Ingeniro

TECH-B
Section TECH-B
Pump Application Data
TECH-B-1 Corrosion & Materials of Construction
Selecting the right pump type and sizing it correctly are critical to the
success of any pump application. Equally important is the selection
of materials of construction. Choices must be made between metals
and/or non-metals for pump components that come into contact with
the pumpage. Inaddition, gaskets and O-ring material selections
must be made to assure long leak-free operation of the pump's
dynamic and static sealing joints. To assist in proper selection,
included in this section is a brief discussion of specific types of
corrosion and a general material selection guide.
Corrosion
Corrosion is the destructive attack of a metal by chemical or electrachemical
reaction withits environment. It is important to understand
the various types of corrosion and factors affecting corrosion rate to
properly select materials.
TYPES OF CORROSION
(1) Galvanic corrosion is the electro-chemical action produced
when one metal is in electrical contact with another more noble
metal, with both being immersed in the same corroding medium
called the electrolyte. A galvanic cell isformed and current flows
between the two materials. The least noble material called the anode
will corrode while the more noble cathode will be protected. It is
important that the smaller wearing parts in a pump be of a more
noble material than the larger more massive parts, as in an iron
pump with bronze or stainless steel trim.
Following is a galvanic series listing the more common metalsand
alloys.
(2) Uniform Corrosion is the overall attack on a metal by a corroding
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 grainboundaries 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 pitformation 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 concentration
difference in these cells. The resulting corrosion may
progress rapidly leaving the surrounding area unaffected.
(6) Stress Corrosion isthe failure of a material due to a combination
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 aliquid 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 hydrogenion concentration
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 Iron, Stainless Steel,
4 to 6 Bronze, Carbon...