METAL SURFACE PREPARATION

16.4 METAL SURFACE PREPARATION

Many test results and a great deal of engineering data over many years have shown that the most important single factor infl uencing the life of a paint is the proper preparation of the metal surface. This factor is, generally, more important than the quality of the paint that is applied. In other words, a poor paint system on a properly prepared metal surface usually outperforms a better paint system on a poorly prepared surface. A well - prepared surface is the foundation on which the paint system is built.

Adequate surface preparation consists of two main processes.

16.4.1 Cleaning All Dirt, Oils, and Greases from the Surface

Initial cleaning can be accomplished by using solvents or alkaline solutions. Solvents.

Mineral spirits, naphtha, alcohols, ethers, chlorinated solvents, and so on, are applied by dipping, brushing, or spraying. One such solvent is Stoddard solvent, a petroleum base mineral spirit with a fl ash point of 40 – 55 ° C (100 – 130 ° F), suffi ciently high to minimize fi re hazard, and it is not particularly toxic.

Chlorinated solvents, on the other hand, although nonfl ammable, are relatively toxic and contribute to pollution. In addition, they may leave chloride residues on the metal surface that can later initiate corrosive attack. They are used largely for vapor degreasing (tri - or perchlorethylene), in which the work is suspended in the vapor of the boiling solvent. Care must be exercised in the vapor degreasing of aluminum, ensuring that adequate chemical inhibitors are added and maintained in the chlorinated solvent in order to avoid cata- strophic corrosion (see Section 21.1.4.1 ) or, in the extreme, to avoid an explosive reaction.

Alkaline Solutions. Aqueous solutions of certain alkalies provide a method of removing oily surface contamination that is cheaper and less hazardous than the use of solvents. They are more effi cient in this particular function than sol- vents, but perhaps less effective for removing heavy or carbonized oils. Suitable

solutions contain one or more of the following substances: Na 3 PO 4 , NaOH, Na 2 O · n SiO 2 , Na 2 CO 3 , borax, and sometimes sodium pyro - or metaphosphate and

a wetting agent. Cleaning may be done by immersing the work in the hot solution [80 ° C (180 ° F) to b.p.] containing about 30 – 75 g/liter (4 – 10 oz/gal) alkali. The hot solution, in somewhat more dilute concentration, may also be sprayed onto the surface. Electrolytic cleaning in alkaline solutions is also sometimes used; this process makes use of the mechanical action of hydrogen gas and the detergent effect of released OH − at the surface of the work connected as cathode to a source of electric current.

If the metal is free of mill scale and rust, a fi nal rinse in water and in a dilute chromic – phosphoric acid ensures both removal of alkali from the metal surface,

ORGANIC COATINGS

which would otherwise interfere with good bonding of paint, and also temporary protection against rusting.

16.4.2 Complete Removal of Rust and Mill Scale

Rust and mill scale are best removed by either pickling or sandblasting. Pickling.

The metal, cleaned as described previously, is dipped into an acid (e.g., 3 – 10% H 2 SO 4 by weight) containing a pickling inhibitor (see Section 17.3 ) at a temperature of 65 – 90 ° C (150 – 190 ° F) for an average of 5 – 20 min. Oxide next to the metal surface is dissolved, loosening the upper Fe 3 O 4 scale. Sometimes, sodium chloride is added to the sulfuric acid, or HCl alone is used at lower tem- peratures, or a 10 – 20% H 3 PO 4 is used at temperatures up to 90 ° C (190 ° F). The latter acid is more costly, but has the advantage of producing a phosphate fi lm on the steel surface that is benefi cial to paint adherence. Some pickling proce-

dures, in fact, call for a fi nal rinse in dilute H 3 PO 4 in order to ensure removal from the metal surface of residual chlorides and sulfates that are damaging to the life of a paint coating.

Sometimes, the fi nal dip is a dilute solution of chromic (30 – 45 g/liter; 4 – 6 oz/ gal) or chromic – phosphoric acid, which serves to prevent rusting of the surface before the prime coat is applied. Use of chromates is less popular than formerly because their toxicity places restrictions on disposal of spent solutions. Hexava- lent chromium (Cr 6+ ) is a potent human toxin and known cancer - causing agent

[7] . A relatively nontoxic pickle for steel consists of hot 3 – 10% ammoniated citric acid followed by a dilute alkaline sodium nitrite solution to minimize superfi cial rusting before application of paint [8] .

Blasting. Using this procedure, scale is removed by high - velocity particles impelled by an air blast or by a high - velocity wheel. Blast materials usually consist of sand, or sometimes of steel grit, silicon carbide, alumina, refractory slag, or rock wool byproducts.

Other methods of removing mill scale include fl ame cleaning , by which scale spalls off the surface through sudden heating of the surface with an oxyacetylene torch. Weathering for several weeks or months is also possible; the natural rusting of the surface dislodges scale, which can then be further removed by wire brush- ing. But these procedures are less satisfactory than complete removal of scale and rust by pickling or blasting.

The poor performance of paints on weathered steel exposed to an industrial atmosphere was shown in tests reported by Hudson [9] (Table 16.1 ). The rela- tively long life of paint shown for intact mill scale would probably not be achieved in actual service. It would, for example, be diffi cult to keep large areas and various shapes of mill scale from cracking before or after painting. Fragmentation of mill scale allows paint to become dislodged, particularly after electrolytic action has occurred between metal and scale by aqueous solutions that have penetrated to the metal surface.

APPLYING PAINT COATINGS

T A B L E 16.1. Effect of Surface Preparation of Steel on Life of Paint Coatings [9] Surface Preparation

Durability of Paint (years) in Sheffi eld, England

2 Coats Red Lead + 2 Coats

2 Coats Red Iron

Oxide Paint Intact mill scale

Red Iron Oxide Paint

8.2 3.0 Weathered and wire - brushed