Technical Resources Area
Main Site  |  Technical Index   |  Data Sheets  |  Processing Guides  |  Enquiry Form  |  Service Conditions		Tel: +44 (0) 1252 733777	Close window

Plascoat PPA 571 Pre-treatment

Metalwork Pre-treatment

It cannot be stressed too highly that correct metal preparation and pre-treatment are fundamental to all successful metal finishing operations, including plastic coating. In order to achieve the optimum coating performance, all of the surfaces of the metal parts to be coated must be free of rust, grease, mill scale and any other contamination because these might inhibit the adhesion of the coating. This in turn can give rise to a premature failure of the coating.

Several metal pre-treatment systems exist that are used prior to plastic coating applications. The most commonly used methods are degreasing followed by mechanical pre-treatment or chemical pre-treatment.

See also ISO 12944 Paint 4 Types of surface and surface preparation.

2.2.1 Degreasing.

2.2.2 Mechanical pre-treatment.

2.2.3 Chemical pre-treatment of steel.

2.2.4 Pre-treatment of Zinc plated or galvanised steel.

2.2.5 Pre-treatment of cast iron.

2.2.6 Pre-treatment of aluminium

For pre-treatment of stainless steel, please refer to Plascoat PPA 571 - Pre-treatments for stainless steel.

More than one pre-treatment process may be necessary for some metal components. This will depend on the design of the part, the application for which it is intended and the coating performance specification that is agreed for the job. Typically, mechanical pre-treatment, such as grit blasting, can follow degreasing.

2.2.1 Degreasing

The first stage of any metal work pre-treatment is the removal of oil and greases from the metal surface. This is essential if the subsequent pre-treatment processes are not to be contaminated and if the initial and long term adhesion is to be optimised.

Degreasing can be done using either solvents or aqueous solutions.

For solvent degreasing; Items are immersed into the hot vapour of a suitable solvent. The vapours condense on the cold metal and so washes off the soluble oils and greases. The solvent degrease process is usually used prior to mechanical pre-treatment. This process will not remove stearates which may have been used in the cold drawing process to produce metal sheets and in tube and pipe forming processes. This solvent degrease process is usually used prior to mechanical pre-treatment.

The environmentally preferred aqueous degreasing systems use either alkalis or detergents solutions or a combination of these. The items are immersed in a hot bath of the solution or they are passed through a spray cabinet. The solution reacts with the contaminants and dissolves or releases them from the surface. It is usual to follow this by washing in hot water prior to further aqueous chemical pre-treatments.

2.2.2 Mechanical Pre-treatment

For items that require to be plastic coated, the most commonly used method of mechanical pre-treatment is grit blasting or shot blasting. The objective of blasting is to clean the surface of the metal substrate of rust, oxides or mill scale. Also by roughening the surface, the surface area is increased which results in increased adhesion. If any grease or oil is present on the metal surface, grit blasting alone is not sufficient as a pre-treatment. These components should be degreased prior to grit blasting.

Various blasting media are available. The decision of which type to use will depend on the metalwork that is to be cleaned. Sharp edged media will abrade the surface and will create a profile on the metal substrate containing undercuts. These will generally enhance the level of adhesion achieved by the coating to the substrate. Ensure that the grit media remains clean. Dirty or oily grit will transfer this dirt or oil onto the metalwork being processed. Chilled iron shot leaves a deposit of graphite on the metal surface and should not be used since this can interfere with the bond between the coating and the metal. In our experience the best results are obtained using none-metallic abrasives.

For the majority of Plascoat PPA 571 external coating applications, the following pre-treatment guideline is reasonable. The metal should have a visual surface cleanliness of between Swedish Standard Sa 2.5 and 3, in accordance with ISO 8501 - 1, 1988, and a surface profile of 30 microns or 10% of the required film thickness whichever is the lowest. Therefore Plascoat would recommend a profile of 20 microns for an electrostatically applied coating at 200 microns.

