Blast cleaning
Blast
cleaning methods can be divided into three different categories:
airblasting
turbine blasting
wet blasting
6.1 Airblasting
Airblasting is a method whereby the abrasive blasting material is
propelled onto the steel surface with the help of compressed air. The abrasive originally
used for this purpose, sand, was later replaced by other materials, but the
process is still often referred to as sandblasting. The abrasives now used can be
divided into abrasives for one-time use and recyclable abrasives. When blasting
large structures in the open air, the abrasive is used only once. Corundum is
the best abrasive and can be used several times. Due to its higher price, it is
not often used for outdoor blasting work, as it can then not be recovered.
Chilled iron shot or grit and steel shot or grit are also used, especially for
turbine blasting. Although mobile blasting equipment is available, for example
for repair work, the construction industry mainly uses fixed installations
which are installed on site at metal treatment firms.
6.2 Turbine blasting
In turbine blasting, the abrasive material is propelled onto the steel
surface with the help of turbines. This type of equipment can be integrated
into entire processing lines, in which large numbers of objects are coated with
a primer immediately after the blasting process in order to prevent new rust
formation. As the effect of turbine blasting depends upon the weight of the
propelled material, coarser abrasive grades of steel grit or shot, whereas for
manually operated blasting cabins chilled iron grit is also used.
6.3 Wet blasting
Wet blasting, also referred to as liquid honing, is not commonly used in
the construction industry. Special equipment is available for wet blasting.
This method can be used for mechanically cleaning new as well as old steel
surfaces and for roughing up old intact coatings in order to prepare them for
the application of new coating layers. In wet blasting, water is sprayed onto
the steel surface under high pressure, with or without the addition of an
abrasive. As the steel surface will very quickly begin to rust again after the
treatment, additional treatment is quickly necessary. Wet blasting is also used
when the formation of fine particles must be avoided or when the formation of
sparks must be avoided due to the risk of fire and/or explosion.
6.4 High-pressure water
blasting
High-pressure wet blasting differs from other blasting methods, as it
uses the energy of the water which is jetted on to the steel surface under high
pressure (500 - 2000 bar). High pressure water blasting, with or without the
addition of an abrasive, makes it possible to completely clean a steel surface.
When treating existing objects, this method also makes it possible to
remove only the damaged top layer and leave the base layer intact.
High-pressure water blasting has the additional advantage that it also removes
any (invisible) residues of chlorides and sulfates from the metal surface.
This blasting method, also referred to in the trade as "Hydro
Jetting,” is gaining in popularity.
6.5 Special blast cleaning
methods
There are quite a number of special blast cleaning methods, many of which have found only limited application until now. These special blasting methods can be divided into the following categories:
blasting in a wet environment to prevent fine particle formation
zinc blasting with zinc-coated abrasive in order to simultaneously apply
a protective coating
blasting and phosphatising in a single process step
blasting with a flame in order to achieve simultaneous drying
high-pressure water blasting without using an abrasive agent
blasting and coating simultaneously
ice blasting
6.6 Blasting standards
In view of the large diversity of blasting agents and methods, it is
only logical that various requirements have been formulated which must be
complied with. These requirements also depend to a large extent on the treatment
applied after the blasting process, although the latter generally consists of
the application of a finish coating of paint. Various standards exist in this
regard which also differ to some extent. In the
6.7 ISO-8501-1:1988
ISO-8501-1:1988 standard for the rust grading of steel surfaces and
standards for the preliminary treatment of these surfaces before they are
treated with (corrosion-control) paint. This standard was formulated by the Swedish Corrosion Institute in
cooperation with the American Society for Testing and Materials
(ASTM), and the Steel Structures Painting Council
(SSPC), and is used in specifications regarding the pre-treatment of surfaces
before they are painted. The specifications in this standard apply to:
the surfaces of hot-rolled steel in four different rust grades (A, B, C
en D)
the same surfaces which have been pre-treated in accordance with two
different surface quality standards: hand tool cleaning (St 2) and power tool cleaning (St 3) with
power wire brushes, power impact tools, power sanders etc.
the same surfaces which have been pre-treated in accordance with four surface quality standards for various types of blast cleaning (Sa 1, Sa 2, Sa 2½ and Sa 3).
