Page 84 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

62 INTRODUCTION AND FORMS OF CORROSION

1.7.34 Stress Ratio
Rates of CF crack propagation generally are enhanced by increased stress ratio R,
which is the ratio of minimum stress to maximum stress.
Some other factors, namely, the metallurgical condition of the material such as
composition and heat treatment, and the loading mode, such as uniaxial, affect fatigue
crack propagation (4).
Environmental factors to consider are: type of environments (gaseous, liquid,
liquid metal) temperature, partial pressures of corrosive species in gaseous envi-
ronments, concentration of corrosive species in aqueous or liquid media, corrosion
potential, pH, conductivity, halogen or sulfide ion content, viscosity of the medium,
oxygen content, solution composition, inhibitors, coatings (4), and so on. Corrosion
products on or within fracture surfaces are identified. CF cracking of high-strength
steel exposed to water vapor leading to hydrogen damage may be difficult to
distinguish from other forms of hydrogen damage. At high frequencies, the fracture
surface features because of CF crack initiation and propagation do not differ
significantly from those produced by fatigue in nonaggressive environments (24).
The usual test of fatigue of metals in air is affected by oxygen or humidity and
represents a measure of CF. Some tests showed the CF endurance limit for copper is
14% higher in vacuum than in air. Oxygen has very little influence on initiation but
considerable effect on crack propagation (8). Brackish waters have a greater effect
on CF of steel than that of copper. Controlled changes in the potential of a sample
can lead to either the complete elimination or the dramatic increase in brittle fatigue
cracking (4).

1.7.35 Material Factors
The most important metallurgical properties are: alloy composition, distribution of
alloying elements and impurities, microstructure and crystal structure, heat treatment,
mechanical working, preferred orientation of grain and grain boundaries (texture),
mechanical properties such as strength, fracture toughness, and so on. (9).
A fatigue test involves subjecting a metal sample to alternate cyclic stresses,
compression–tension of different values, and measuring the time (number of cycles,
N) before rupture of the sample. A short characteristic of the fatigue test is known
as the C–N curve, giving the number of cycles N to rupture. The value of maximum
stress for which an infinite number of cycles can be supported without rupture
is known as endurance limit or fatigue limit. The fatigue limit exists for steels,
but not necessarily for other metals and generally equals half the tensile strength.
Nonferrous metals such as Al, Mg, Cu alloys do not have a fatigue limit and are
8
assigned fatigue resistance to an arbitrary number of cycles, such as 10 cycles (69).
Crack growth rates are influenced by metallurgical variables such as compositional
impurities, microstructure, and the cyclic deformation mode. In carbon steels, cracks
often originate at hemispherical corrosion pits and often contain significant amounts
of corrosion products. The cracks are often transgranular with possible branching.
Surface pitting and a transgranular fracture path are not prerequisites for CF cracking