Pitting is a form of extremely localized attack that produces damage characterized by surface cavities. (i.e. produces holes-pits in the metal).  It is an especially insidious form of corrosion because it causes equipment to fail after only a small percentage of the designed-for weight loss.  Use materials with alloying elements designed to minimize pitting susceptibility.  Other methods for preventing pitting corrosion include reducing exposure to aggressive ions by shielding the part; reducing the concentration of these ions in the environment surrounding the part, and minimizing the effects of and exposure to corrosive factors on those design elements that must not be weakened by pitting.

 8. Selective leaching.   The removal of one element from solid-solution alloy by corrosion processes is called selective leaching.   When selective leaching occurs, the alloy is left in a weakened, porous condition.   

The most common example is the removal of zinc from brass, however cobalt, chromium, iron, aluminum and other metal can also be removed.

A special form of selective leaching is dealloying.     This is a corrosion process whereby one constituent of metal alloy is preferentially removed from the alloy, leaving an altered microstructure.   A number of alloy systems are susceptible to this process such as yellow brass.   The removal of zinc from brasses is called dezincification.    The net damage is in most cases a mechanical failure, because the metal remaining after dealloying is weak and "spongy", an effect similar to intergranular attack corrosion. Another form of selective leaching is graphitization, which is the dissolution of iron from gray cast irons leaving only the graphite.   It is difficult to control and it is one of the most costly forms of selective leaching.   Graphitization occurs in corrosive soils, cider fill, and waste contaminated soils.

In addition to these basic forms of corrosion of metals here are others that are worth mentioning:

a)  Corrosion fatigue.  The reduction of fatigue strength by a "corrosive environment".   When metals are subjected to corrosion during repeated cyclic loadings, specimen liftime and the endurance limit are significantly reduced.  

Special corrosion-resistant coatings, such as cadmium or zinc may be required for these applications.   If the magnitude of the applied stresses is varied during service, the fatigue response of the metal becomes very complex.   

While a few high-stress cycles may substantially reduce the expected part lifetime, low-stress cycles are less damaging to the part itself.   These load variations and their impact on subject parts are of significant importance to and under continued study by materials testing engineers and structural design engineers.  Parts under these kinds of varying stresses occupying corrosive environments must be coated with appropriate inhibitor or cathodic protection to mitigate fatigue.    Cyclic stresses in an environment capable of causing corrosive attack must be avoided.   Surface corrosion causes stress concentrations that increase local stress above the fatigue limit of material.

 b)   Filiform corrosion.   Occurs under organic coatings on metals and manifests as fine hairlines in the surface finish.

c)  Stray current corrosion. Corrosive attack induced by electric currents traveling   through irregular patterns.

d)   Microbiological corrosion.    Corrosion activated by the presence and growth of living organisms.  It is most common in soils and cooling water systems.  Biocides are the remedy for this type of corrosion.

Corrosion of Stainless Steel

By the late 1920s most of the stainless steels in use today were commercially produced in Europe and the US.   Stainless steels are alloys of iron, chromium (at least 10.5%), nickel, molybdenum, manganese, titanium and other elements that are corrosion resistant.  The corrosion resistance of the martensitic, ferritic, PH alloys and austenitic stainless steels is imparted by the formation of a strongly adherent chromium oxide on the surface.   

 In general stainless steels perform best in oxidizing environments.   These steels have limitations and weaknesses such as their susceptibility to local corrosion in the form of pitting, interganular attack when sensitized, crevice corrosion and galvanic corrosion.    Some are prone to stress corrosion cracking and uniform attack in certain environments and atmospheres.  All grades are however resistant to atmospheric corrosion.  Stainless steels are in general not resistant to halide environments, reducing environments and bleaches, however these steels are the front runners of all other material systems in usefulness in corrosive environments.   For example: at room temperature in sulfuric, nitric and phosphoric acids show low corrosion rates.   At higher temperatures and high concentrations the rate may increase based on the solution and environment.  In neutral water and gasoline most grades show no attack and have excellent corrosion resistance when gasoline is uncontaminated.    Salt water can lead to stress corrosion cracking and pitting in temperatures above 125 degrees F if tensile stresses are present.   Stainless steels are not recommended for use in solutions of hydrofluoric and hydrochloric acids. These concentrations cause rapid attack of most types.   Most stainless steels have good corrosion resistance in acetic acid and food product environments.   When improperly preheated or heated during welding, stainless steels can become subject to intergranular attack.  In the event that the protective oxide film is deteriorated and cannot be reformed, as in some shielded areas, pitting, crevice corrosion and stress cracking corrosion may be initiated at such locations.    Stainless steel must be always passivated to remove iron pickup from fabrication processes to avoid pitting under the iron deposits.

Corrosion of Plastics, Composites and Ceramics.

Plastics, composites and ceramics are becoming more and more competitive in the applications of engineering design materials because of their weight, aesthetics, and ease with which many forms and shapes may be manufactured.   Ceramics, composites and plastic materials do not rust.  Corrosion deterioration is in the form of loss of properties.  Bonds between the organic molecules making up these “plastic” structures can be adversely affected by various corrosive environments. Many of the corrosion processes that occur in metals are electrochemical (i.e. they require flow of electrons).   Plastics and ceramics as well as composite materials are poor conductors and are not susceptible to electrochemical corrosion as metals.   They corrode but not as readily, and the corrosion occurs when unstable molecules in the environment react chemically with the molecules making up these non-metal materials.

