Page 36 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

14 INTRODUCTION AND FORMS OF CORROSION

bonding of nearby structure and adding additional anodes and increasing rectifier
capacity (13). Stray currents follow paths other than their intended circuit. They leave
their intended path because of poor electrical connections or poor insulation around
the intended conductive material. The escaped current then will pass through the soil,
water, or any other suitable electrolyte to find a low-buried path such as metallic pipe.
Stray currents cause accelerated corrosion when they leave the metal structure and
enter the surrounding electrolyte. These corrosion sites can be several hundreds of
meters away.
At the points where the current enters the structure, the site becomes cathodic
in nature because of changes in potential, and the area where the current leaves
becomes anodic. Electric railways, cathodic protection, electric welding machines,
and grounded DC electrical sources are prone to stray current corrosion (14).
Although the damage because of stray current is localized in a part of the system,
stray current may lead to uniform corrosion of this part of the system and hence
considered as a general form of corrosion. The attack because of stray current is
generally more localized, sometimes leading to a concentration of pits. Stray current
corrosion can cause penetration along the boundaries or a selective attack of the ferrite
within the matrix of gray cast iron. Aluminum and zinc (amphoteric metals) can show
signs of corrosion at cathodic portion of the metals because of the localized alkalinity.
Buried power lines can give rise to AC stray currents. In general, AC currents cause
less damage. DC and the stray current corrosion decreases with increasing frequency.
However, damage to passive alloys such as stainless steels and aluminum alloys is
important because of the alternating reduction and oxidation of the passive or barrier
layer on the surface, leading to porous and nonprotective passive layers (14).
The escaping current can be monitored by measuring the current before it enters
the soil as around the electrolyte. Good electrical connections and insulation can stop
the current leakage from the metallic structure to the ground. The stray current con-
ductor is connected with the source ground via a separate conductor resulting in the
elimination of the need for the current to leave the metal and enter the soil. Sacri-
ficial anodes may be used to prevent stray current corrosion. The insulation should
be used with care to reduce the current to a negligible value. Coatings are not useful
as cracks, pinholes, or pores will promote localized corrosion. However, coatings on
cathodically protected structures are useful and make the stray current less severe and
more easily controlled.

1.4 LOCALIZED CORROSION

This is the most insidious form of corrosion as it is less predictable than general
corrosion and can cause serious failures. All the forms of general corrosion that result
in a nonuniform surface can be considered as localized corrosion. Figure 1.9 shows
the various forms of localized corrosion.
The two major types of localized corrosion to be discussed are pitting corro-
sion and crevice corrosion, including filiform corrosion. Although the morphological
appearance of these two types of corrosion is different, the electrochemical basis of