Page 42 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

20 INTRODUCTION AND FORMS OF CORROSION

Breakdown of passivity is the first stage in pitting corrosion. Corrosion passi-
vation consists of pit growth and repassivation. Metals show different patterns of
passivation. Aluminum and copper are not passive in strongly acidic solutions while
Fe, Ni, and steels are passive even in strong acidic electrolytes in disagreement with
the predictions of Pourbaix diagrams. Localized acidification by the hydrolysis of
metal cations may be a stabilizing factor for pitting in some metals. Addition of alloy-
ing elements such as Cr and Mo in steel can produce a beneficial effect on the oxide
film and result in effective passivation. For austenitic steels, the barrier oxide layers,
the salt deposit layers, and the alloy surface layers play an important role in passivity
and the breakdown of passivity. It is useful to note that modeling passivation can lead
to insights into improved performance and development of new corrosion-resistant
alloys (25, 26).
There are two types of pitting that follow the breakdown of a passive metal at
surface and can be distinguished as pitting at low and high potential. Pitting at low
potential is influenced by cathodic or self-activation leading to emerging etch pits that
can eventually lead to general corrosion with etching. High-potential pitting results
in hemispherical pits corresponding to anodic dissolution in the electrobrightening
mode. This requires a random dissolution because of the presence of film breakdown
factors but independent of the crystal structure of the metal. This occurs through a
random defective solid film, which is a very good ion conductor. Pits of both types
result in occluded corrosion (2). This is also in agreement with the level of potential
of the passive metal, that is, noble or active potential (27).
Recently, statistical and stochastic approaches involving Gaussian and Poisson
distribution of localized corrosion have been reviewed (28). Application of Pois-
son distribution to pit generation was successful. Different pit generation rates were
observed as a function of time. The models consider either pit generation rate alone or
pit generation and repassivation. The three models that are concerned with initiation
process leading to passive film breakdown are (i) adsorption and adsorption-induced
mechanisms where chloride adsorption is the main process; (ii) ion migration and
penetration models, and (iii) mechanical film breakdown model (29).

1.4.6 Coatings and Localized Corrosion
This type of corrosion occurs when protective coatings are applied over metal and
where there is a break in the coating so that the large coated area acts as a cathode
and the small defective area as the anode (15).

1.4.7 Electrochemical Studies of Localized Corrosion
There are numerous electrochemical methods to determine the electrochemical con-
ditions of pitting and crevice corrosion. Here we are concerned with pitting corrosion.

1.4.7.1 Cyclic Potentiodynamic Polarization Method Electrochemical studies of
pitting corrosion indicate that pitting occurs within or above a critical potential or
potential range. The electrochemical methods involved in pitting corrosion studies are