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Anodizing - Sulfuric vs Chromic vs Hard

I would like to know the reasons for choosing one over another. Specificly the hardness difference between Sulfuric or Chromis vs Hard. Why does one typiclly specify hard when it is more expensive?

Also or the sealing colors going to come out the same between the three?

Steven E. Myers
- Seattle, Washington, USA
2005

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First of two simultaneous responses -- 2005

Check out letter 36552, we discussed the differences between type II (conventional) and type III (hard) anodize pretty well in there. Chromic acid anodize is much thinner and is ussually only used when sulfuric is a poor choice for one reason or another (that is my understanding, at any rate). These reasons might include fear of chemical entrapment (the sulfuric, if not completely rinsed, will damage the anodize. The chromic won't) or fear of decreased fatigue strength. Becuase of the environmental and health risks associated with chromic acid, most people are moving away from it.

All colors, including any colors caused by the sealing process, are greatly affected by the operating conditions of the bath that produced them. Since the conditions between a type II and a type III are not identical, it would take a very skilled operator to make the appearance come out the same - if it was even possible.

Jim Gorsich
Compton, California, USA

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Second of two simultaneous responses --

You will get three different colors. Chromic is not for decorative applications. Chromic is for aircraft and military corrosion resistance, paint bonding, and adhesive bonding. Soft coat is what you want on wheels. Hard coat is just thicker and harder, however, also darker and not as pretety when dyed. Hardcoat would withstand more road debris bashing but would not be as clear or as pretty if dyed.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2005

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To put it very basically,
Chromic acid anodizing is very thin, and cannot be colored. Its use is usually limited to parts where materials are mated or have tight hems, and bleed-out of entrapped chemistry will tend to self heal.

Sulfuric anodize is your every day anodize. Good cosmetics and corrosion resistance are obtained. Parts can be dyed a variety of colors.

Hard anodize is about the most abrasion resistant coating available for aluminum alloys. If superior wear resistance is required a hard anodize would be appropriate.

HTH
Bill.

Bill Grayson
- San Jose, California
2005

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A lot of dye is sold to dye chromic anodized films. Admittedly it is not very pretty but covered by the Mil Spec and also suitable for color coding parts.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2005

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Robert- I would call it ugly, but it is darn functional as a color seperator of part sizes or alloy's.

Also, what no one has told the newbie is that the first couple of batches run in a new chromic acid will come out green if the tank is not fed some aluminum before use.

James Watts
- Navarre, Florida
2005