International Catalogue

Corrosion is a chemical reaction in which metal is decomposed. The less noble the
metal (“electrochemical potential“), the more intense is the material damage. In
this process it is either converted into flaking rust or worn away in places. Different
appearance patterns are differentiated here. The most frequent types of corrosion in
fixings and anchors include:
Surface corrosion: In this case, the metal corrodes relatively uniformly over the
entire surface or over a part of the surface. An example of this is the invisible rusting
due to condensation of a screw in the transition area from anchor plate to hole. The
result: The connection that appears completely intact from the outside fails abruptly.
Contact corrosion: If metals with a different nobility contact each other in a
conductive medium, the less noble metal always corrodes (the anode). As a
consequence, stainless steel is usually not endangered. What is decisive is the surface
ratios of the two types of metal: the greater the surface area of the most noble metal
in comparison to the less noble, the greater the corrosion becomes. For example, if
large stainless steel sheets are screwed with galvanised screws, the screws will be
highly attacked within a very short time. In contrast, using stainless steel screws for
galvanised sheets is not critical.
Stress corrosion cracking: If lasting internal or external tensile stresses occur, there
can be strain and corrosion of the metal. In this process, a crack develops due to
mechanical stresses, which grows under increasing loads and thus prepares a path
for progressive corrosion. For example, it occurs with A4 steel in an atmosphere
containing chlorine (indoor swimming pools, etc.). Generally stress corrosion cracking
is not visible with fixings and usually leads to sudden failure of the anchoring.
Corrosion - Fundamentals
In 1985, the suspended concrete ceiling of
an indoor swimming pool collapsed in Uster,
Switzerland. The ceiling attachments of
stainless steel exhibited no external defects
whatsoever, but inside were completely
destroyed in some cases due to stress
corrosion cracking.
Example of trans-crystalline stress corrosion
cracking on stainless steel 1.4401 with high
chloride concentration.
Basic Knowledge of Fastening Technology
Corrosion protection
There are different methods for protecting fastenings from corrosion.
The most important are:
The galvanised zinc coating (or even electrolytic zinc coating) with subsequently
started passivation is to reach a corrosion protection with the usually applied process
in the metal refinement. Layer thicknesses between 3 µm and 10 µm can be achieved.
Since the galvanising is worn off over time, it offers adequate corrosion protection only in
dry interior rooms.
Hot-dip galvanising is the application of a metal zinc coating by dipping it in molten
zinc (at approx. 450 °C). Zinc layer thicknesses of 45-80 µm offer an excellent corrosion
protection for moisture rooms and outside applications.
Stainless steel fixings of the corrosion resistance class III e.g. A4 material no.
1.4401, 1.4362 are suitable for fastenings in damp rooms, in open air, in industrial
atmospheres or near the sea (but not directly in sea water). These steels are alloys with a
chrome content of at least 12% that forms a passive layer on the steel surface that protects
against corrosion.
Stainless steel fixings of highly corrosion resistant steel of the corrosion resistance
class IV e. g. material no.1.4529 are used in especially aggressive environments like
atmospheres containing chlorine (indoor swimming pools), in road tunnels or with direct
sea water contact. In this case, the chrome content of normal stainless steels drops
below 12%. The protective passive layer disappears and the anchor becomes susceptible
to corrosion. On the other hand, the special alloys are very corrosion resistant in these
highly aggressive media, due to their relatively high percentage of molybdenum. With
an alloy percentage of 50%, they clearly surpass the usual unalloyed, low alloyed or high
alloyed steels with maximum 30% alloy percentages. This means the steel 1.4529 alloyed
with chrome, molybdenum and nickel has an alloy percentage of 58%. The rest consists
of iron and carbon. Because of this high percentage of expensive alloy additives, the
manufacturing of these steel types are correspondingly costly.
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