"Dishwasher Safe" Anodized Aluminum?

Q. We are looking for a black anodize finish that does not come off after a caustic wash. The components must be anodized first, then there is a secondary brazing operation to attach a tab. Then the whole assembly is put into a caustic wash to remove any flux residue. Is there any black anodize out there that can survive this caustic bath?

A. My opinion is very probably not. The pH, temperature and time will have a great effect on how bad it gets. I would look at doing the braze operation, including flux removal and then mask the tab and anodize. Masking will have to be excellent if it is not aluminum.

Any braze operation is going to destroy some or all of the anodize, or at a minimum degrade the anodize, so it is another reason to anodize after braze.

A. The alkalinity of dishwashing solution may have dissolved the anodic coating. If you can specify non-alkaline detergent, perhaps it will stay dishwasher safe.

A. Boy, that's a good one! Basically, an anodized surface is an anodized surface and is subject to attack from the strong caustic detergents that might be used in a dishwasher. Maybe a teflon impregnated anodizing or an autoclave-safe anodized surface or a thicker hardcoated surface might help. I think you might be into an area of proprietary coatings to obtain dishwasher-safe ratings though.

A. caustic soda [affil links] and Caustic Potash DISSOLVES BARE ALUMINUM AND ALSO DISSOLVES Aluminum Oxide. In a "bleach" strength the dissolution will be slow, but noticeable.

A. Hi S.

Alkaline materials are widely used to strip anodizing from aluminum, so I strongly doubt that there is any hardcoat which is dishwasher safe. However, it is very common these days for hard coated cookware to have a non-stick Teflon-like polymer coating; in fact it's very hard today to find any that doesn't. So I suspect that it is the polymer coating which is dishwasher safe.

Regards,

A. Joel, are you sure about the 14 ph measurement? When I read that I had to check for myself, as that sounded too high. I measured our commercial grade supply and came up with a pH of 10.5, which makes much more sense.

You didn't mention your sealing time. Increasing that time would be the first place I'd start.

Just another thought, if your ph meter is giving you a pH reading of 14 on the 409 solution, perhaps it's giving you inaccurate readings on your seal pH as well.

A. That depends (1) on the alkalinity of the detergent as anything over 8.0 will start to deteriorate the seal first and then leach the dye, finally down to aluminum metal and (2) the temperature as over 80 °C will crack the coating that has 1/5th the expansion coefficient as the underlying metal.

  Thank you. Stainless steel is looking more and more like the safe and sensible option at a time when most people want the convenience of dishwasher cleaning without worry about damage to the product.

A. Hi Zac,
Over many many years of the military, the Scouts, and the general camping gear industry trying to figure out how to do just that- keep Aluminum utensils from corroding without getting the user sick, even WITHOUT the added challenge of 'dishwasher safe' requirements- the Titanium utensil market exploded. I'm not trying to discourage you, but poke around Amazon on any given day and you'll see how saturated the market is for Titanium backpacking utensils, and even chopsticks (which I covet; they are super trick)!
The sad truth is that the tradeoff between durability, weight, and cost means titanium for the fancy pants campers and stainless steel (or the free plastic sporks that they give you at KFC) for the rest of us bums.
But if you have a unique and marketable product, why not see about Titanium as a material, and try to get it out there before the knockoff artists steal your design ? Good luck!