CORROSION GUIDE 181108_new table content format ... - Reichhold

Common Types of Metal Corrosion
Hydrogen Embrittlement
Atomic hydrogen can diffuse or become adsorbed into
steel. It then reacts with carbon to form methane or
microscopic gas formations which weaken and detract
from ductility. Usually this happens at high temperature
under conditions where FRP is ordinarily not considered.
The same type of mechanism of attack is associated
at lower temperatures with various forms of galvanic
or stress induced corrosion. Quite often hydrogen
embrittlement can be a problem for steel which has
been electroplated or pickled, especially when done
improperly or ineffi ciently. Some of these matters are
receiving more attention due to future considerations of
hydrogen in fuel cell and other energy applications.
Sulfate Reducing Bacteria and Microbially Induced
Corrosion (MIC)
Colonies of microorganisms, especially aerobic and
anaerobic bacteria contribute greatly to corrosion of
steel through a wide variety of galvanic and depositional
mechanisms. Usually the corrosion is manifested in the
form of pitting or sulfi de induced stress cracking. Perhaps
the most signifi cant type of such corrosion involves
sulfate-reducing bacteria (SRB), which metabolize
sulfates to produce sulfuric acid or hydrogen sulfi de.
Such bacteria are prolifi c in water (including seawater),
mud, soil, sludge, and other organic matter.
These bacteria are a major reason why underground
steel storage tanks are corroded, and this has
lead to widespread use of FRP as an alternative or
as an external protective barrier to steel. Various
manifestations of MIC are seen far-and wide, including
industrial environments which inadvertently serve as
warm or nutrient-rich cultures for biological growth.
FRP is unaffected by many of the mechanisms
associated with MIC.
Apart from sulfate reducing bacteria, other forms of
microbial corrosion which affect metals include acid
producing bacteria, slime forming organisms, denitrifying
bacteria which generate ammonia, and other corrosion
associated with various species of algae and fungi.
It is expected that biologically induced corrosion will
receive increased attention as more applications and
technologies evolve in the fi eld of energy production
associated with biomass and renewable resources.
Processing will include such things as aerobic and
anaerobic digestion, fermentation, enzymatic hydrolysis
and conversion of cellulose, lignin, or polysaccharides
to sugars, which in turn may be converted to ethanol.
Carbon and stainless steels are not the only metals
affected by MIC. Also routinely corroded are copper and
various alloys as well as concrete. The most common
example of which involves sewage and waste treatment
applications in the presence of the thiobacillus bacteria,
which oxidizes H 2 S to sulfuric acid. FRP has a long
history of successful use in these environments.
48