Corrosion handbook
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Electrogalvanizing
Process description
Electrical current is passed through an aqueous solution containing
zinc ions leading to the deposition of zinc metal on the steel
substrate. Prior to this step, the parts usually undergo a cleaning
and pickling process and subsequent passivation after the
deposition of zinc. It is an excellent way to protect small threaded
parts due to the formation of homogeneous and dense coatings.
Coating thickness It can vary from 5 to 15 µm.
Corrosion behavior and further
information
Due to the limitations of the achievable coating thickness, Zn
electroplated parts without further corrosion protection should only
be used in dry indoor conditions. Using electroplating, it is also
possible to deposit Zn alloys such as ZnNi coatings.
Electrogalvanizing can lead to hydrogen uptake. Hilti’s high-
strength fasteners such as nails for direct fastening are therefore
baked after electroplating (kept at temperatures of around 180 °C
for 24 h) to remove hydrogen.
Hot-dip galvanizing
Process description
During this process, steel parts are dipped into a bath of molten
zinc. Large parts with a size of several meters can be coated using
this technique. Small parts like bolts and anchors are centrifuged
after hot-dip galvanizing in order to remove excess zinc from the
threads.
Coating thickness
The typical thickness is between 35 and 100 µm, depending on
the material thickness and the steel composition. The duration of
dipping is usually several minutes.
Corrosion behavior and further
information
The molten zinc reacts with the steel substrate forming a ZnFe alloy
layer with a thinner pure Zn layer on top (see Fig. 30).
Corrosion products of hot-dip galvanizing may look brownish due
to Fe in the zinc coating. This, however, is not necessarily a sign
of corrosion of the steel substrate (see Fig. 31). HDG is applicable
for some outdoor environments, depending on the thickness of the
zinc coating and the exposure conditions
Continuous hot-dip galvanizing / Sendzimir galvanizing
Process description
During this process, sheet metal from coils is drawn continuously
through a bath of molten zinc after the surface has been cleaned
and it has been subjected to a special annealing (heat treatment)
process. The zinc bath contains small amounts of Al. The Al reacts
with the steel surface to create an inhibition layer with a thickness
of a few nanometers, which inhibits the formation of ZnFe phases.
The coating consists mainly of pure zinc.
Coating thickness
It can vary from 10 to 70 µm on both sides and it is controlled by
removing excess zinc with a jet of air.
Corrosion behavior and further
information
Apart from pure Zn, also coatings consisting of Zn alloys such
as ZM can be produced by continuous dip galvanizing. Typical
coatings have around 2–4 % of Al and Mg and show increased
corrosion protection, which is around two times higher than that of
a Zn coating with the same coating weight.
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Fig. 29: Typical service life of Zn and ZM coated steel in various environments.
Fig. 30: Micrograph of hot-dip
galvanized steel.
Fig. 31: Brown staining on a hot-dip
galvanized channel after 200 h in the
salt spray test. No corrosion of the
steel yet.
Zn-phase
ZnFe-phase
steel