Successful Metal Surface Treatment

No matter what metal surface needs to be painted, successful performance begins with proper cleaning and surface preparation. This article will concentrate on the essential issues in the manufacturing process necessary to ensure successful metal treatment and resultant coating performance. As there are hundreds of surface treatments, we will address the major factors that influence phosphate metal pretreatment which are one of the most widely used pretreatment chemistries. Phosphate treatments are used on steel, zinc and aluminum substrates.

The pretreatment process for metal surfaces provides multiple benefits as it is the foundation of the paint layering system. A quality metal treatment process enhances adhesion between the metal and paint layers by providing a more uniform surface and provides greater corrosion resistance with less undercutting of the paint film.

Table I – Typical Spray or Immersion Process involved in Phosphate Pretreatment

PROCESS STEP ORDER	PURPOSE	CHEMICALS	POTENTIAL PROBLEM(S)
1. Cleaning (see Figs I & II)	Remove soils, mill oil, lubricating oil and drawing compounds, dissolution of metal oxide(s), precipitate hard water deposits	Alkaline Cleaner	Incomplete removal, synthetic oils can be more difficult to remove than natural oils Contaminated cleaning process, cleaning chemicals spent Temperature too low Poor tank maintenance Inadequate mechanical action Change in time, temperature, pressure (for spray cleaner) or cleaner concentration
2. Water rinse(s)	Remove residual detergents and deposits	Quality tap water and/or reverse osmosis (R/O) water	Drag out water sensitive deposits from the cleaning process Tap water that contains hard water may deposit moisture soluble compounds on the metal surface
3. Rinse Conditioner (see Fig III)	For Phosphate- Aids in the development of the proper phosphate crystals on the metal surface	Colloidal Titanium Salts and additives	Destabilization of the Ti Colloid: pH too low or too high High heat Contamination Poor water quality (too hard)
4. Phosphate Step (See Fig IV)	Forms a microcrystalline coating to enhance paint adhesion and corrosion resistance	Phosphoric and nitric acid Zn, Ni, Mn Fe cations Fluoride, Surfactants and accelerator	Must continually remove iron phosphate sludge for proper control Ensure optimum recirculation rate for tank process or spray nozzle pressure for spray process
5. Rinse	Stops the chemical reaction on the metal surface	Water	Water must be clean
6. Post Rinse	Fill voids in pretreatment	Hexafluorozirconqic acid	Proper control of pH, time , temperature and pressure (spray)
7. Deionized (D.I.) Rinse(s)	Remove any residual chemicals and to provide a clean surface for coating	D.I. Recirculating rinse, followed by a D.I rinse	Carry over of chemicals and other contaminants from previous steps. Must ensure that D.I. water quality is maintained

To read the rest of the article, written by Ron Lewarchik, please click here to head over to UL Prospector.

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Ron Lewarchik, Author of article & President of Chemical Dynamics

As a contributing writer, Ron pens articles on topics relevant to formulators in the coatings industry. He also serves as a consultant for the Prospector materials search engine, advising on issues related to optimization and organization materials within the database.