Conversion coated steel

Travinski, D. L. et al. "Adhesive Bonding to Conversion Coated Steel Surfaces." 15th National SAMPE Technical Conference. October 4-6. 1983. 

Trawinski DL, McNamara DK, Venables JD (1984) Adhesive bonding to conversion coated steel surfaces. SAMPE... 

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

In Section 4.3 several recently developed and experimental surface preparation methods for steel such as conversion coatings, plasma spray and sol-gel that attempt to simultaneously improve durability and bond strength over grit blasting will be discussed. 

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 possible passivation of iron and steel surfaces this can be a problem prior to chemical conversion coatings. 

The processes are dealt with fully in Chapters 11, 14 and 15. Because many paint systems include an initial surface pretreatment, e.g. chromated aluminium or phosphated steel, BS4479 1990, Part 3 deals with conversion coatings and should be consulted by designers. Whatever the method of treatment, liquids must be able to drain quickly and freely from the surfaces. Crevices where liquids can become entrapped are best avoided. The surface configuration needs to be such that active solutions can be washed away, leaving the surface to be painted completely free from unreacted pretreatment solution. Failure to achieve the requisite level of freedom from the surplus chemicals causes paint failure, e.g. osmotic blistering. 

Conversion coating Conversion coatings are chemical solutions which react with the metal surface to create a corrosion-resistant layer onto which the coating can bond. For mild steel iron phosphate is used to attain good adhesion, but it does not give the underfilm corrosion resistance which can be obtained using zinc phosphate. Zinc coatings can be treated with either zinc phosphate or chromate. Aluminium is usually treated with chromate... 

Steel, unless adequately protected with a film of oil subsequent to rolling mill operations, has a tendency to form surface rust rather quickly. This rust on the surface of the metal prevents proper conversion coating. A traditional method of removing rust is an acid applied by power spray equipment. The spraying action cleans both by physical impingement and the etching action of the acid. The power spray action is followed by a brush scrub, which further removes soil loosened by the acid. The brush scrub is followed by a strong alkaline spray wash, which removes all traces of the acid and neutralizes the surface.1-5... 

Phosphate conversion coatings provide a highly crystalline, electrically neutral bond between a base metal and paint film. The most widespread use of phosphate coatings is to prolong the useful life of paint finishes. Phosphate coatings are primarily used on steel and galvanized surfaces but can also be applied to aluminum. Basically, there are three types of phosphate coatings ... 

Wastewater generation occurs for each basis material (steel, galvanized and aluminum) and for each functional operation (cleaning, conversion coating, and painting). The wastewater generated by the three functional operations may be handled in one of the following ways ... 

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