Effects of Isoprep 35 (Caustic Etch) on Aluminum 6061

[rocketscientist628]

I am experimenting with using caustic etch to aid with deburring some otherwise difficult to deburr parts. This has yielded some encouraging results, but I need to know if there will be any effects on the material properties. Our manufacturing procedure calls for on a one minute soak (max), but I am finding an acceptable amount of deburring will occur at or around about a 3 minute soak. This still yields an acceptable surface finish as well. These parts will be brazed at a higher level and though these are NOT the parts that have the clad on them, I am concerned about the integrity of the brazing. Currently we are only dipping the parts in caustic etch for 2 seconds and we are getting acceptable results with the brazing. If the material properties will not be otherwise degraded, I could possibly run some sample brazing tests to test the integrity of the brazing. Any help would be greatly appreciated.

 

[kenvlach]

What exactly is your concern?
Surface roughness, preferential etching, loss of temper, weaker braze joint?
What is the temperature, concentration and dissolved Al content of your caustic etch?

As long as you de-smut after the caustic etch (e.g., by nitric acid) to get a water break-free surface, I don’t see any problems. Al 6061 doesn’t smut much, anyway.

As a 3 minute etch still leaves an acceptable surface finish, you are not losing significant tolerance or creating excessive roughness. If the parts were very smooth to start with, a little roughening may actually improve the mechanical strength of the braze.

I doubt that any bulk properties will be harmed by a few minutes at a typical etch temperature of 160[sup]o[/sup]F. Certainly, less than by brazing. Will you quench from the brazing furnace and then artificially age?

Sample tests are a good idea, worth more than opinions. Let us know the results.

 

[rocketscientist628]

Thanks very much for your reply!
My main challenge is selling this idea to program team members that need to be convinced that the aluminum 6061-T651 will not be harmed in any way. The concentration of the isoprep 35 when I did the samples was between 7.7 oz/gal and 7.2 oz/gal. I maintained the temperature of the bath at around 145 deg F. I did not measure the AL content of the bath. After the soak I immersed the parts in de-oxidizer (isoprep 184) for 30 seconds and then rinsed with de-ionized water.

I am somewhat unfamiliar with isoprep 35 and what exactly it does to the aluminum. My main concern is will it effect any of the following properties of the material: 1) corrosive resistance, 2) Electrical and thermal conductivity, 3) strength and hardness, and 4) resistance to wear, creep, stress relaxation or fatigue? With respect to hardness, our deburr operator indicated that the increased soaking "softens" the burrs. Does this soften the aluminum as well?
Thanks again for your continued help!

 

[kenvlach]

A smoother surface is more corrosion-resistance, so in that regard etching is detrimental. Also, not all components of an alloy etch at the same rate, so it can set up small galvanic cells. For these reasons, when metal finishers prep test panels for chemfilm (chromating) for monthly salt spray qualification testing for MIL-C-5541C, the caustic etch step is skipped or greatly reduced.

A buildup of dissolved Al will of course reduce the etch rate, but there are some simple titrations for analysis.

The burrs become 'softer' because of reduced metal thickness. Sharp edges have higher surface energy and are thus more rapidly attacked.

Over-etching could coarsen the surface enough to reduce fatigue properties.

None of the bulk alloy properties should be affected.

 

[visht]

I am trying to replace the deburring operation with etching process for surface prep before the painting process. I am a new engineer. The current deburring process takes a lot of time. Can anybody please direct me as to how to approach the problem. What kind of chemical should be used? Immersion tank? paste? Most of the parts we do are made of aluminum.
Thanks