Metallurgical Failure Analysis
Corrosion Failures
Solving Corrosion Failures
"It failed by corrosion." This is not a complete failure analysis. How does one solve a corrosion failure? For a corrosion failure to be properly solved it isn't enough to just conclude: "It corroded." Metalurgical Failure Analysis uses a more rigorus method of solving corrosion problems. Using these steps will help to avoid future failures.
1. Know the forms of corrosion. Metallurgical Failure Analysis has expertise in the many forms of corrosion failures. A typical, poor solution to a corrosion failure is to use a stainless steel alloy to hopefully solve the problem. This may not always work as the corrosion of stainless steels may be accelerated in some common environments.
2. Identify the specific form of corrosion. Years of experience and the use of sophisticated analytical equipent allows Metallurgical Failure Analysis to identify the form of corrosion failure. A complete failure analysis combined with good engineering judgement is the key to the identification of the form of corrosion. If the form of corrosion failure is not properly identified then the assurance of a good solution is not certain.
3. Have all the facts about the environment, temperatures and service. An understanding, in depth, of the service environment is a must.
4. Solve the problem. Metallurgical Failure Analysis will make engineering recommendations for the sucessful solution of the corrosion failure by: materials selection, process changes or design improvements. Practical, real world recommendations for good solutions have satisified clients needs. Results are available by clear reports, excellent presentations or verbal methods.
Metallurgical Failure Analysis has expertise in identifying and solving all forms of corrosion failures. The following are some general examples of metals, and their form of corrosion.
Forms of Corrosion Failures
GENERAL, UNIFORM CORROSION
GALVANIC CORROSION
INTERGRANULAR CORROSION
EROSION / CORROSION
EXFOLIATION CORROSION
SELECTIVE LEACHING (DEALLOYING)
CORROSION FATIGUE
PITTING CORROSION
CREVICE CORROSION
STRESS CORROSION CRACKING
HYDROGEN EMBRITTLEMENT
HIGH TEMPERATURE CORROSION
MICROBIOLOGICALLY INFLUENCED CORROSION
General, Uniform Corrosion
Corrosion is an on-going attack found in both metals and materials. Uniform, general corrosion is the form we encounter most frequently. It occurs in vehicles, infrastructure, pipelines, and in most things we come into contact with. Coatings, metallurgical alloy improvements, and good maintenance practices are common ways to minimize general corrosion.
The metal loss is uniform in nature, and readily apparent. Often general corrosion problems can be solved by a cadding orrosion allowance;. The designer can increase the material thickness in a bridge, tank or pipe line so its life expectancy is reached before there is an unacceptable loss of metal.
Metals that are susceptible to general corrosion are steels, and aluminum and copper based alloys.
Galvanic Corrosion
This corrosion can be used to our advantage. Galvanized steel, zinc electrodes in marine environment and sacrificial electrodes in tanks and water heaters are examples of using galvanic corrosion to an advantage. The attached chart lists metals and alloys in order of their electromotive potential. Knowing the distance between metals, measured in millivolts, is the key to understanding galvanic corrosion. The noble metals gold and silver will have a large potential difference when placed in contact with steel or aluminum. The metals toward the anodic end of the chart will corrode to protect metals that are more cathodic, toward the gold and silver. Three factors must be present for galvanic corrosion to occur: 1. Dissimilar metals. The greater their distance on the chart will drive the corrosion of the more the anodic metal and likewise the more cathodic metal will be protected. In some metals, stainless steels for example, there may be a potential difference between areas on the same component. One area will corrode, one will be protected. 2. An electrical circuit must be completed for a current to flow. As the current flows there is a loss of metal on the anode. 3. Both metals must be in contact with an electrolyte. If the joint is dry then there will be no current flow and therefore no corrosion. Galvanic corrosion can be controlled by removing one of the three factors. Don't use dissimilar metals, use an insulator to completely insolate the joint and keep the joint free of moisture, oil and debris, all electrolytes are practical rulres of controlling galvanic corrosion. There is an advantage of galvanic corrosion in that the cathode is protected by the anode. Therefore a large anode will protect the relatively small cathode. The loss of metal is spread over the large anode area. Galvanized steel when scratched will not allow the steel substrate to corrode. Instead the loss of metal is spread over the surface of the zinc and there will be no steel rusting as long as is some zinc remains in the immediate area of the scratch.
Intergranular Corrosion
This is a localized form of corrosion found in some stainless steels. Welding, casting and heat treating are processes that put enough heat into the stainless steels that carbon will migrate to grain boundaries and form chromium carbides. The grain boundary areas will be attacked because the chromium is not available to form the needed corrosion resistant chromium oxide film. Controlling the heat input, keeping carbon low, alloying with large amounts of chromium and other alloys and solutionizing the component after processing will all prevent intergranular corrosion.