2.2.3 Chemical Pre-Treatment of Steel

The important aspect of chemical pre-treatment is that it leaves a thin corrosion resistant barrier layer on the metal surface to which the coating can adhere.

Always apply chemical pre-treatments in accordance with the manufacturer's instructions. Particular attention should be paid to details of the processing times and temperatures, as well as to the recommended chemical solution strengths. Such a pre-treatment system will include an appropriate degreasing stage.

Several chemical pre-treatment systems are available for use with plastic coating. These include zinc phosphate, iron phosphate and chromate. Some pre-treatment chemicals decompose at high temperatures. Therefore when selecting the pre-treatment process it is important to take this into consideration especially when using the fluidised bed coating process which requires relatively high metal temperatures.

The best pre-treatments are the chrome based systems but these have environmental issues. Zinc phosphate is the next best process but may be damaged by the high preheat temperatures normally required for fluid bed applications. Therefore Plascoat recommend the use of iron phosphate for fluid bed applied PPA 571 and the use of Zinc phosphate or chromate for electrostatically applied PPA 571ES. The zinc phosphate should be followed by a passivator rinse. Your pre-treatment chemicals supplier will be able to offer more details information for your specific requirement. For suggested pre-treatment systems see "PPA 571 - Recommended Pre-treatment for Galvanised Steel".

2.2.4 Pre-Treatment of Zinc Plated or Galvanised Steel

With any coating system it is often difficult to achieve good adhesion on hot dipped galvanised or zinc plated surfaces. It is therefore recommended that a test coating is conducted on such surfaces to ensure that the metal surface is suitable for coating.

When Plascoat PPA 571 is applied over a galvanised substrate, the following guidelines may be helpful, in order to achieve a satisfactory coating:

Plascoat recommend a mechanical pre-treatment using a non ferrous medium such as aluminium oxide. A particle size of 0.2 to 0.5 mm would be suitable. Do not use angular iron grit which may embed itself into the zinc layer and cause premature failure due to bimetallic corrosion. The blasting pressure should not exceed 0.3 Mpa (40 psi) and we would recommend a nozzle diameter of about 6 mm and a nozzle to work piece distance of 300 mm.

If a chemical pre-treatment were felt to be necessary then we would recommend an acid pickle followed by phosphating or chromating. See "PPA571 - Recommended Pre-treatment of Galvanised Steel".

Where hot dip galvanising is being used under PPA 571ES it is essential to pre-bake the metal at 30°C above the coating temperature for about 30 minutes to remove volatiles.

2.2.5 Pre-treatment of Cast Iron

There are several different qualities and types of cast iron and therefore the required pre-treatment will vary. Therefore this information is for guidance only. The main problems associated with any powder coating onto cast iron are poor adhesion and pinholes due to gassing from the pores in the cast iron. These problems are overcome by pre-baking the items at 30°C above the preheat temperature for one to two hours. When cool the items are grit blasted. During blasting graphite powder dust may be released. This must be removed using an air blast and suitable dust extraction system. Any large holes can be filled in using a fire cement. The items can then be preheated and coated in the usual way.

2.2.6 Pre-treatment of Aluminium (Aluminum)

PPA 571 bonds extremely well to clean aluminium and salt spray tests (ISO 7253) have consistently shown that there is no loss of adhesion on scribed plates even after 1000 hours. However many aluminium parts, especially aluminium extrusions, will have lubricating soaps on the surface. These must be removed before coating. To do this we recommend an acid cleaner such as Chemetall's Pyroclean 208. The contacts for Chemetall can be found at www.chemetall.com.

If mechanical pre-treatment is the only pre-treatment option then we would recommend a non-metallic grit such as aluminium oxide. This is because metal grits may leave metal particles embedded in the surface leading, later, to bi-metallic corrosion.

2.2.7 Plascoat PPA 571ES over Oxsilan pre-treatements

See: Plascoat PPA 571ES over Oxsilan pre-treatements

Plascoat PPA 571 Data Sheet