Conversion Coating

Common conversion coatings processes are briefly discussed in this section, including oxide coatings, phosphate coatings, and chromate coatings.

Oxide Coatings: The oxide coatings are in fact corrosion products which is a thin, usually less than 2.5 um (.00001 in) oxide with good adhesion. The oxide treatments are done by heat, chemicals, or electrochemical reactions.

 

  • Gun-bluing-type oxidations are done by heating the metals, generally steel, at 370 Deg C (700 Deg F) in a steam atmosphere. An oiled gun bluing provides some atmospheric corrosion resistance, but little protection on wear and other corrosion.
  • Chemical baths produce coatings similar to a gun bluing coating by immersion techniques.
  • Black oxide treatments are done by proprietary chemicals. Some pastes can be rubbed on surfaces to produce similar results. Black oxide can be applied on steel, copper, and most stainless steel.
  • Anodizing is produced by electrochemical conversion. The anodizing process, usually performed on aluminum for protection and cosmetic purposes, builds up both on the surface as well as into the metal. Thin coatings, 2 um to 25 um (100 uin to 1000 uin) can be coated on most aluminums. Thick coatings from 25 to 75 um (1000 to 3000 uin) are more durable and abrasion resistant than above chemical conversion oxide coatings. This oxide layer can be made in different colors depending on the post chemistries that are employed. The anodized parts are quite durable and do not tarnish and maintain their cosmetic appearance for a long period of time. Anodized coatings are usually dielectric in nature.

Phosphate Coatings: Phosphate coatings are processes of chemical conversion on a metal surface to produce thin adherent phosphate compound coatings. The phosphate crystals formed on the surfaces of materials can be iron, zinc, or manganese phosphates. Among these phosphates, manganese phosphate is more suitable for wear applications. Phosphate coatings are usually applied to carbon steel, low-alloy steel, and cast iron. They can also be applied to zinc, cadmium, aluminum, and tin. Phosphate processes are hard to apply on high alloys for these alloys are likely immune to the phosphoric acid. In short, phosphating is one of the most useful non-metallic coatings.

Chromate Coatings: Chromate coatings, similar to phosphate coatings, are processes of chemical conversion. But the chromate coatings are formed by the reaction of water solutions of chromic acid or chromium salts. The coatings can be applied to aluminum, zinc, cadmium, and magnesium. The coatings usually have good atmospheric corrosion resistance. Chromate coatings are widely used in protecting common household products, such as screws, hinges, and many hardware items with the yellow-brown appearance.

Coating Considerations

 

Materials Nominal Thickness �m (�in) Rating (1=poor : 10=good) CR: Corrosion Resistance SA: Solderability
Anodizing on Aluminum 2.5 – 25 ( 100 – 1000 ) CR: 9 after sealing, SA: 2. Can be made in different colors. Electrically insulative.
Chromate Conversion (Alodine, Irridite) on Aluminum ~ 0.025 ( ~1 ) CR: 6, SA: 2. Good electrical conductivity, good base coating for subsequent coatings.

Sinotech offers an exceptionally wide range of secondary processes that are applied to metals formed in hot or cold processes. Sinotech’ Supplier Quality Engineers determine the availability of the process within the metal forming facility, as well as the quality. If the internal secondary processes do not meet Sinotech standards then the processes are carried out in Sinotech-audited and qualified off-site secondary processing facilities. Sinotech has audited, qualified and worked with QS-9000 and ISO certified secondary processing facilities in China, Taiwan and Korea for over 12 years. Sinotech is dedicated to managing your project on-site and delivering parts to you at lower prices but the same quality, service and terms as a domestic supplier.

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