Corrosion Control Through Organic Coatings Part 4 potx

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4 Blast Cleaning and Other

Heavy Surface

Pretreatments

In broad terms, pretreatment of a metal surface is done for two reasons: to remove

unwanted matter and to give the steel a rough surface profile before it is painted.

“Unwanted matter” is anything on the surface to be painted except the metal itself

and — in the case of repainting — tightly adhering old paint.

For new constructions, matter to be removed is mill scale and contaminants.

The most common contaminants are transport oils and salts. Transport oils are

beneficial (until you want to paint); salts are sent by an unkind Providence to plague

us. Transport oil might be applied at the steel mill, for example, to provide a

temporary protection to the I-beams for a bridge while they are being hauled on a

flatbed truck from the mill to the construction site or the subassembly site. This oil￾covered I-beam, unfortunately, acts as a magnet for dust, dirt, diesel soot, and road

salts; anything that can be found on a highway will show up on that I-beam when

it is time to paint. Even apart from the additional contaminants the oil picks up, the

oil itself is a problem for the painter. It prevents the paint from adhering to the steel,

in much the same way that oil or butter in a frying pan prevents food from sticking.

Pretreatment of new steel before painting is fairly straightforward; washing with an

alkali surfactant, rinsing with clean water, and then removing the mill scale with

abrasive blasting is the most common approach.

Most maintenance painting jobs do not involve painting new constructions but

rather repainting existing structures whose coatings have deteriorated. Surface prep￾aration involves removing all loose paint and rust, so that only tightly adhering rust

and paint are left. Mechanical pretreatments, such as needle-gun and wire brush,

can remove loosely bound rust and dirt but do not provide either the cleanliness or

the surface profile required for repainting the steel. Conventional dry abrasive blast￾ing is the most commonly used pretreatment; however, wet abrasive blasting and

hydrojet cleaning are excellent treatment methods that are also gaining industry

acceptance.

Before any pretreatment is performed, the surface should be washed with an

alkali surfactant and rinsed with clean water to remove oils and greases that may

have accumulated. Regardless of which pretreatment is used, testing for chlorides

(and indeed for all contaminants) is essential after pretreatment and before applica￾tion of the new paint.

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