Page 89 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

ENVIRONMENTALLY INDUCED CRACKING (EIC) 67

deaeration of saline solution with mild steel brings back the CF limit to the value
in air (84). Addition of 200 ppm of sodium dichromate to the city water reduces
the CF of normalized 0.35% carbon steel to a lower level than the value in air.
4. Cathodic protection, provided the material is not prone to embrittlement.
Sacrificial zinc (galvanized) coatings may be used.
5. Surface treatments such as hot peening, nitriding of steels, sandblasting of the
surface of the metal and other treatments that produce constraints of compres-
sion are beneficial.
6. Organic coatings, which can impede CF. The coatings must contain inhibitory
pigments in the primary layer. Local defects in the coatings reduce the CF
strength of carbon steel (9).
7. Noble metal coatings, which are useful as long as they remain unbroken and are
of high density and thickness. Electrolytic deposits of tin, lead, copper, silver,
or steel are useful as protectors of CF (60).

1.8 ENVIRONMENTALLY INDUCED CRACKING (EIC)

In some environments and under certain conditions, a microscopically brittle fracture
of materials can occur at levels of mechanical stress that may be far below the level
required for general yielding or those that cause significant damage in the absence
of an environment. The susceptibility also depends on the chemical composition and
microstructure of the alloy. This form of corrosion requires an interaction between the
electrochemical dissolution of the metal, hydrogen absorption, and the mechanical
loading conditions (stress, strain, and stain rate) (73). The nature of these fracture
modes varies from one class of material to another. However, all fracture modes are
largely similar to one another.

1.8.1 Testing of CF
Some of the factors that must be considered in CF testing are the following:

1. Stress intensity range, load frequency, and stress ratio.
2. Electrode potential in aqueous media and intended environment composition.
3. Metallurgical factors such as alloy composition microstructure and yield
strength (4).

The scientific basis for the reliable estimate of fatigue life for intended conditions
and situations remains elusive in spite of expenditures of the order of billions of dol-
lars in combating fatigue. The fatigue test consists of subjecting a sample to a certain
frequency of alternate cyclic stresses (compression-tension) of different values and
plotting a C–N curve. The presence of rust or other corrosion products do not indi-
cate a decrease in fatigue. It is necessary to carry out fatigue tests and determine the
decrease in fatigue resistance (17).