Page 32 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

10 INTRODUCTION AND FORMS OF CORROSION

TABLE 1.3 Comparative Features of HIC and SSC (10)
Phenomenon Hydrogen-Induced Cracking Sulfide Stress Cracking
Crack direction Parallel to applied stress Perpendicular to stress
Applied stress No effect Affects critically
Material strength Primarily in low-strength Primarily in high-strength
steel steel
Location Ingot core Anywhere
Microstructure Trivial effect Critical effect Quenching and
tempering enhances SSC
resistance
Environment Highly corrosive conditions, Can occur even in mildly
considerable hydrogen corrosive media
uptake

potential has a tendency to corrode when it is in contact or connected to a metal with
more positive or noble potential.
The galvanic series is useful in giving a qualitative indication of the possibility of
galvanic corrosion in a given medium under some environmental conditions.

1.2.3 The Nature of the Metal/Solution Interface
Cast iron corrodes because of the exposure of graphite content of cast iron (graphitic
corrosion), which is cathodic to both low alloy and mild steels. The trim of a valve
must be cathodic to the valve body to avoid pitting attack. Thus in aggressive media
valve bodies of steel are preferred to cast iron bodies. Steel bolts and nuts coupled to
underground mild steel pipes or a weld rod used for steel plates on the hull of a ship
should always be of a low nickel, low chromium steel, or from a similar composition
to that of the steel pipe (8).

1.2.4 Polarization of the Galvanic Cell
The different phenomena of polarization of the anodic and cathodic reactions (activa-
tion, diffusion, convection) should be well known as a function of the evolution and
change of the properties of the interface as a function of time. The polarization behav-
ior of the cathode and anodic reactions on the two electrodes should be examined. In
natural atmospheres, the cathodic reaction controls frequently the rate of attack. The
diffusion of oxygen is an important parameter to avoid control and polarization of the
corrosion by the rate of the cathodic reaction (8) (Fig. 1.8).
The resistance overpotential of the cell IR is mainly a function of the conductivity
of the electrolyte solution and the distance between the electrodes as the electrolytic
resistance is more important than the electric resistance of the metals. Thus if dissim-
ilar pipes are butt-welded with the flow of electrolyte, the most severe corrosion will
occur near the weld on the active metal. In soft water, the critical distance between