Corrosion conversion coatings

The basic objective of the conversion coating process is to provide a corrosion-resistant film that is integrally bonded chemically and physically to the base metal and that provides a smooth and chemically inert surface for subsequent application of a variety of paint films. The conversion coating processes effectively render the surface of the basis material electrically neutral and immune to galvanic corrosion. Conversion coating on basis material coils does not involve the use of applied electric current to coat the basis material. The coating mechanisms are chemical reactions that occur between solution and basis material.1-4... 

Key words rare earths, inhibitors, corrosion, conversion coatings, supply and demand, availability, metal finishing. 

Design of corrosion conversion coating on Al alloy substrate. 

A primer On metal, the purposes of a primer are to enhance corrosion protection and to give excellent adhesion. The primer will contain anticorrosive pigments, such as strontium chromate or zinc phosphate, which will slowly release ions that can repair damage or faults in the underlying conversion coating. 

Whatever application method is used, the maximum corrosion resistance can only be achieved if the metalwork is properly prepared. This preparation consists of dressing, blasting and conversion coating. 

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... 

The selection of basis material, conversion coating, and paint formulation is an art based upon experience. The variables that are typically involved in the selection are appearance, color, gloss, corrosion resistance, abrasion resistance, process line capability, availability of raw materials, customer preference, and cost. Some basis materials inherently work better with certain conversion coatings, and some conversion coatings work better with certain paint formulations. On the whole, however, the choice of which combination to use on a basis material is limited only by plant and customer preferences.1-4... 

Chemical conversion coatings are applied to previously deposited metal or basis material for increased corrosion protection, lubricity, preparation of the surface for additional coatings, or formulation of a special surface appearance. This operation includes chromating, phosphating, metal coloring, and passivating. 

In order to obtain maximum corrosion protection for painted metal articles, the metal parts are pretreated with an inorganic conversion coating prior to the painting operation. These zinc or iron phosphate coatings greatly increase both paint adhesion and corrosion protection. Traditionally, a chromic acid post-treatment has been applied to these phosphatized metal surfaces to further enhance corrosion protection. 

A large segment of the metal parts produced by industry are painted for both decorative purposes as well as to increase the corrosion resistance and extend the useful life of the product. To obtain maximum quality from painted metal articles, it is of paramount importance to pretreat the metal parts with a conversion coating process. ( 1,2) Pretreatment processes contribute a significant improvement in corrosion protection and durability to metal articles by ... 

A conversion coating provides an insulating, often non-conducting coating which both inhibits corrosion once painted and provides a better surface for paint adhesion. 

The "Mannich" adduct synthesized from the condensation of formaldehyde, 2-(methylamino)ethanol and poly-4-vinylphenol as shown in Structure I, has been evaluated as a function of molecular weight versus corrosion resistance as measured by salt spray and humidity tests on Bonderite 1000, an iron phosphate conversion coating. The molecular weight of the polymer was varied from approximately = 2,900 to 60,000. The corrosion resistance results were essentially equivalent over the molecular weight range evaluated. 

Chromate conversion coatings perform poorly in environments containing acidified chloride. In salt/S02 spray tests the substrate metal is heavily pitted after three to four days of exposure. In this work, a new coating was developed which itiproved the corrosion resistance of the conventional chromate coating remarkably. 

Recent studies on monitoring of a carrier environment( 3 ) have shown that its corrosion severity parallels to a laboratory cyclic environment of 5% NaCl spray (fog) with 30ppro of SO2. The development of a new chemical conversion coating which is also resistant to this environment would be of great inportance. 

Almost all aluminum structures are painted with organic polymers for corrosion protection. The purpose of incorporating an inhibitor interface (chromate conversion coating) between the substrate and tlie paint film is to ensure protection vAien paints fail to perform( ). It has been generally acc ted that no matter what Icind of paint system, and how well it is applied, it always will have some porosity defects and will degrade with tine during service. 

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