Erosion/Corrosion
Solids and fluids at high velocities will remove metal from the surface, that is erosion. Erosion corrosion also has metal loss due to corrosion. The synergistic effects of the erosion and corrosion cause metal loss faster than would be expected for either one by itself. An example of this is brass in a pump which will acceptably handle concentrations of chlorine in the range of 10 ppm. If excessive chlorine is added to the processed water before it was pumped then there will be erosion/corrosion. Areas of higher fluid flow in the eye of the impeller and on the shroud will show a corresponding higher metal lost.
In instances of high erosion/corrosion hardfacing can be applied by welding.
Pitting Corrosion
Pitting corrosion is highly localized. Stainless steels, copper and some aluminum alloys are subject to pitting. Localized corrosion will occur when there is a potential difference from one area to another on the same component, (a galvanic cell). The small anode will rapidly corrode in order to protect the surrounding metal. The pitting is rapid and sometimes undetected until the wall has been penetrated. Debris on the bottoms of stainless steel tanks will initiate the galvanic cell. Also a lack of oxygen to replenish the protect chromium oxide film will allow some stainless steels to pit.
Alloy selection, avoiding stagnant, oxygen deprived water, and not allowing debris to settle will control pitting corrosion.
Crevice Corrosion
Crevice corrosion is a close cousin to pitting corrosion. In the same manner that a lack of a protective oxide film will pit; a crevice between faying surfaces will set up a similar oxygen deprived surface. There does not have to be dissimilar metals for crevice corrosion. The area underneath a bolt or washer, under a gasket or between pieces of metal in conjunction will allow for crevice corrosion. Normal varieties of stainles steel are very susceptible to crevice corrosion. Factors to control crevice corrosion are use of high alloys, designing to avoid crevices, and control of temperatures and environments.
Corrosion Fatigue
When a component is in a corrosive environment and also subject to fatigue loading the combination of both corrosion and fatigue will accelerate the failure. One characteristic of corrosion-fatigue is multiple initiation cracks. These grow by the crack propagating by fatigue and also from corrosion. Eventually one crack will predominate and it will grow to a greater extent that the remaining cracks. This predominate crack will lead to the failure. Springs, shafts and fasteners are subject to corrosion-fatigue.
Material selection, control of service environment, design and inspection are methods to control corrosion-fatigue.
Exfoliation Corrosion
This mainly occurs in some aluminum alloys. The aluminum alloy is rolled into a shape and in so doing the grains are elongated and normally have good mechanical properties. When the material is cut, drilled or machined to expose the grains corrosion will act to separate the layers of grains. This expansion will distort the component, sometimes bread the fasteners ans will result in a failure. Some aircraft aluminum alloys are subject to exfoliation corrosion. Washing and keeping corrosives off the material is the common solution for the aircraft grades.
Selective Leaching (Dealloying)
Brasses are the most famous alloys for dealloying. This corrosion happens when one phase or alloying element is preferentially lost from the matrix. In some instances there is no warning of the loss and the failure is sudden. Aluminum may be lost from aluminum bronze, carbon (in form of graphite) can be lost from cast iron and zinc can be lost from high zinc alloys of brass. In World War I the British Fleet had more battleships out of service due to dezincification in their boiler tubes than were lost due to action with the German Fleet. Good material selection, alloy additions, minimizing other phases and heat treating are used to control dealloying.
Stress Corrosion Cracking
Given the right corrosive environment all alloys will fail by stress corrosion cracking, (SCC). Common alloys that are known to have failed by SCC are steel, stainless steels, brasses, aluminum alloys and titanium alloys. The combination of a stress, temperature and environment will cause SCC. The stress could be residual from welding and processing or it could be from service applied loading. The temperatures vary on alloy but 304 grade stainless steels will fail by SCC in the presence of chlorides at temperature about 140F. SCC can be controlled by alloy selection, keeping stresses below a certain level, and the most difficult being avoiding the corrosive environment.
Microbiologically Influenced Corrosion
Bacteria such as iron reducing bacteria will severly pit and corrode vessels, tanks and pipelines. The attack has the appearance of pitting and one has to be careful to verify the absence or presence of the Microbiogically Influenced Corrosion, (MIC). The areas adjacent to weldments in stainless steels, in the presence of chlorides, may be subject to MIC. The selection of high alloys or copper based alloys are ways to control MIC. Care in cleanging and grinding of weldments is useful in stainless steels.
Hydrogen Embrittlement
High strength steels that have been plated, welded or exposed to nacent hydrogen at elevated temperatures will be embrittled by hydrogen. The single hydrogen atoms are small and will diffuse with energy from the heat of welding. They tend to concentrate at the tip of a crack or point of high stress. Ths concentration of hydrogen atoms will cause to metal to loose ductility and there by become embrittled.
Avoid using high strength steels unless necessary. Post weld heat treat to drive out the hydrogen after welding is common. High strength fasteners that have been plated are required to be baked for hours or even days to drive out the hydrogen.
A visual examination can properly identify the cause, with some materials it is necessary to use metallurgical microscopes and analytical equipment to make a good identification of the corrosion type. Corrosion testing to duplicate and thereby verify the type of corrosion is justified on some projects.
