222 Fundamentals of Corrosion
means to achieve this end. It is important, therefore, that the painter or the
person doing the surface preparation be knowledgeable. It is important that
the various pieces of equipment be sized properly; that air and abrasives (if
used) be cleaned, graded, and free of moisture, oil, and other contaminants;
and that ambient conditions be controlled, or at least closely monitored.
Surface preparation techniques have changed drastically. Silica sand has
been banned in all Western countries except the United States and Canada
(although there is a strong movement to ban it in these countries as well) as
a blast-cleaning abrasive. To prevent environmental damage caused by the
leaching into water supplies of lead, chromate, and other toxic paint pigments
removed during the course of blast cleaning, many localities require the
safe containment and disposal of spent blast-cleaning abrasives. Although
a paint layer over a properly cleaned surface still acts as a barrier against a
corrosive environment, in many cases the components that form the barrier
have changed considerably.
In some environments, certain surface preparation and coating applica-
tion techniques are not permissible. For example, many companies do not
permit open blast cleaning where there is a prevalence of electric motors or
hydraulic equipment. Reneries, as a general rule, do not permit open blast
cleaning — or for that matter, any method of surface preparation that might
result in the possibility of a spark, static electricity buildup, or an explosion
hazard. During the course of construction or erection, many areas requiring
protection are enclosed or covered, or so positioned that access is difcult or
impossible. Consideration must be given to painting these structures prior
to installation.
Some methods of coating must be done at a specialized facility because the
equipment used is not readily transportable to eld sites. Typical methods
include most chemical cleaning, including pickling and acid etching; auto-
matic rotary wheel blasting; and automatic spraying, electrostatic, or high-
speed roller coating application.
7.6.1.1 Abrasive Cleaning
Abrasive cleaning is undertaken after oily soils have been removed. Rust and
corrosion are removed by media blast, hand or power sanding, and hand or
power blasting. Media blasting is accomplished by propelling, under pres-
sure, materials such as sand, metallic shot, nut shells, plastic pellets, or dry
ice crystals so that they impinge on the surfaces to be cleaned. High-pressure
water jet cleaning is similar to media blasting.
7.6.1.2 Detergent Cleaning
Aqueous solutions of detergents are used to remove oily soils. They are applied to
metals by immersion or spray. After cleaning, the surfaces are rinsed with clean
water to remove the detergent. Detergents will not remove rust and corrosion.