Phosphate coatings nature

Nature of phosphate coating and of sealing coat Concentration in terms of CrO (%) ... 

The main value of salt-spray tests is in the evaluation of the effectiveness of phosphate coatings in restricting the spread of rust from scratches or other points of damage in a paint film. This feature is of particular interest to the motorcar industry, as vehicles are often exposed to marine atmospheres and to moisture and salt when the latter is used to disperse ice and frost from road surfaces. Great care is needed in the interpretation of a salt-spray test, as it has been found to favour thin iron phosphate coatings more than is justified by experience with natural weathering. In the motorcar industry the present custom is to use zinc phosphate coatings on the car bodies and all other parts exposed to the outside atmosphere. 

Surmenev, R.A., Surmeneva, M.A., and Ivanova, A.A. (2014) Significance of calcium phosphate coatings for the enhancement of new bone osteogenesis - a review. Acta Biomater., 10, 557—579. Westheimer, F.H. (1987) Why nature chose phosphates. Science, 235, 1173—1178. Zhang, S. (ed) (2013) Hydroxyapatite Coat-ings for Biomedical Applications, Advances in Materials Science and Engineering, CRC Press, 469 pp., ISBN 978-1-4398-8693-9. 

As with chemical etches, developing optimum conversion coatings requires assessment of the microstructure of the steel. Correlations have been found between the microstructure of the substrate material and the nature of the phosphate films formed. Aloru et al. demonstrated that the type of phosphate crystal formed varies with the orientation of the underlying steel crystal lattice [154]. Fig. 32 illustrates the different phosphate crystal morphologies that formed on two heat-treated surfaces. The fine flake structure formed on the tempered martensite surface promotes adhesion more effectively than the knobby protrusions formed on the cold-rolled steel. 

The nature of the setting reaction was finally elucidated by Wilson et al. (1970a), who established that formation of an aluminium phosphate gel was responsible although siliceous gel was also formed it merely coated the partly reacted glass particles. 

Wet adhesion phenomena represent a potentially fruitful area of research since so little is known. Some of the important questions are (1) How does one measure quantitatively the magnitude of the adhesion when the coating is wet (2) What is the governing principle that determines whether or not water collects at an organic coating/metal interface (3) What is the thickness of the water layer at the interface and what determines the thickness A recent paper (1.) correlates the wet adhesion properties of a phosphated surface with the crystalline nature of the zinc phosphate at the metal surface. 

Cyclic voltammetry was employed to determine the nature of the anodic and cathodic reactions of bare and coated copper in aerated 0.1M HClOjj (pH=1) and phosphate buffer (pH=5.6) solutions. Steady-state measurements were made in air to obtain a more... 

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