Most plastics corrode by direct photochemical attack, dissolution, permeation, weathering and aging.  As an example  plastics and carbon fiber fabrics deteriorate under extensive exposure to ultraviolet light.    Rubber corrosion ensues as tires craze and crack with the aging of their polymers.  Ceramics that in the late 1980's were touted as the “new steels” were found to corrode in various chemical environments at high temperatures.  Ceramics will probably never replace steel in the manufacture of wide-flange type beams used to support buildings and bridges.  Concrete and glass are the most popular ceramic structural materials. Many other ceramic materials are used largely for machine and small scale (non-building) structural parts.  The corrosion resistance of ceramics and glasses decreases rapidly with increase in temperature.

CORROSION CONTROL AND PREVENTION  

Regardless of the metal (or other material) to be used or the corrosive environment it is designed to occupy, it is the designers responsibility to consult available corrosion data before selecting a specific type of material.   If data is limited, it may be the designer’s responsibility to order stress and corrosion tests to be performed prior to the selection of the optimal material designed for immersion in specific chemical environments.

Steel rusts, aluminum pits, concrete stains, wood rots, composites degenerate by sunlight.      There are many approaches to corrosion control depending upon the materials under consideration, the functions they will be designed to perform and the environment in which they will be performing.    Corrosion prevention usually involves one of three approaches:

a)     Use of a barrier material to insulate or separate the metal from the corrosive environment.

b)      A change in the properties of the metal surface through a surface reaction that makes it less susceptible to corrosion.

 c)      Contact with another metal, which will corrode sacrificially to protect the metal of interest.

All approaches involve, in equal parts: environment comprehension and if possible, control;

materials research/testing; and then material design and selection.  Material selection is not only dependant on the design environment.  Materials are also evaluated based on the viability of applied corrosion inhibitors available for each material under consideration.The degree of protection and its durability are dependent upon a large array of factors. With coatings this includes application, surface preparation, porosity, thickness and its resistance to the environment.   When using metalliccoatings for corrosion protection a metal that is anodic to substate should be used so that pitting will not occur in pinholes and scratches.    Nonmetal coatings should be checked with conductive solutions and an electrical continuity test.   Thin coatings (less than 0.12 mm) will never provide long-range corrosion protection.  Many materials can be electroplated on metal substrates with Zn, Sn, Cu, Cr, Ni, Sn among the most commonly applied.   Immersions - These coatings are formed by chemical displacement after the immersion of the metal in a chemical solution.   The metal ions from the solution to form extremely thin coatings, with the reaction ceasing when the surface is covered.    Spraying - paint and droplets of molten metals can be sprayed to protect against corrosion.    Hot-dipped coatings are obtained by dipping the material into a molten bath of the coating material such as: Zn, Pb, Al, Sn.   Galvanizing and ternecoating arethe most common examples of these coatings.  

Cladding is a process of metallurgically bonding one metal to another in order to form a composite material.   The core metal is selected for corrosion protection by sacrificial or other means.   Chemical or electrochemical conversion coatings are formed by the metal surface with constituents in a solution to form an inorganic, inert film that is used as a substrate to increase corrosion resistance.   Chromate, phosphate and oxide coatings are typical of those formed by this process.   Vapor deposition metal coatings and cementationare other protective processes that are used, however these coatings do not usually equal the quality rendered by the other processes.   

Organic coatings include materials such as: enamels, epoxies, lacquers and varnishes, being used as physical barriers between the substrate and the environment.    Rust prevention petroleum base coatings such as: resin, grease, wax, metallo-organic and asphaltic are used for undercoating or to augment the protection of other coatings.   

Semiconductor coating technology in the last ten years made remarkable progress in the field of corrosion prevention.   This technology prevents corrosion by inhibiting the flow of electrons required by corrosion and consists of a semiconductor coating, applied in a single coat, and an electronic control unit.   The coating thickness depending on the application. Both the UBC and IBC have provisions for structures and metal alloys when exposed to corrosive environments.   Structures that are exposed to extremely corrosive conditions or for aesthetich reasons arepainted.   In addition to the code provisions treatmentand painting of the structure in accordance with the US Military Specifications MIL-T-704 and MIL-M-10578 is also acceptable.    Contact with dissimilar materials.   Where aluminum alloys come in contact with, or are fastened to steel members or other dissimilar materials, the aluminum shall be kept from direct contact with the steel, or other dissimilar material by painting.    In general one coat of zinc chromate primer in accordance with Federal Specification TT-P-645 or the equivalent, or one coat of nonhardening joint compound capable of excluding moisture from the joint during the years of service.    Where severe corrosion conditions are expected such as: research facilities, manufacturing and chemical facilities, additional cost effective protection can be obtained by applying the joint compound in addition to the zinc chromate primer.    The steel surfaces placed in contact with aluminum are painted as well with primer paint, such as zinc chromate primer in accordance with TT-P-645, followed by one coat of aluminum paste pigment (ASTM Specification D96266, Type 2, Class B).    Stainless steel, hot-dip galvanized or electrogalvanized steel placed in contact with aluminum in general needs not to be painted.

In the case of metals, both chatodic and the newer more sophisticated anodic protection play equally large roles.   Corrosion can be controlled largely by proper research and design. In fact, the biggest payback is always in designing, engineering and specifying the proper material and the appropriate inhibitor(s) as dictated by the design environment.

Desi J. Kiss, M.S.,P.E. - is the principal of DJK Engineering, and served as Part-Time  Faculty at the California State University Fullerton, Civil and Environmental Engineering and Mechanical Engineering Departments. Mr. Kiss obtained his Master Degree from CSUF and has over 30 years of practical Experience. He can be reached at 949/458-9169