×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Anodic Growth vs. Dissolution

Anodic Growth vs. Dissolution

Anodic Growth vs. Dissolution

(OP)
I am aware that anodizing involves two competing reactions; the growth of the the anodic layer, and the dissolving (dissolution) of that layer by the action of the acidic (sulfuric acid) electrolyte.

It's obvious that the rate of layer growth is proportional to current density, but I have been led to believe that the dissolution reaction is independent of current density. Is this true? I would have thought that dissolution would have been accelerated by the flow of current.

I hope I'm posting this in the right forum.

RE: Anodic Growth vs. Dissolution

What you were told is correct. While the dissolution of a bare metal is normally accelerated by an anodic current (ionization: Me --> Me+ + e-), this is not so for an oxide (Me+ = Me+).

The dissolution rate of the anodic layer is primarily dependent upon temperature and acid concentration; also, upon solution agitation, dissolved aluminum concentration, and anodizing additives.  The Arrhenius-type temperature dependence is 1 of the 2 main reasons that hard anodizing is typically conducted at low temperatures, it gives a more dense oxide since the acid electrolyte causes less increase of the pore diameters.  The other reason for low temperatures is to remove the large amount of heat generated without risk of ‘burning,’ which is a sort of electrochemical dissolution of hot points (especially a problem in intermetallic-containing alloys such as Al 2024).

The temperature effect is why a conventional sulfuric anodize coating (MIL-A-8625F, Type II) is softer and the thickness is 2/3 ‘in,’ 1/3 ‘out’ or ‘buildup.’ E.g., for 1 mil total anodize, penetration into original surface is 2/3 mil.  Whereas, for hard anodize (Type III), penetration is ~ ½ of the total anodic thickness.

A consequence of the competing growth and dissolution processes is that a maximum anodic thickness will be reached for a given current density.  Continued anodizing will maintain a constant anodic thickness, while the substrate metal becomes thinner.  Sheasby and Pinner, in The Surface Treatment of Aluminum and Its Alloys, 6th Edn., p. 967, (2001) mention that for certain Al-Mg & Al-Mg-Zn alloys, the anodic coating can also become thinner.

Hope this helps,
Ken

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

eBook - Integrating the Engineering Ecosystem
Aras Innovator provides multiple options for integrating data between systems, depending on the scenario. Utilizing the right approach to meet specific business requirements is vital. These needs range from authoring tools, federating data from various and dissimilar databases, and triggering processes and workflows. Download Now
White Paper - Industry 4.0 and the Future of Engineering Education
With industries becoming more automated, more tech-driven and more complex, engineers need to keep their skills and knowledge up to date in order to stay on top of this wave—and to be prepared for the Industry 4.0 future. The University of Cincinnati offers two online Master of Engineering degree programs designed specifically for practicing engineers. Download Now
White Paper - Comparing Multi-Patterning at 5nm: SADP, SAQP, and SALELE
Self-aligned multi-patterning techniques such as SADP, SAQP, and SALELE are increasingly popular at advanced nodes, but each process has its pros and cons. IMEC and Mentor, a Siemens business collaborated to identify potentially less-obvious process and design limitations and trade-offs between the three SAMP techniques. Learn more in this paper. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close