Corrosion handbook

20 06/21
3.1 Corrosion and corrosion protection of carbon steel
The unalloyed steel (i.e. mild steel or carbon steel) from which the majority of
our fastening and installation products are manufactured requires corrosion
protection. In most environments, the corrosion rate of carbon steel (typically
around 20µm/a in a rural outdoor atmosphere and rising to more than 100 µm/a
in coastal environments) is usually too high for a satisfactory outdoor application.
The product design does not generally account for a material loss. Hilti therefore
offers a wide range of suitable, cost-efficient corrosion protection solutions for
carbon steel products.
In alkaline surroundings like when covered by concrete, however, iron and steel
usually remain stable. This explains why, for example, reinforcing bars made of
carbon steel are already very well protected against corrosion in the alkaline
environment of the surrounding concrete.
Phosphating
Process description
Steel is dipped into an acidic solution containing metal (Zn, Fe)
phosphate salts. The solution reacts with the steel surface forming
a micro-crystalline layer of phosphates on the surface (see
Fig.28). This results in a rough surface with excellent oil-retaining
properties.
Corrosion behavior and further
information
Oil applied for corrosion protection is intended to stay long
enough on the surface in order to provide protection during
normal transport and deliver slightly increased general corrosion
protection. Such products can be used only in dry indoor
environments. Hilti uses phosphatizing on drywall screws.
Zinc and Zn alloy coatings
Zinc is an excellent choice for the corrosion protection of carbon steel. Several
suitable processes are available for the application of zinc coatings on steel parts
ranging from small screws to channels of several meters in length.
The corrosion rate of zinc is more than ten times lower than that of steel, ranging
at around 0.5 to 1 µm/a in rural/urban atmospheres and rising to up to around
5 µm/a in coastal environments. Zinc alloy coatings such as electroplated ZnNi
or continuously hot dip galvanized ZM have a significantly reduced corrosion
rate, which in most applications is only about half that of zinc. Fig. 29 provides
an overview of the typical service life of zinc-coated steel and ZM coated steel
under various conditions. The low corrosion rates are the result of the formation of
stable layers of corrosion products containing carbonates (from CO
2 in the air) and
chlorides (if they are present in the atmosphere). Conditions where the formation of
such insoluble corrosion products is not possible will lead to much higher corrosion
rates and hence will limit the suitability of zinc as a protective coating. These
include permanently wet conditions or exposure to high concentrations of industrial
pollutants such as sulfur dioxide. In these environments, soluble corrosion products
are formed preferentially and they can be washed off by rainfall.
In addition to decreased corrosion rates, zinc also provides cathodic or sacrificial
protection to the underlying steel. Where scratching, chipping or any other
damage to the zinc coating exposes the steel, a special form of galvanic corrosion
takes place (see section 1.2.6). Zinc, being a less noble metal than steel, corrodes
preferentially, thereby helping to keep the exposed steel surface protected.
Zinc coatings are consumed quite homogenously during atmospheric corrosion.
Accordingly, in a given application, doubling of the coating thickness usually also
doubles the time until the zinc is consumed and red rust on the steel substrate
occurs. Zinc is not stable in alkaline environments and is readily attacked in
solutions with a pH-value of 10 or higher. Likewise, on the jobsite, the spilling
of aggressive construction materials on zinc products, e.g. cement or fresh
concrete, should be avoided.
Fig. 28: Phosphate crystals on a steel
surface at 3000X magnification in the
scanning electron microscope.
5 μm