TCI Application & Troubleshooting Guide

Chapter Six: Pretreatment

Perhaps one of the most important steps in powder coating is pretreatment. Pretreatment allows for the proper adhesion of the powder to the substrate and ensures the long term adhesion of the coating to provide lasting resistance to corrosion. The process requires three steps:

Cleaning:

Removes soils, oils, oxides, smut, and contaminants

Factors to consider include accurate levels to improve soil removal, decrease the cleaning time and provide an optimum surface for the phosphate layer. Titration readings measure the total alkalinity or acidity levels. Increased temperatures will improve the action of the detergent and allow for less time in the cleaning process. Be careful to watch for sludge or soil build up in tanks that will reduce the cleaning activity even with good titration readings.


Rinse:

Removes any remaining contaminated solutions from the surface while diluting the chemicals to stop their action.

It also adjusts the pH for the next step.

 

Chemical Conversion of the Surface:

Common Types Include:

  • Iron Phosphate for Ferrous Metals
  • Zinc Phosphate for Ferrous Metals
  • Chromate Conversion for Aluminum
  • Manganese Phosphate

Conversion coatings are a complete film which changes the physical and chemical nature of the metal surface. These coatings deposition weights are measured in milligrams and are grey to blue iridescence or blue to gold iridescence. A clean surface is required for the reaction.


Seal

Neutralizer stabilizes the conversion coating.

The temperature of the rinse increases the solubility of the residues and improves dryoff time. Proper flow patterns and rinse volume promote a more thorough rinse. Overflows and screens are used for the removal of the soils, oils, and other contaminants.


Remember to WATCH:

  • Water
  • Action
  • Time
  • Chemicals
  • Heat

Cross Section of Phosphated
and Coated Steel
Cross section of phosphated coated steel

Mechanism of Phosphating
Mechanisms of phosphating, how phosphating works
 
 
Phosphate Comparisons
  Iron Phosphate Zinc Phosphate
pH 4.5-5.5 1.5-3.5
Composition Iron Phosphates
Iron Oxides
Phosphates of Zn, Fe, Mn, Ni
Coating Weight 20-80 mg/ft2 100-350 mg/ft2
Sludge Minimal Low-High
Accelerators Increasing coating weight Decreasing coating weight
Adhesion:
Forming Type
Scrape Type

Excellent
Fair to Good

Fair to Good
Good to Excellent


Phosphate Control Factors
Control factors to consider when phosphating, phosphate control factors



 

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