Chemistry of in-situ phosphating coating

Suresh Darmarajan (2005). Chemistry of in-situ phosphating coating PhD Thesis, School of Engineering, The University of Queensland.

       
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Author Suresh Darmarajan
Thesis Title Chemistry of in-situ phosphating coating
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 2005
Thesis type PhD Thesis
Supervisor A/Prof. Andrejs Atrens,
Dr. Jason Nairn
Total pages 298
Language eng
Subjects 030603 Colloid and Surface Chemistry
030699 Physical Chemistry not elsewhere classified
Formatted abstract

The main focus of this thesis was SD-100, a commercially successful single component wash primer manufactured by Wattyl Paints Pty Ltd. The critical ingredients of SD-100 are polyvinyl butyral resin, epoxy resin, zinc phosphate modified with chromium (VI) anticorrosive pigment and phosphoric acid. The first objective of this project was to develop an improved version of SD-100 using commercially available raw materials. The primary target was the development of a chromium free anticorrosive wash primer. The second objective of the project was to understand the various reaction mechanisms that provided SD-100 with excellent adhesion and anticorrosive properties despite only having a dried film thickness of 20 microns. 

 

Commercially available polyvinyl butyral resins, epoxy resins, chromium-free anticorrosive pigments, silane coupling agents, different levels of phosphoric acid, phosphating additives and water were trialled to improve the performance of SD-100. All developmental work on improving the performance of SD-100 was carried out at the Development and Design laboratory of Wattyl Paints Pty Ltd (Queensland). Variations of the standard SD-100 formulations were made and tested in accordance with Wattyl Paints Pty Ltd (Queensland)'s in-house quality control test methods. Expected improved formulations of SD-100 were first screened in the laboratory for anticorrosive and adhesion properties. The following screening tests were conducted to select the final formulations for outdoor exposure testing; pull off adhesion test (ASTM D4541- 93), cross cut test (AS 1580.408.93), humidity testing (AS1580.452.1), total water immersion testing (ASTM D870-92), and salt spray testing (ASTM B117-94). Based on the performances in these screening tests, new formulations were made and exposed at two outdoor testing sites at Woongoolba and Rocklea, which represented marine and industrial environment respectively. 

 

It was found that the direct replacement of the current chromium (VI) based zinc phosphate anticorrosive pigment with a zinc aluminium tripolyphosphate on a volume solids basis yielded a chromium free product with comparable anticorrosive and adhesion properties. The direct replacement of both the current epoxy resin and chromium (VI) based zinc phosphate anticorrosive pigment with a lower molecular weight epoxy resin and zinc aluminium tripolyphosphate anticorrosive pigment also yielded a chromium free product with comparable anticorrosive and adhesion properties. 

 

In order to achieve the second objective of the thesis, the bulk chemistry, interfacial chemistry and anticorrosive chemistry of SD-100 was studied. The bulk chemistry entailed studying the reactions take took place in the coating by solubility studies, infrared spectroscopy, 13C nuclear magnetic resonance, small angle x-ray scattering and electron paramagnetic resonance. There was evidence of crosslinking between the epoxy and polyvinyl butyral resins under the influence of phosphoric acid. It was believed that the reactions between the epoxy and polyvinyl butyral resins improved the cohesive strength and impermeability of the coating towards aggressive corrosion stimulants such as water or dissolved salts. Furthermore it was shown that the uppermost clear layer of the coating contained only polyvinyl butyral resin, whose chemical structure was altered to improve the impermeability of the coating. 

 

The interfacial chemistry between SD-100 and the steel substrate was studied by x-ray photoelectron spectroscopy, time of flight secondary ion mass spectroscopy, scanning electron microscopy and x-ray diffraction. It was found that under the influence of the phosphoric acid there was an increase in the formation of primary valence bonds containing organometallic complexes at the interface. Carboxylic acids also formed at the interface only in the presence of phosphoric acid. The increase in the adhesive strength of SD-100 was attributed to the formation of strong bonds between the metal surface and these carboxylic acids. It was also shown that only the polyvinyl butyral resin was responsible for improvement of the adhesive strength. 

 

The main anticorrosive properties of SD-100 were characterised by potentiodynamic polarisation and auger spectroscopy. There was evidence to suggest that the chromium (VI) based anticorrosive pigment was behaving both as an anodic and a cathodic inhibitor. The main component of the passivation film formed by the chromium (VI) based anticorrosive pigment and the tripolyphosphate anticorrosive pigment was zinc hydroxide. It was believed the presence of zinc oxide as an auxiliary component of the anticorrosive pigments contributed to the formation of zinc hydroxide.   

 

A comprehensive understanding of the chemistry and action of SD-100 was gained through combined studies of the interfacial layer, bulk layer and anticorrosive properties. 

Keyword Phosphate coating

 
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Created: Wed, 01 Aug 2012, 16:49:17 EST by Talha Alam on behalf of The University of Queensland Library