An introduction to
surface preparation for
corrosion protection
Cost-effective corrosion protection of structural steelwork can
be achieved through the appropriate application of paint and
metallic coatings and should present little difficulty for common
applications and environments if the factors that affect durability
are recognised at the outset.
10 NSC
February 18
In this article NSC looks at surface
preparation which is the essential first stage
treatment of a steel substrate before the
application of any coating, and is generally
accepted as being the most important factor
affecting the total success of a corrosion
protection system.
Why surface preparation is essential to
ensure effective corrosion protection?
The performance of a coating is significantly
influenced by its ability to adhere properly
to the substrate material. The initial surface
condition of steel can vary in terms of the
amount of residual millscale and degree of
initial rusting. However, generally it is an
unsatisfactory base upon which to apply
modern, high performance protective coatings.
Preliminary treatments
Residues of oil, grease, marking inks, cutting
oils etc. after fabrication operations will
seriously affect the adhesion of applied
coatings and must be removed. It is erroneous
to think that subsequent cleaning operations
will remove such contaminants and it is
bad practice to permit them to remain
on the surface. Failure to remove these
contaminants before blast cleaning results in
them being distributed over the steel surface
and contaminating the abrasive. Suitable
organic solvents, emulsion degreasing agents
or equivalents should be applied to remove
contaminants in preparation for subsequent
descaling treatments.
Blast cleaning
A range of methods of preparation and
grades of cleanliness exist, but by far the
most significant and important method used
for the thorough cleaning of millscaled and
rusted surfaces is abrasive blast cleaning.
The standard grades of cleanliness for
abrasive blast cleaning in accordance with
ISO 8501-1 are:
• Sa 1 – Light blast cleaning
• Sa 2 – Thorough blast cleaning
• Sa 2½ – Very thorough blast cleaning
• Sa 3 – Blast cleaning to visually clean steel
The surface preparation process not only
cleans the steel, but also introduces a suitable
surface profile and amplitude to receive the
protective coating. High build paint coatings
and thermally sprayed metal coatings need
a coarse angular surface profile to provide
a mechanical key. This is achieved by using
grit abrasives. Shot abrasives are used for thin
film paint coatings such as pre-fabrication
primers.
A very wide range of abrasives is available.
These can be non-metallic (metal slags,
aluminium oxide, etc) and metallic (steel shot
or grit, etc)
The particle size of the abrasive is also
an important factor affecting the rate and
efficiency of cleaning. In general terms, fine
grades are efficient in cleaning relatively
new steelwork, whereas coarse grades may
be required for heavily corroded surfaces.
The removal of rust from pitted steel is more
easily effected by fine grades and, depending
upon the condition of the steel surface, a
mixture of grades may be required initially to
break down and remove millscale and clean
in pitted areas.
After abrasive blast cleaning, it is possible
to examine for surface imperfections and
surface alterations caused during fabrication
processes, e.g. welding. Certain surface
imperfections introduced during the original
processing of the steel may not be detrimental
to the performance of a coating in service,
particularly for structures in relatively low
risk environment categories. However,
depending upon the specific requirements
of the structure, it may be necessary to carry
out additional surface treatments to remove
surface imperfections on welds and cut
edges, as well as soluble salts to produce an
acceptable surface condition for painting.
Acid pickling
This process involves immersing the steel in
a bath of suitable inhibited acids that dissolve
or remove the millscale and rust but do not
appreciably attack the exposed steel surface.
The cleaning can be 100% effective. Acid
picking is normally only used for structural
steel intended for hot-dip galvanizing.
Andrew Harrison, Sales & Marketing
Director at Wedge Group Galvanizing Ltd,
explained: “Before the hot-dip galvanizing
process can take place, steel is put through a
chemical clean which removes all rust, oil and
millscale from its surface. Once submerged
in molten zinc, the steel is then coated inside
and out, meaning narrow gaps and corners
are easily protected.”
Sector Focus: Protective Coatings
Sponsors
Protective Coatings
Gold: Wedge Group Galvanizing
Silver: Jack Tighe Ltd
Bronze: Hempel; Jotun Paints;
Joseph Ash Galvanizing;
Sherwin-Williams
Galvanized steelwork
is used in numerous
industrial applications
such as supporting silos,
as pictured
/Surface_preparation#Initial_surface_condition
/Surface_preparation#Initial_surface_condition
/Surface_preparation#Abrasive_blast_cleaning
/Surface_preparation
/Surface_preparation#Surface_profile_and_amplitude
/Paint_coatings
/Fabrication
/Welding
/Surface_preparation#Additional_surface_treatments
/Surface_preparation#Acid_pickling
/Metallic_coatings#Hot-dip_galvanizing