In short, no. Alocrom/Alodine or any other chemical conversion coating is not the same as anodising.
Anodising is an electrolytic process which coats a metal substrate with a protective oxide layer. The anodised coating on aluminium increases corrosion resistance and wear resistance, is electrically insulative, provides a good key for paint or adhesive and is often used as a finish that is decorative as well as protective. Anodising has an impact on the dimensions of a part, ranging from a few microns (chromic anodising) up to tens of microns (hard anodising).
Alocrom/Alodine/Iridite NCP/Surtec 650 are normally referred to as conversion coatings or chromate conversion coatings. They essentially change the surface chemistry of a substrate to give the desired properties to a metal substrate. Unlike anodising, these processes are not electrolytic in their application and are normally requested through a colour spectrum or coating weight to give the required finish specification. These coatings have very little effect to final dimensions of a part and as a result designers do not normally need to factor this into their calculations.
Conversion coatings can serve a wide variety of design applications including:
- Corrosion resistance
- Base for priming or painting
- Electrically conductive
- Lower cost than anodising in most cases
While anodising itself is occasionally called a conversion coating, it is an entirely different mechanism (electrolytic) and end result. In every day industry use, conversion coatings refer to non-electrolytic conversion coatings.
For some applications it’s likely that both anodising and conversion coatings are both adequate solutions, but in the majority of cases one will be more suitable in terms of cost or performance. We can help you to decide at the design stage.
Common Design Considerations
Below are some potential design cases and typical solutions. All design cases here are simplistic rules of thumb to illustrate the sort of considerations that may lead to selecting anodising or a selecting conversion coating. It should be considered a starting point for further research and prototyping specific to your design goals and end use.
|Design Case||Typical Solutions|
|Maximum corrosion resistance||Sulphuric anodising, sealed.|
|Wear resistance or scuff resistance||Anodising, especially hard anodising.|
|Electrical contact/bonding||Whole part or contact faces with conversion coating. Conversion coating may be applied at low coating weight to minimise electrical contact resistance further. If at risk of corrosion, non-contact areas are often painted or selectively anodised. More info on anodising and electrical bonding and selective anodising.|
|Reducing galvanic corrosion risk||Anodising. Provides electrically insulative coating. More info on anodising to prevent galvanic corrosion|
|Electrical insulation||Anodising. Dielectric breakdown voltage increases with thickness. Designers must consider failure modes, e.g. physical damage to coating creating conductive path.|
|Pretreatment before applying paint systems or adhesives||Unsealed anodising or conversion coating. Best adhesion achieved by applying primer without delay.|
|Low cost protection in low corrosion environments||Conversion coatings|
|Cosmetic finish||Sulphuric anodising, sealed and optionally dyed.|
|Protection for parts unsuitable for anodising||Conversion coatings, especially Alocrom 1200 or Surtec 650V.|
|Protection for parts exposed to elevated temperatures||Anodising often preferred, but other factors such as temperature, alloy, cost, overcoating and environment will be relevant.|
|Touch up: repair of minor damage to anodic coatings (anodising), when permitted||Conversion coatings, especially Alocrom 1200 or Surtec 650V.|
|Temporary protection in storage before further machining/assembly or for single use items.||Conversion coatings|
If you know what specification you are looking for but not which treatment, you may find our specification list useful. Please do not hesitate to contact us, our technical team will be able to help you find the treatment that you need.