Page 233 - Corrosion Engineering Principles and Practice

208 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 209

Impurities in the metal
Orientation of grains
Grain boundaries
Differential grain size
Differential thermal treatment
Surface roughness
Local scratches or abrasions
Difference in shape
Differential strain
Differential pre-exposure to air or oxygen
Differential concentration or composition of solution
Differential aeration
Differential heating
Differential illumination
Differential agitation
Contact with dissimilar metals
Externally applied potentials
Complex cells

TABLE 7.1 Causes of Corrosion Currents

There is a vast body of information relating practically all the
previously listed factors to actual field observations and subsequent
analysis of failed components. These failure investigations are
typically carried out in a detailed mechanistic “bottom-up” manner
whereby a failed component would be sent to the laboratory where
analytical techniques would then be used following well-established
protocols. Chemical analysis, hardness testing, metallography, optical
and electron microscopy, fractography, x-ray diffraction, and surface
analysis are specialized tools used in such investigations.
However, this approach alone provides little or no insight into the
real causes of failure. Underlying causes of serious corrosion damage
often include human factors such as lack of corrosion awareness and
inadequate training and poor communication. Further underlying
causes may include weak maintenance management systems,
insufficient repairs due to short-term profit motives, a poor
organizational “safety culture,” defective supplier’s products, or an
incorrect material selection.
It is thus apparent that there can be multiple causes associated
with a single corrosion mechanism. Clearly, a comprehensive failure