4.4 Environmentally Induced Failure
4.4.1 Classification of Corrosion
Corrosion is the most commonly used generic terminology for all environ- mentally induced degradation. Strictly speaking, corrosion is a chemical and electrochemical reaction between a material and its surrounding environ- ment that results in a deterioration of the mechanical and physical proper- ties of the material. The electrochemical nature of a corrosion process is best demonstrated in many automobile batteries, as shown in Figure 4.18. Severe corrosion appears in the positive post and other areas with direct contact between the battery and the frame. However, the term corrosion is also loosely applied to mechanically assisted corrosive attack, such as fretting corrosion and erosion corrosion. In many cases, metal embrittlement, such as cadmium embrittlement, or hydrogen embrittlement, and oxidization are also referred to as corrosion. There is no unified terminology used to describe the forms of corrosion. The following discussions are based on the terminology acceptable to most engineering communities.
The overlapping characteristics of various corrosion forms and mecha- nisms made it very challenging to completely separate one mechanism from another. The most frequently observed corrosion forms are categorized into the following seven classes based on how the corrosion process mani- fests itself: uniform or general, galvanic, crevice, pitting, intergranular, ero- sion, and stress corrosion cracking. Figure 4.19 is a photo of a mining cart exhibited at the Bingham Canyon Mine Visitors Center of Kennecott Utah Copper in Bingham Canyon, Utah, that shows general corrosion in the form of rust, resulting from exposure to the atmosphere. This type of uniform
environmental degradation is observed in many outdoor exhibits and struc- tures. Any reverse engineered part is expected to demonstrate sufficient rust resistance when it is used outdoors. The corrosion processes are typi- cal electrochemical processes. Intergranular corrosion is heavily influenced by alloy metallurgical properties. Erosion corrosion is only observed in the presence of moving corrosive fluid. Stress corrosion cracking is a combined effect of corrosive environment and applied stress. Further subclassification
FIgurE 4.18 (See color insert following p. 142.) Corrosion due to electrochemical reaction.
FIgurE 4.19 (See color insert following p. 142.) General corrosion observed on a mining cart.
is also used to define the unique corrosive attack under these primary cor- rosion forms. For example, the term exfoliation corrosion is widely used to identify a unique corrosion class in aluminum alloys caused by intergranu- lar corrosion.
The uniform or general corrosion is characterized by corrosive attack pro- ceeding evenly over the entire or most of the surface area. Compared to most other corrosion mechanisms, the uniform corrosion is more predicable. The measurement of weight loss is commonly used to quantitatively calculate the corrosion rate of uniform corrosion.
Galvanic corrosion is an electrochemical process between two dissimilar metals in which one corrodes preferentially. There are three necessary con- ditions for galvanic corrosion to occur. First, two electrochemically different metals are present: one functions as an anode and the other as a cathode.
Second, an electrically conductive path exists between these two metals.
Third, a conductive path of metal ions is available between the anodic and cathodic metals. The corrosive interactions between two metals are often ref- erenced in a galvanic series table or chart. This table ranks the metals in the order of their relative nobility in a corrosive environment such as seawater.
This table begins the list with the most active anodic metal and proceeds down to the least active cathodic metal. In a galvanic couple that consists of two dissimilar metals, the metal higher in the series, representing an anode, will corrode preferentially. The galvanic series table provides very useful guidance to galvanic corrosion protection in joint metals. The closer two metals are in the series; the more electrochemically compatible they are, and therefore less a chance the galvanic corrosion will occur when they are in contact. Conversely, the farther apart the two metals are, the worse the galvanic corrosion that occurs will be. A galvanic series applies only to a specific electrolyte solution. Different galvanic series tables are used for dif- ferent environments and different temperatures.
Crevice corrosion is a localized corrosion occurring in narrow openings such as crevices. There are many of these crevices in the part joint areas or in a machine itself, such as the areas under gaskets or seals, or inside cracks and seams. These crevices are often filled with muddy deposits, solid sediments, or slushy precipitates. These sludge piles can develop a local chemistry of the electrolyte that is very different from that of the surroundings. The diffu- sion of oxygen into the crevice is usually restricted. As a result, a differential aeration cell can establish between the crevice and the external surface. An electrochemical potential drop in the crevice might also occur because of deoxygenation of the crevice and a separation of electroactive areas, with net anodic reactions occurring within the crevice and net cathodic reactions occurring exterior to the crevice. Unfortunately, this local corrosive envi- ronment stagnates because of lack of electrolyte flow, and induces crevice corrosion. In contrast to galvanic corrosion where corrosion occurs between two dissimilar metals immersed in one electrolyte, crevice is a corrosive
action that occurs between two metal parts made of the same alloy while surrounded with two different electrolytic environments.
Figure 4.20 shows a localized corrosion. However, it is not a pitting corrosion.
Carbon steel typically does not pit; the observed localized corrosion is break- through of a galvanized coating that allows red-rust formation from the under- lying steel substrate. Pitting is a corrosion confined to a small area, penetrating deep into the metal surface. It appears as small and irregular pit holes on the surface. Pitting is most likely to occur in the presence of chloride ions, com- bined with such depolarizers as oxygen or oxidizing salts. The distinct features typifying pitting corrosion have long classified it as a unique form of corro- sion. However, the driving force of pitting corrosion is very similar to galvanic corrosion. In pitting corrosion, the lack of oxygen around a small area makes this area anodic, while the surrounding area with an excess of oxygen becomes cathodic. This leads to a localized galvanic corrosion that corrodes into the part and forms pit holes. These tiny pit holes limit the diffusion of ions and further pronounce the localized lack of oxygen. The formation of pit holes makes the mechanism of pitting corrosion very similar to that of crevice corrosion.
Intergranular corrrosion is also referred to as intercrystalline or interden- dritic corrosion. The detailed microstructure characteristics of intergranular corrosion can only be examined under a microscope. However, the accu- mulated damage, such as part exfoliation, is readily visible when the inter- granular corrosion just underneath the surface expands and blists the part surface. Exfoliation corrosion is most often observed on extruded or rolled aluminum products where the grain thickness is relatively shallow. It may also occur on parts made of carbon steel. Without proper microstructure analysis the actual grain morphology of the plates used for the box shown in Figure 4.21 can not be absolutely confirmed. Nonetheless, the subject box does show distinct macro characteristics of exfoliation corrosion.
FIgurE 4.20 (See color insert following p. 142.) Localized corrosion.
Intergranular corrosion occurs along the grain boundaries or immediately adjacent to the grain boundaries, which usually have a different crystallgo- graphic structure and chemical composition than the interior grain matrix.
Figure 4.22 shows the grain morphology of an aluminum alloy with various second phases attached to the grain boundaries. Heat-treated stainless steels and aluminum alloys are noticeably susceptible to grain boundary corro- sion attack, partially due to the segregation and precipitation induced by heat treatment. Such segregation or precipitation can form a zone in the immediate vicinity of grain boundary, leading to preferential corrosive attack. The inter- granular precipitation of chromium carbides (Cr23C6) during a sensitizing heat treatment or thermal cycle often causes the intergranular corrosion of auste- nitic stainless steels. Intergranular corrosion occurs in many aluminum alloys either due to the presence of some chemical elements or second phases anodic to aluminum or due to copper depletion adjacent to grain boundaries in cop- per-containing alloys. Small quantities of iron segregation to the grain bound- aries in aluminum alloys can induce intergranular corrosion. Precipitation of some second phases, such as Mg5Al8, Mg2Si, MgZn2, or MnAl6, in the grain boundaries will also cause or enhance intergranular attack of high-strength aluminum alloys, particularly in chloride-rich media.