MEFE—Mitchell Engineering Food Equipment Pty Ltd
23 Storie Street Clontarf QLD 4019 Australia
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5.4
Corrosion
Although, the passive layer is formed on stainless steel surfaces,
there are cases when it corrodes due to the following condi-
tions:
-
Influence of hydrochloric acid and chlorine,
-
No ongoing maintenance which leads to the formation of strong acid solutions on steel walls (water evaporates and an acid
solution remains),
-
An environment more aggressive than the steel provided for it;
-
Contamination during installation and manufacture (lime, cement, foreign metallic inclusions as a result of using angle grind-
ers nearby or unsuitable assembly tools),
-
Contact with normal carbon steel (scratching with black steel during transportation or storage),
-
Insufficient room ventilation or even their absence in aggressive environments (there must always be an air flow in ventila-
tion ducts).
Symptoms of stainless steel corrosion are different. One can notice that corrosion appears usually on various types of internal
(non
-
metallic inclusions, separations, deformations) and external (edges, scratches, dents, residues of scale, sediments, etc.)
surfaces, while smooth and homogenous surfaces are much more resistant to corrosion. Therefore, it is necessary to ensure
proper pickling and passivation of a surface.
The mechanism of destruction and the type of corrosion depend on a specific environment and steel affected by it. The fol-
lowing types of corrosion may occur depending on the environment and stainless steel:
-
Surface (uniform), Pitting, Intergranular,
Stress, Crevice.
Pitting corrosion is the most frequently occurring corrosion caused by inappropriate stainless steel maintenance, more specifi-
cally due to the use of chlorinated cleaning agents.
Pitting corrosion is a form of a localised environmental attack leading to local losses (pits) in material. It is caused by the influ-
ence of galvanic cells formed between a passivated steel surface and clearly localised non
-
passivation areas laying on it. Oxy-
gen or oxidising substances in a cathodic cell areas (passivated) are required for pitting corrosion to occur. If they are absent,
cathodic areas polarize and the cell stops functioning.
Solutions which cause pitting corrosion of stainless steels most frequently are chlorine solutions. For this type of corrosion,
condition of a steel surface is very important. The smoother and cleaner the surface, the smaller the intensity of said corro-
sion.
A quality assessment of the effects of this type corrosion is difficult, since damage can be very serious at a minor weight loss. It
is assumed that an average number of pits per area unit and their greatest depth can serve as some indicators.
5.5
Maintenance and Cleaning
When using stainless steel furniture and equipment, a layer of chromium oxide present on the steel surface must be taken
care of. Compounds used in food industry which disturb the passive layer (chromium oxide) include compounds containing
chlorides –
salt, disinfectants and acids (water from sauerkraut, cucumbers, acid juices, vinegar, etc.). Water is the most effec-
tive neutraliser of chlorides and weak acids.
Stainless steel is not an exception to this rule and each user must be aware that regular cleaning and maintenance of stainless
products is required.
