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Plascoat PPA 571
Recommended Pre-treatment for Galvanised Steel

No matter what organic coating is being applied onto any galvanised surface good adhesion is always difficult without the proper pre-treatment. It is therefore recommended that a test coating is conducted on a sample of the pre-treated surface in order to ensure that it is suitable for coating and that adhesion has been obtained.

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

For fluidised bed coatings of PPA 571 Plascoat recommend a mechanical pre-treatment using a non ferrous medium such as fine copper slag, J Blast or Carborundum powder. A particle size of 0.2 to 0.5 mm would be suitable. Do not use angular iron grit which may 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. High pressures may damage the galvanised surface by weakening the bond between the zinc and the iron/zinc alloy below.

Alternatively phosphating, chromating or Gardobond 4590 can be used for PPA 571 or Plascoat PPA 571ES applications. For high stoving temperatures (i.e. in fluid-bed application) Gardobond 4590 or calcium-modified zinc phosphates such as Bonder 25/1 can be used. However if the galvanised surface has been treated at the galvanising plant with a resin phosphate system it will be necessary to remove this treatment prior to the pre-treatments recommended below. This can be done using a strong alkaline stripper such as Stripalene 504 (25% at 85°C for 5 minutes) obtainable from Henkel Metal Chemicals, England (Phone +44 (0) 1753 811108, Fax +44 (0) 1753 811195). It is also possible to remove it using a solvent stripper based on NMP obtainable from Chemetall (see below for details).

Excellent initial adhesion and salt spray results have been obtained on galvanised steel pre-treated with the following systems:-

• Phosphoric acid based degrease-cleaner-etch e.g. Walterclean 6231
  5 mins at 50°C
  Cold water rinse X 2.Resin based phosphate coating e.g. Walterisation RCT101 1 min at 20°C
  Dry at 120°C for 5 mins
  
  With this system it is important that the conversion coating has not cross-linked too much because adhesion to the PPA 571ES will be poor.
  
  All these products are supplied by Walterisation (UK) Ltd.
  Phone +44 (0) 181 688 2791, Fax +44 (0) 181 688 2727.
  
  
• Acid based degrease-cleaner-etch e.g. Pyroclean 205 at 20°C for 2 mins
  Cold water rinse X 2
  Conditioning rinse based on Titanium salts to create fine phosphate crystals e.g.
  Parcolene X for 1 min at 20°C
  Zinc phosphate plus nickel salts to minimise build up e.g. Bonderite 35 Demineralised water rinse
  Dry at 150°C for 10 mins
  
  These products are supplied by Chemetall. The contact numbers in England are:
  Phone +44 (0) 1908 649333. Fax +44 (0) 1908 373939. Other country contacts can be found at www.chemetall.com.
  
  
• Acid based degrease-cleaner-etch e.g. Pyroclean 205 at 20°C for 2 mins
  Cold water rinse X 2
  Hexavalent Chromate e.g. Ardrox 160 at 20°C for 1 min and dry at 50°C for 10 mins
  
  These products are supplied by Chemetall. The contact numbers in England are:
  Phone +44 (0) 1908 649333. Fax +44 (0) 1908 373939. Other country contacts can be found at www.chemetall.com.
  
• Pre-degrease      Gardolene 517         2% 45-50°C     55 secs
  Degrease             Gardolene 517         2% 45-50°C     1 min 50 secs
  Wash                     Water                         Amb                    55 secs
  Activation              Gardolene Z/1          1.5% Amb         55 secs
  Phosphate           Bonder 25/1 Form   4% 2 min 9 sec
  Wash                    Water                          Amb                    55 secs
  Wash                    Water                          Amb                    55 secs
  
  
• Gardobond 4590
  These products are supplied by Chemetall. The contact numbers in England are:
  Phone +44 (0) 1908 649333. Fax +44 (0) 1908 373939. Other country contacts can be found at www.chemetall.com.

© Plascoat Systems Limited, MMIII	Plascoat Systems Ltd. Trading Estate, Farnham, Surrey GU9 9NY, UK Tel: +44 (0) 1252 733777, Fax: +44 (0) 1252 721250 email: sales@plascoat.com