Steel Surfaces Coatings Systems

Good surface preparation is the most decisive factor for coating successfulness. Another decisive factor is the selection of a suitable finishing coat adapted to the end use requirements. Metals exposed to severe weather conditions will be treated differently from these exposed to mechanical stresses or to chemicals.

Metals are classified under two broad headings, ferrous (iron and steel) and non-ferrous metals (galvanized steel, aluminum, etc...).

STEEL:

Iron and steel may rust if they come in contact with moisture and oxygen. The presence of certain contaminants such as chloride and sulfate ions as well as the gaseous oxides of sulfur and nitrogen, which are present in marine and industrial atmospheres, accelerates the corrosion rate of steel.

The durability of paint applied to steel is affected by the surface condition of steel immediately prior to painting. Three principal factors are to be considered when coating steel structures:

1. The presence of rust and / or mill scale;
2. The presence of surface contaminants including salts, dust, oils, greases, and loose paints;
3. The surface profile (too smooth “ glossy surface”).

Different standards have been prepared to provide methods of assessing these factors, the Old Swedish standard SIS 05 59 00 – 1967, the German standard DIN 55928), the British standard BS7079, and the International standard ISO 5801-1: 1988.

The Old Swedish standard SIS 05 59 00 – 1967 and the ISO 5801 are used on worldwide scale for visual assessment of rust grades “rust and mill scale levels that are commonly found on surfaces of uncoated steel”. They are used also for visual assessment of preparation grades “the visual cleanliness after surface preparation of uncoated steel and of steel surfaces after overall removal of any previous coating”. ISO 5801 includes 28 representative photographic examples, 24 of the photographs originate from the Old Swedish standard SIS 05 59 00 – 1967 and the other 4 photographs originate from the German standard DIN 55928.

The ISO 8501-1 identifies four levels of rust grades and mill scale that are commonly found on uncoated steel surfaces and on steel surfaces after overall removal of any previous coating. The levels of visual cleanliness are related to the methods of surface cleaning that are used prior to coating.

1-Mechanical methods

Different preparation grades are provided by the international standard ISO8501-1 indicating the method of surface preparation and the degree of cleaning. Two mechanical methods are mostly used:

Prior to blast-cleaning, steel surfaces must be cleaned from oil, grease, and dirt. Heavy layers of rust should be removed by chipping.

Sa 1 Light blast-cleaning:

Sa 2 Thorough blast-cleaning

Sa 2½ Very thorough blast-cleaning (to near white metal)

Sa 3 Blast-cleaning to visually clean steel (to white metal)

B-Hand and power tool cleaning, St (Scrapping, wire-brushing, machine-brushing and grinding)

Prior to hand and power tool cleaning, steel surfaces must be cleaned from oil, grease, and dirt. Heavy layers of rust should be removed by chipping.

St 2 Thorough hand and power tool cleaning

The surface should be free from visible oil, grease and dirt, and from most of the mill scale, rust, paint coatings and foreign matter.

St 3 Very thorough hand and power tool cleaning

As for St2, but the surface should be treated thoroughly to give a metallic sheen arising from the metallic substrate.

1- In general and depending on the recommended end use, cleaning of large or flat areas to Sa2½ or Sa2 or even St2 could be sufficient. While cleaning of corners and small areas require higher degree of cleaning such as to Sa3 or St3.

2- Steel surfaces coated by the same material will last three times if blast cleaning (to Sa3) is used instead of wire brushing (to St3).

3- After blasting or hand and power tool cleaning, the surface shall be cleaned from loose dust and debris.

2-Chemical  methods:

Table 1: Ferrous and Steel substrate treatment

Cleaned and blasted steel surfacesrequire priming and coating in order to protect the steel surface from corrosion. Painting operations or at least the application of a protective primer have to be performed within few hours of blasting to ensure that surface rusting does not recommence.

Primer thickness depends on the profile of the roughened steel. With coarser profiles, it is important to ensure an adequate paint coating thickness in order to ensure that peaks of the roughened surface are coated.

As steel surfaces are recommended to be coated for different applications such as home and office equipment, structural members for buildings, pipes, tank exteriors, storage tanks and reservoirs, marine equipment, oil refineries, …etc. The selection of a suitable paint system for each of this diversity of end uses, each with their own specific performance requirements, is a must.

ALUMINIUM And GALVANISED STEEL:

Aluminum and galvanized steel are difficult to paint metals. They must be degreased with a special cleaner such as Gel Cleaner then rinse well with water. Prime dry substrate with a tie coat such as wash primer (WP) in order to improve the adhesion between the metal and the paint system.

Different kinds of anti-corrosive primers are provided by Azmeh Co. and listed in Table2. They differ in their anti-corrosive strength, adhesion and chemical nature (Figure 1).

Figure 1

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FINISHING COAT

Finishes for steel are based on materials selected to withstand the conditions likely to be experienced in the proposed service environment. Steel surfaces exposed to severe weather conditions or to mechanical wears or to chemicals require different coating systems.

Pipe and tank lining systems are listed in Table3:

Table 4 shows the recommended coating systems for interior and exterior exposure. Exterior exposure can vary from mild to severe and extremely severe or hard weathering.