Page 276 - Corrosion Engineering Principles and Practice
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250 C h a p t e r 7 C o r r o s i o n F a i l u r e s , F a c t o r s , a n d C e l l s 251
The following are general recommendations to avoid the initiation
of SCC due to the formation of corrosion cells in service.
• Residual stresses may be minimized by careful thermal stress
relief appropriate to the alloy
• Compressive stresses may be introduced by controlled shot
peening
• Stress raisers should be eliminated or at least minimized
• Open crevices should be eliminated, as well as areas where
deposits can accumulate and build up tremendous internal
pressure
• Conditions conducive to evaporation and concentration of
corrosive species should also be minimized
7.6 Visualizing Corrosion Cells
The existence and location of anodes and cathodes in a corrosion
cell can be demonstrated by the changes in color of certain reagents.
Such color changes have been very helpful in the early days of
corrosion science to study the local interplay of local anodes and
cathodes on apparently homogeneous steel surfaces exposed to a
corrosive environment. As noted by Cushman and Gardner in their
1910 textbook, it is a matter of common observation that iron
usually corrodes rapidly at certain weak points in an effect known
as pitting [24].
The interest in using color changes to reveal subtle corrosion
mechanisms is still quite modern as attested by two papers recently
published in a reputable corrosion journal. In one paper, the corrosion
of aluminum and aluminum alloys in chloride-containing agar gels
was studied by using a broad-range pH indicator [25]. Distinct changes
in pH were observed at low-pH anodic sites and at high-pH cathodes.
There was a definite edge effect that occurred when gels were placed
directly on the metal, which dominated the pattern of corrosion. Wet-
abraded surfaces initially showed a general type of corrosion, whereas
dry-abraded surfaces showed localized corrosion under the gel.
In a second paper published recently, paint systems containing
color-change or fluorescing compounds were found to be sensitive to
underlying corrosion processes by reacting to the pH increase
associated with the local cathodic reaction [26]. The sensitivity of
acrylic-based coating systems for detection of cathodic reactions
associated with corrosion was determined by applying constant
cathodic current and measuring the charge at which color change or
fluorescence was detected.
As explained in much detail in Chaps. 2 and 3, the cathodic
reaction in a corrosion process generally produces an increase in the
concentration of hydroxyl ions as a result of removal of hydrogen