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5. Stress corrosion.
6. Erosion corrosion. CHEMICAL ENGINEERING
7. Corrosion fatigue.
8. High temperature oxidation.
9. Hydrogen embrittlement.
Metallic corrosion is essentially an electrochemical process. Four components are
necessary to set up an electrochemical cell:

1. Anode the corroding electrode.
2. Cathode the passive, non-corroding electrode.
3. The conducting medium the electrolyte the corroding fluid.
4. Completion of the electrical circuit through the material.

Cathodic areas can arise in many ways:
(i) Dissimilar metals.
(ii) Corrosion products.
(iii) Inclusions in the metal, such as slag.
(iv) Less well-aerated areas.
(v) Areas of differential concentration.
(vi) Differentially strained areas.

7.4.1. Uniform corrosion
This term describes the more or less uniform wastage of material by corrosion, with no
pitting or other forms of local attack. If the corrosion of a material can be considered
to be uniform the life of the material in service can be predicted from experimentally
determined corrosion rates.
Corrosion rates are usually expressed as a penetration rate in inches per year, or mills
per year (mpy) (where a mill D 10 3 inches). They are also expressed as a weight loss
in milligrams per square decimetre per day (mdd). In corrosion testing, the corrosion rate
is measured by the reduction in weight of a specimen of known area over a fixed period
of time.
12w
ipy D 7.1
tA
where w D mass loss in time t,lb,
t D time, years,
2
A D surface area, ft ,
3
D density of material, lb/ft ,
as most of the published data on corrosion rates are in imperial units.
In SI units 1 ipy D 25 mm per year.
When judging corrosion rates expressed in mdd it must be remembered that the
penetration rate depends on the density of the material. For ferrous metals 100 mdd
D 0.02 ipy.
What can be considered as an acceptable rate of attack will depend on the cost of the
material; the duty, particularly as regards to safety; and the economic life of the plant. For