Page 66 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

44 INTRODUCTION AND FORMS OF CORROSION

biofilm on the metal surface (57); (ii) direct observation of the community struc-
ture of the biofilm; (iii) electrochemical corrosion measurements such as polarization
resistance; (iv) water quality and redox potential measurements; (v) identification of
microorganism in water and on metal; (vi) identification and analysis of corrosion
products and biofilms; (vii) evaluation of morphology, form, and type of corrosion
after removal of corrosion products.
Risk assessment of carbon steel pipelines, on the basis of the details of water chem-
istry and operation parameters, has been documented.
Prevention of biocorrosion problems varies with the types of materials of con-
struction, environment, economics, and duty cycle of equipment. The most common
approaches to prevention of biocorrosion involve the use of sterilization, coatings,
cathodic protection, and proper selection of materials. A general rule is to start with
a clean system and keep it clean.
Sterilization of the system by gamma or UV irradiation for disinfection of mate-
rials and environments may be useful to mitigate biocorrosion (51). Sterilization by
chemical methods such as the use of biocides to control biofilm formation in closed
systems such as heat exchangers, cooling towers, and storage tanks will mitigate bio-
corrosion (51, 52).

1.7 MECHANICALLY ASSISTED CORROSION

Mechanically assisted degradation can consist of the following types of corrosion:
erosion–corrosion, water drop impingement corrosion, cavitation erosion, erosive
and corrosive wear, fretting corrosion, and corrosion fatigue (CF) (Fig. 1.14).
Erosion–corrosion consists of the corrosion process enhanced by erosion or wear.
Fretting corrosion consists of the wear process enhanced by corrosion. CF consists
of the combined action of fluctuating or cyclic stress and a corrosive environment.

1.7.1 Corrosion and Wear
The progressive deterioration, because of corrosion and wear, of metallic surfaces
leads to loss of plant efficiency, and in the worst case, to shutdown. For instance,
both direct and indirect costs to the US economy have been estimated to be nearly
$300 billion per year. The wear of materials has been estimated to cost $20 billion a
year (in 1978 dollars) compared to $80 billion for corrosion during the same period
(9, 58).
The combined action of wear or abrasion and corrosion results in more severe dam-
age than with either mechanical or corrosive attack alone. The metal removed from
the surface can be metal ions, or as particles of solid corrosion products or as elemen-
tal metal. The erosion–corrosion process ranges from primarily erosive attack such
as sandblasting, filing, or grinding of metal surface to essentially corrosion failures
devoid of mechanical action.
Corrosion can occur in the absence of mechanical wear, but the opposite is rarely
true. Corrosion often occurs in a wear process to a certain extent in all environments,