Page 44 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

22 INTRODUCTION AND FORMS OF CORROSION

technique has been described for aluminum alloys in ASTM G1 as a method for Al
alloys (22).

1.4.8 Potentiostatic Methods
When the breakdown potential is determined by cyclic potentiodynamic polarization
methods, polarization of the sample at potentials below and above the breakdown
potential gives information on the initiation and propagation of pits at different levels.
Another method involves initiation of pits above the pitting or breakdown poten-
tial and then shifts to lower values above or below the protection potential. It is
assumed that at imposed values below the protection potential the current is expected
to decrease until complete repassivation.
The critical pitting potential (E ) lies between the breakdown potential and pro-
cpp
tection potential and may be determined by scratch repassivation method. The scratch
repassivation method for localized corrosion involves scratching the alloy surface at
a fixed potential. Then the change in current is monitored as a function of time, which
will show the effect of potential on the induction and repassivation times. An informed
choice of the level of potential between the breakdown potential and the critical pitting
potential leads to the critical potential for a particular value in the given conditions
(22).

1.4.9 Prevention of Localized Corrosion
Knowledge and data on the various physical and chemical aspects of passivity such
as composition, thickness, structure, growth, and properties of passive layers may be
used in the studies of localized corrosion knowledge of the surface reactions in the
formation and composition of passive films, passivation/repassivation, which is useful
in the development of highly corrosion-resistant alloys (CRA). A good understanding
of metallurgical factors and a rational use of alloying elements can be useful in the
control of pitting corrosion. It is also useful to know the mechanism of failure or
breakdown of passive films in pitting corrosion for safety purposes (2).
Design-to-prevent is a useful approach to avoid pitting and crevice corrosion.
Some examples of surface treatments and coatings to mitigate pitting and crevice
corrosion are the following:

(i) Sacrificial zinc coatings for steel.
(ii) Anodizing and sealing accompanied by painting.
(iii) Steel phosphating before painting or using corrosion inhibitors such as chro-
mate, phosphate, molybdate, or zinc rich primers (70–75 μm thick).
(iv) Light barrier protective paint coating ∼100 μm thick or heavy 6–13 mm thick
to acid proof brick lining.
(v) A thin organic coating for sacrificial zinc can extend life by a factor of 10.
A comparable organic coating together with cathodic protection is probably
the best solution.