RoHS COMPLIANT CONVERSION COATING ON ALUMINIUM & ITS ALLOYS PRIOR TO POWDER COATING
ABSTRACT
The extent of added corrosion protection and base for paint / powder coating adhesion derived from conversion coating of aluminium is primarily a function of chemistry of conversion coating bath, the coating weight/film thickness, post drying condition and time lag between conversion coating & powder coating.
Our efforts to develop trivalent chromium & Cr-free conversion coating chemical in response to crying need of present environmental regulations have been discussed highlighting process parameters, process control and evaluation criteria needed to get optimum properties of RoHS Compliant conversion coating chemicals.
This paper also give emphasis on the suitability of chrome free coating as an alternate to chrome conversion coating for regular application on aluminium & its alloys.
INTRODUCTION
The conversation coating as used in the metal finishing industry, refers to the conversion of metal surface into a less reactive surface that will more easily accept applied subsequent powder coating and or provide for a more corrosion resistant surface.
If the conversion coating is done by chromium free chemicals then it is called chrome free conversion coating [1]. Chromate conversion coating which is based on hexavalent chrome plays a major role to provide both corrosion protection and base for paint & powder coating and are found to be very cost effective [2]. However, due to the environmental issues / health hazardous and consequent imposition of different regulations on the use of hexavalent chromium like end of the life vehicle directive (ELV), waste of electrical & electronic equipment directive (WEEE) and restriction of hazardous substance directive (RoHS) aluminium powder coating / paint manufacturers are now looking for new technologies to effectively replace and deliver coating that meet corrosion resistance & paint adhesion requirement and simultaneously complying with the environmental mandates.
Many options have been proposed to replace hexavalent chromium from the conversion coating chemicals but the following systems are considered to be useful as alternate to hexavalent (Cr6+) conversion coating of aluminium.
- Trivalent chrome (Cr3+) based chemical [3]
- Chrome free chemicals based on titanium / zirconium or a mixture of both [4, 5]
The discussion will be limited to both types of RoHS compliant conversion coating Cr3+ & Cr-free and Cr6+ may come during discussion for comparison purpose.
EXPERIMENTAL
Substrate: Aluminium sheet / profile purchased from the market (grade – 6063).
Conversion coating chemicals (proprietary):
Hexachrome base conversion coating chemical
Trichrome base conversion coating chemical
Cr-free conversion coating chemical for clear coating
Operational condition of bath for RoHS compliant conversion coating chemicals are given in Table-1.
| Conversion chemical type | Concentration | pH | Application temperature | Application time | Coating weight |
|---|---|---|---|---|---|
| Cr3+ | 10 – 30% v/v | 3.5 – 4.0 | 35 – 40°C | 1 – 3 minutes | 200 – 400 mg/m² |
| Cr-free colourless | 10 – 25 g/L | 2.5 – 4.0 | 25 – 35°C | 2 – 4 minutes | 75 – 150 mg/m² |
| Cr-free yellow colour | 35 – 45 g/L | 2.4 – 3.1 | 25 – 35°C | 4 – 8 minutes | 300 – 750 mg/m² |
Table 1.0 : Operating parameters of RoHS compliant conversion coating chemical
The appearance of RoHS compliant chemical & duly conversion coated aluminium profile is given in Figure – 1.0.
Cr3+ / Trichrome | Cr-free (colourless) | Cr-free (Yellow)
Fig. – 1.0
Chrome free coatings are mostly clear & colourless & hence inspection of coating during processing is difficult. A special technology has been introduced in Cr-free yellow chemical which allows visual assessment / inspection of the coating development and coating quality during processing.
SEM / EDAX study of conversion coated Al sheet
The SEM & EDAX study on Cr-free conversion coated aluminium sheet before & after SST hrs have been done and the SEM image & compositional analysing by EDAX are given in the following (Fig. – 2.0 : chrome-free clear & Fig. – 3.0 : chrome-free yellow).
| Elemental Composition (%) of conversion coating by EDAX | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| A1 | 74.3 | 73.3 | 74.6 | 72.5 | A1 | 61.9 | 61.6 | 60.8 | 64.5 |
| O | 18.6 | 19.1 | 17.8 | 20.2 | O | 20.8 | 21.7 | 22.3 | 20.5 |
| C | 6.5 | 6.9 | 6.5 | 6.5 | C | 16.4 | 16.4 | 16.2 | 16.4 |
| Ti | 0.7 | 0.8 | 0.6 | 0.7 | Ti | 0.5 | 0.4 | 0.4 | 0.4 |
The average titanium % in the Cr-free clear conversion coating panel is 0.7 whereas the Titanium % on 5% white rust coated panel is 0.425 which indicates the SST resistance is mainly due to the Titanium deposited on the panel.
The same logic is also applicable for Cr-free yellow conversion coated panel [6]. Here both titanium & organic content (C %) is lower in panel after SST studies compared to that of before SST (Fig. – 3.0).
Testing after powder coating
All tests after powder coating on duly RoHS compliant conversion coated aluminium panel as per Qualicoat standard have been passed and well accepted.
CONCLUSION
Both Cr3+ & Cr-free conversion coating are highly suitable as an alternate to chromate (Cr6+) conversion coating on aluminium & its alloys.
Many options have been attempted to replace chromate (Cr6+) from conversion coating chemical but Cr3+ conversion coating based on Cr3+ & zirconium compound and Cr-free based on titanium or mixture of titanium & zirconium are found to be very useful alternative to chromate (Cr6+) conversion coating for satisfactory corrosion resistance, environmental acceptability and good adhesion characteristics to powder coating.
ACKNOWLEDGEMENT
The authors thank Sanjib Adhikary & Kuhelika Nath for experimental studies & developmental work on RoHS compliant conversion coating chemicals.
REFERENCE
- F. W. Eppensteiner, M. R. Jenkins, Metal Finishing Guidebook & Directory, 2006, P 419.
- M. A. Heine and M. J. Pryor, J. Electrochem. Soc. Vol. 114, No. 10, 1967, P 1001 – 1006.
- Saha R, Nandim R, Saha B, J. Coord. Chem 2011, 64, P 1782 – 1806.
- John. W. Bibber, Plating & Surface Finishing, March 2003, P 40 – 43.
- Markus Beeker, Corros Rev. 2019; 37(4), P 321 – 342.
- US Patent 4,054,466 Oct. 18, 1977.
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