Removal of rust, mill scale and contaminants
To
ensure good results, it is essential for rust, mill scale and contaminants such
as grease to be removed from the steel before application of the protective
coatings.
5.1 Rust
The microporous structure of rust allows it to attract and store a great
many contaminants from the air, which further accelerate the process of
corrosion. The relative softness of the surface rust layer would lead one to
think that it can easily be removed. However, its adhesion to the steel surface
is such that the last remainders of rust in particular can be removed
effectively only via the appropriate work methods. A complete removal of rust
is needed to ensure that, after further follow-up treatments, no so-called
subsurface rust develops, as this would destroy the protective coating applied
later.
5.2 Mill scale
Mill scale is a kind of skin that develops on the surface of steel as a
result of the high temperature used when steel is rolled out. It is grey-blue
to black in colour. This skin is generally composed of layers of various iron
oxides and is comparable to the iron oxide flakes that break off in the smithy
(hammer scales) or the layer of oxides formed when steel undergoes heat
treatment or welding. Mill scale and hammer scales are significantly thicker
than the oxide layers formed when steel undergoes heat treatment or welding,
but all these layers have a very hard and compact structure. If these oxide
layers were continuous and unbroken, they would provide the metal with an
excellent form of protection. However, due to the formation of shrinkage
cracks, the oxide skin does not completely enclose the metal. In addition, the
oxides are nobler than steel, which means that they can also cause the steel to
corrode. As is the case with rust, all scale must therefore be completely
removed, as no protective coating would be resistant to particles left behind
of rust, hammer scale, mill scale, heat scale or welding scale. The performance
of the protective coatings applied at a later stage therefore depends largely
on the effectiveness with which the steel surface is initially cleaned.
The removal of rust and mill scale can be carried out by:
pickling
flame blasting
mechanical cleaning
wet blasting
high-pressure water jetting
special blast cleaning methods
5.3 Pickling
Pickling is a chemical method for removing the layer of mill scale. When
sulfuric or phosphoric acid is used, it takes place in heated baths. When
hydrochloric acid is used, it is done in unheated baths. Large acid-resistant
pickling tubs are therefore needed for the process. A complete treatment always
consists of three immersions: first in one of the above-mentioned acid baths,
followed by a warm water bath to rinse any remaining acid from the steel
surface, and finally a bath in a dilute solution of warm phosphoric acid to
create a thin layer of iron phosphate on the surface of the steel. This
phosphate layer slows down rust formation and provides a better surface for
adhesion during further treatments. In order to prevent acid from accumulating
in specific parts of an object, these parts are dealt with accordingly before
the object is assembled. Pickling in hydrochloric acid is used primarily for
galvanized steel.
5.4 Flame blasting
Flame blasting has various disadvantages, which is why this method is no longer very common. By subjecting the steel to a hot flame fed by a mixture of oxygen and acetylene or oxygen and propane, the layer of mill scale suddenly expands and breaks while at the same time the moisture present is quickly converted into steam, thereby removing the rust. However, this never removes all the rust, which is why this method is only suitable as a form of preliminary cleaning. But even in that case, flame blasting still suffers from the disadvantage that the steel may become distorted as a result of intense heat applied locally.
