Page 108 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

86 INTRODUCTION AND FORMS OF CORROSION

1.8.10.27 Hydrogen-Induced Plasticity Two models have been proposed on the
basis of the fact that hydrogen increases the local plasticity. Two models have been
advanced on the basis of adsorption and absorption of hydrogen.
The adsorbed hydrogen-induced model is based on the fact that the adsorbed atoms
weaken interatomic bonds at crack tips and thereby facilitate the injection of dislo-
cations (alternate slip) at crack tips. Crack growth occurs by alternate slip the crack
tips, which promotes the coalescence of cracks with small voids nucleated ahead of
the cracks.
The second absorbed hydrogen-enhanced local plasticity mechanism is based on
the fact that the local decrease of the flow stress by hydrogen leads to highly localized
failure by ductile processes, while the local macroscopic deformation remains small.
Shear localization results from local hydrogen absorption, resulting in a macroscop-
ically “brittle” fracture related to microscopic localized deformation.

1.8.10.28 Prevention of SCC Although several factors are the causes of SCC, the
measures to prevent or reduce the risk of SCC can be divided as a function of stresses,
environmental factors, metallurgical properties, and surface treatments.

(i) Stresses. The sum of stresses in service and residual stresses, including fab-
rication stresses, should be below the threshold level, which, in the absence
of reliable data, should be evaluated by testing as a percentage of the tensile
yield strength. This can be achieved by a design that avoids initial concentra-
tion of stresses, reducing operating stresses, relieving fabrication stresses by
heat treatment, and by choosing appropriate dimensions of the loaded alloy.
(ii) During use, avoiding stress concentrators by increasing susceptibility to dif-
ferent forms of degradation in service, especially localized corrosion, galvanic
corrosion, and erosion–corrosion will be helpful. Accidental or nondesigned
cyclic loading also should be avoided.
(iii) Bolted or riveted joints can produce high local stresses that can cause SCC,
so that proper joint design and construction can be done. Some examples are
the use of performed parts; avoid overtorqueing of bolts, provision of adequate
spacing, and edge margins for rivets.

Environmental Considerations:
(i) Eliminating potential aggressive ions that cause SCC such as dissolved gases,
heavy metal ions, and impurities by chemical or physical methods such as
degasification, demineralization will prevent the risk of SCC.
(ii) Addition of inhibitors to form a strong and adhesive film will also help. Phos-
phates and other inorganic and organic inhibitors decrease the effects of SCC.
A minimal amount of oxidizing inhibitor such as sodium nitrite is absolutely
necessary to avoid pitting.
(iii) Mixture of inhibitors may be useful for corrosion control. Capillary condensa-
tion can fill crevices and result in corrosion that can be avoided by filling the
crevices with corrosion-inhibited putty.