Corrosion & Materials of Construction
- 2. Uniform Corrosion is the overall attack on a metal by a corrod-ing
liquid resulting in a relatively uniform metal loss over the exposed
surface. This is the most common type of corrosion and it can be
minimized by the selection of a material which offers resistance to
the corroding liquid.
- 3. Intergranular corrosion is the precipitation of chromium
carbides at the grain boundaries of stainless steels. It results in the
complete destruction of the mechanical properties of the steel for the
depth of the attack. Solution annealing or the use of extra low carbon
stainless steels will eliminate intergranular corrosion.
- 4. Pitting Corrosion is a localized rather than uniform type of
attack. It is caused by a breakdown of the protective film and results
in rapid pit formation at random locations on the surface.
- 5. Crevice or Concentration Cell Corrosion occurs in joints or
small surface imperfections. Portions of the liquid become trapped
and a difference in potential is established due to the oxygen con-centration
difference in these cells. The resulting corrosion may
progress rapidly leaving the surrounding area unaffected.
- 6. Stress Corrosion is the failure of a material due to a combina-tion
of stress and corrosive environment, whereas the material would
not be affected by the environment alone.
- 7. Erosion-Corrosion is the corrosion resulting when a metal?s
protective film is destroyed by high velocity fluids. It is distinguished
from abrasion which is destruction by fluids containing abrasive solid
particles.
pH VALUES
The pH of a liquid is an indication of its corrosive qualities, either
acidic or alkaline. It is a measure of the hydrogen or hydroxide ion
concentration in gram equivalents per liter. pH value is expressed as
the logarithm to the base 10 of the reciprocal of the hydrogen ion
concentration. The scale of pH values is from zero to 14, with 7 as a
neutral point. From 6 to zero denotes increasing hydrogen ion con-centration
and thus increasing acidity, and from 8 to 14 denotes
increasing hydroxide ion concentration and thus increasing alkalinity.
The table below outlines materials of construction usually
recommended for pumps handling liquids of known pH value
| pH Value |
Material of Construction |
| 10 to 14 |
Corrosion Resistant Alloys |
8 to 10
6 to 8
4 to 6
|
Iron, Stainless Steel, Bronze, Carbon Steel |
| 0 to 4 |
Corrosion Resistant Alloys |
The pH value should only be used as a guide with weak aqueous
solutions. For more corrosive solutions, temperature and chemical
composition should be carefully evaluated in the selection of
materials of construction.
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