25 May 2005
General Corrosion. See Figure 4-17 (D). The term "general corrosion"
refers to uniform corrosion of metal surfaces. A single piece of metal will have cathodic
and anodic areas due to differences in impurities and stresses. These areas will change
periodically, causing the metal to corrode over the entire surface at a more or less
Concentration Cell Corrosion. When two pieces of the same metal are
in a solution capable of acting as an electrolyte, and the electrolyte contains different
substances or the same substance in different amounts, such as a salt or a mixture of
salts, or oxygen, an electrical potential difference will develop between them.
Crevice Corrosion. See Figure 4-17 (B). The term "crevice corrosion"
refers to corrosion that occurs in a slight separation between two pieces of metal, such
as at the contact point of two mild or stainless steel plates that have been bolted
together. Water flow is restricted in a crevice and, as a result, oxygen is consumed
faster than it can be replenished. The metal in the crevice functions as an anode and
corrodes. This is a form of concentration cell corrosion, also called "differential oxygen
cell" corrosion. Stainless steel is particularly susceptible to this type of corrosion, which
results in localized or pitting attack.
Under-Deposit Corrosion. See Figure 4-17 (C). The term "under-deposit
corrosion" refers to corrosion occurring under any type of deposit. The underside of a
deposit that has been caused by fouling, bacterial slime, or debris acts in much the
same way as the inside of a crevice. The metal under the deposit becomes anodic and
corrodes. This process is considered another form of concentration cell corrosion
because oxygen cannot easily get under the deposit. All metals are susceptible to this
type of corrosion, which results in localized or pitting attack.
Microbiologically Influenced Corrosion (MIC). See Figure 4-17 (E).
This term refers to metal corrosion associated with microbiological organisms whose
presence contributes to the creation of, or maintenance of, a corrosive environment.
MIC can be either eliminated or prevented to a large degree by the proper use of
Corrosion Rate. The term "corrosion rate" refers to the rate at which the
corrosion action proceeds. The rate is measured in units of mils per year (mpy). A mil is
0.0254 millimeter (one-thousandth of an inch). The rate measurement is performed
using corrosion coupons that have been exposed to cooling water for a short period of
time (i.e., 30 to 90 days). The weight of the coupon is measured before and after
exposure to the water. The thickness of the metal lost due to corrosion over the testing
period is then calculated using a measurement of the weight loss. This weight loss is
extrapolated to give a rate for 1 year and a calculation of the thickness loss is then
performed and the value is reported. Alternatively, this measurement can be taken
using specialized instruments that rapidly measure corrosion rates. Table 4-9 shows the
corrosion rates for corrosion coupons of different metals. Paragraph 6-5.2 provides
detailed information on corrosion testing.