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5. Surface protection
5.1 Corrosion
About 4% of the gross national product of a western
industrial nation is destroyed by corrosion.
5.2 Corrosion types
About 25% of this could be avoided by applying existing
knowledge.
Corrosion is the reaction of a metallic material with its
environment that causes a measurable change to the
material and may lead to an impairment of the function
of a component or of a complete system. This reaction is
usually of an electrochemical nature, but in some cases it
may also be of a chemical or metal-physical nature.
Surface corrosion e.g. rust
We can also observe corrosion in our daily lives:
• Rust on vehicles, railings and fences
• Creeping destruction of road structures, bridges, buildings
• Leaks in water pipelines and heating pipes made of
steel
Crevice corrosion
Corrosion is unavoidable – but the damage caused
by corrosion can be avoided through the correct planning
of suitable corrosion protection measures.
Electrolyte
The corrosion system of a screw assembly should, under
operating conditions, be at least as corrosion-resistant as
the parts that are to be connected.
The design engineer’s job is to decide on the necessary
corrosion protection measures. Here, the wear reserve of
a corrosion protection system and the ambient conditions
have to be taken into account.

+
Contact corrosion
The ways in which corrosion manifests itself can vary
greatly. (See DIN 50900 for corrosion types).
1777
10
Corrosion rates, reference values in μm per year
Medium
Zincnon-chromated
Brass
Ms 63
Copper
CuNi 1.5 Si
Unalloyed steel
unprotected
Country air
1–3
≤4
≤2
≤ 80
Urban air
≤6
≤4
≤2
≤ 270
Industrial air
6–20
≤8
≤4
≤ 170
Sea air
2–15
≤6
≤3
≤ 170
Tab. 1
5.3 Frequently used types of coatings for fasteners
5.3.1 Non-metallic coatings
Designation
Procedure
Application
Corrosion resistance
Rubbing with oil
Workpieces are immersed in oil
Bright steel parts
Suitable for short-term corrosion
protection e.g. during transport
Undefined
Burnishing
Workpieces are immersed in acid Parts of weapons
Salt spray test approx. 0.5 h
or alkaline solutions.
Gauges and measuring technology Corrosion protection oil can
Oxide layers with a (brown) black
increase resistance
colour are created through reaction
No layer development
Purpose: formation of a weak
protective layer on the surface
No hydrogen embrittlement
Phosphatising
Steel component in metal
­phosphate bath or chamber with
metal phosphate solution
5–15 µm layer connected with the
material
Iron/manganese/nickel/zinc
phosphate