Page 189 - Corrosion Engineering Principles and Practice

164 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 165

14. Nature of abrasive treatment and preliminary washing with
various liquids
15. Character of the scratch line
16. Effect of corrosion in the neighborhood (protective action of
a recent scratch-line for a less recent one)
A number of statistical transformations have since then been
proposed to quantify the distributions in pitting variables. Gumbel is
given the credit for the original development of extreme value
statistics (EVS) for the characterization of pit depth distribution [10].
The EVS procedure is to measure maximum pit depths on several
replicate specimens that have pitted, then arrange the pit depth
values in order of increasing rank. The Gumbel or extreme value
cumulative probability function [F(x)] is shown in Eq (6.1), where l
and a are the location and scale parameters, respectively. This
probability function can be used to characterize data sets and estimate
the extreme pit depth that possibly can affect a system.
  x − l 
F x) = exp   −exp   − a  (6.1)
(

6.3.2 Crevice Corrosion
Crevice corrosion occurs in cracks or crevices formed between mating
surfaces of metal assemblies, and usually takes the form of pitting or
etched patches. Both surfaces may be of the same metal or of dissimilar
metals, or one surface may be a nonmetal as shown in Fig. 6.20. It can
also occur under scale and surface deposits and under loose fitting

FIGURE 6.20 Large holes created in a bridge structural steel by the presence
of rocks. (Courtesy Kingston Technical Software)