Corrosion is a chemical reaction of the metal with the environment to form an oxide, carbonate, sulfate or other stable compound. Corrosion is a chemical process and as such is affected not only by the metal but also by the environment to which the metal is exposed and the manner in which the metal is exposed.
In case of carbon steel, the environment has a much greater impact on the corrosion rate than minor changes in the composition of the metal. On the other hand, for corrosion resistant alloys (CRA's) the corrosion rate is as sensitive to metal composition as it is to the environmental composition and the manner in which it is exposed.
Corrosion Resistance
A metal derives its corrosion resistance by forming a protective oxide film on the surface. Metals may be classified in two categories-active and passive, depending on the nature of the oxide film. With active film metals, the oxide film grows, reaches a limiting thickness keeps getting destroyed until the metal is completely consumed. Examples of metals with active oxides are iron, copper and zinc. Passive film metals form an extremely thin oxide layer, in the order of 10-100 atoms thick, then stop growing. This film remains stable until something upsets the equilibrium. Examples of metals with passive films are stainless steel, titanium, gold, platinum, and silver.
Types of Corrosion
Corrosion can be broadly classified in two forms:
- Chemical dissolution of the metal.
- Galvanic, or electrically driven.
Within these two basic classifications there are five types of corrosion:
- General or uniform corrosion.
- Intergranular corrosion.
- Galvanic corrosion, including pitting and crevice corrosion.
- Stress corrosion cracking
- Microbiologically induced corrosion.
General Corrosion
Often, a metal starts to corrode by one mechanism, for example pitting corrosion, and then fails by a second mechanism, stress corrosion cracking.
Uniform corrosion occurs over large areas of the metal surface. This is the most common form of corrosion with steel and copper. It is the easiest form of corrosion to measure, and service lifetime is easy to calculate. This is the only form of corrosion that may be accurately calculated for lifetime before failure and the only corrosion mechanism in which increased section thickness gives longer life. This type of corrosion is measured by corrosion rate, usually reported as mpy (mils per year), mm/y (millimeters per year), ipm (inches per month), or mg/sdm/yr (milligrams per square decimeter per year). This type of corrosion can be minimized in the active metals by painting the surface, and unexpected failures can be avoided by periodic inspections.
Acid cleaning of metals is an exaggerated example of general corrosion. Every time a copper or carbon steel surface is acid cleaned, the metal walls are thinned due to uniform corrosion. Stainless steel is subject to general corrosion in many acids and some salt solutions. Stainless steel is not subject to general corrosion in water.
Uniform corrosion can be reduced or even prevented by proper selection of materials that are resistant to the corrosive environment. Certain elements make the alloy more resistant to different media. For example, high chromium content imparts oxidation resistance. High chromium is useful for high temperature oxidation resistance; so, any stainless steel is better than carbon steel in elevated temperature applications. High copper content in stainless steel imparts resistance to sulfuric acid. High nickel content gives resistance to reducing acids and produces a tightly adhering oxide film in high temperature oxidation.
Galvanic Corrosion
Galvanic corrosion occurs whenever two electrically different metals are connected in a circuit and are in an electrically conductive solution. This type of corrosion requires three conditions: two metals that differ in the galvanic series, an electrically conductive medium between the metals and both metals submerged in the conductive medium. A variation of galvanic corrosion can occur with passive film metals. If the alloy loses the passive film in one spot, then it becomes active in that area. Now the metal has both passive and active sites on the same surface. This is the mechanism for pitting and crevice corrosion.
Crevice Corrosion
Crevice corrosion is another form of galvanic corrosion, which occurs when the corroding metal is in close contact with anything that makes a tight crevice. Crevice corrosion is usually the first to occur and is predictable as to when and where it will take place. Like pitting, a conductive solution must be present; and, the presence of chlorides makes the reaction proceed at a fast rate. Crevice corrosion depends on the environmental temperature, alloy content and metallurgical category of the alloy. Also, there is a relationship between the tightness of the crevice and the onset time and severity of corrosion. There is a "critical crevice corrosion temperature" (CCCT) below which corrosion will not occur.
Pitting Corrosion
Pitting corrosion is a form of galvanic corrosion in which the chromium in the passive layer is dissolved leaving only the corrosion prone iron. The voltage difference between the passive and active layer on an austenitic stainless steel is +0.78 volts. Acid chlorides are the most common cause of pitting in stainless steel. Chlorides react with chromium to form the very soluble chromium chloride (CrCl3). Thus, chromium is removed from the passive layer leaving only the active iron. As the chromium is dissolved, the electrically driven chlorides bore into the stainless steel creating a spherical, smooth wall pit. The residual solution in the pit is ferric chloride (FeCl3), which is very corrosive to stainless steel. This is the reason ferric chloride is used in so many of the corrosion tests for stainless steel. When molybdenum and/or nitrogen is used as an alloying element in stainless steel, the pitting corrosion resistance improves. In an attempt to quantify the effect of alloying elements, a relationship of the various elements responsible for corrosion resistance was developed. The resulting equation is called the Pitting Resistance Equivalent Number, or PREN.
Intergranular Corrosion
All metals are composed of small grains that are normally oriented in a random fashion. These grains are each composed of orderly arrays of atoms with the same spacing between the atoms in every grain. Because of the random orientation of the grains, there is a mismatch between the atomic layers where the grains meet. This mismatch is called a "grain boundary." In a typical stainless steel product, there are about 1,000 grain boundaries that intersect a one-inch (25 mm) line drawn on the surface. Grain boundaries are regions of high-energy concentration. Therefore, chemical or metallurgical reactions usually occur at grain boundaries before they occur within the grains. The most common reaction is formation of chromium carbide in the heat-affected zone (HAZ) during welding. These carbides, formed along the grain boundaries, are called "sensitization." Because the carbides require more chromium than is locally available, the carbon pulls chromium from the area around the carbon. This leaves a low chromium grain boundary zone and creates a new low chromium alloy in that region. Now there is a mismatch in galvanic potential between the base metal and the grain boundary; so, galvanic corrosion begins. The grain boundaries corrode, allowing the central grain and the chromium carbides to drop out as if particles of rusty sand.
Stress Corrosion Cracking
Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. SCC is one of the most common and dangerous forms of corrosion. Stress corrosion cracking (SCC) is characterized by cracks propagating either transgranularly or intergranularly (along grain boundaries). There are several types of stress corrosion cracking (SCC), for example, chloride-induced SCC and H2S-induced SCC. Nickel containing stainless steel is especially susceptible to chloride induced SCC. Stress corrosion cracking (SCC) has three components: alloy composition, environment and the presence of tensile stress. All metals are susceptible to stress corrosion cracking,