I am going to make a replacement grill for my '37 Buick out of stainless steel. The original is made out of pot metal and are notorious for breaking into two or more pieces. Also, like pot metal is apt to do, they pit badly and cost a small fortune to have repaired and replated. I have priced the cost to do it in stainless and it will cost about half of it would to have the grill repaired. In my business I deal in food processing machinery, I build and install the control systems, I don't do the actual building of the equipment. Most of the machines in the food industry are built out of stainless steel for ease of cleaning and other sanitary reasons. I know that the manufacturers passivate the stainless parts after they weld on them. This cleans the minute particles of iron that might have contaminated the surface of the stainless and forms a protective surface on the stainless steel. The use different acids, some pretty nasty like nitric acid and some not so bad and eco friendly like citric acid. It depends on what kind of stainless they are using. I am planning to use 304 stainless and would like to use citric acid because of the environmental friendliness of and also the safest to use. Any one have experience doing this? Any suggestions on what steps to take. I know, make sure the parts are dirt and grease free, rinse with water, soak in the acid and then rinse in water again. Is there a special water? Distilled or regular tap water? How long of a soak in the acid? I read if you soak too long you can get a flash reaction that turns the stainless black, not what I want.
We passivate parts at the Aerospace company I work at. We passivate our stuff to the AMS-2700 specification. I'm actually looking at a copy of it now, you dip your parts in an aqueous solution of nitric acid, sodium dichromate dihydrate and copper sulfate at temperature for certain amounts of time. I believe they use de-ionized water baths. Bottom line, I don't think this is something you could do at home due to the chemicals involved. Try to find a plating company by where you live. Passivation is an excellent finish option. It can withstand some pretty corrosive environments! Good luck!
Do you have any experience using citric acid? From what I have been reading there are conflicting opinions on the effectiveness of citric vs. nitric acid. I read one site about government specs, which seem to be always overdoing it, requiring nitric acid. I read a few sites that say citric acid does a great job. I would like to get the experiences of real people who do this type of work for a living. One place I've done a lot of work at I think sends out most of their stuff but I think I remember them doing some in their parking lot. I'll have to try to stop down there next week to see what they actually do.
For citric acid passivation: 304 stainless is austenitic, so you may not need post treatment in sodium dichromate dihydrate according to this AMS 2700 spec. It says to do an, "aqueous solution of 4 - 10% citric acid with addtional wetting agents and inhibitors as applicable". Also, "Bath temperature shall be 70 - 160°F with an immersion time of not less than 4 minutes for baths operating over 140°F, not less than 10 minutes for baths operating in the 120 - 140°F rnage, not less than 20 minutes for baths operating below 120°F". This spec doesn't call out a certain type of water for the post rinse, so I guess you could use tap water. This is all from the spec, so I guess your challenge would be to obtain citric acid from a chemical supply house. It looks like you could possibly do some backyard passivation with the citric acid method. I'm curious; if you try it out definitely let us know the results!
Citric acid, at least according to a cooking site I found while Googling "citric acid", is a common food product that can be found in specialty food stores. It's used in soda and candy. Thanks for posting the specs, it will be very helpful.
I've only been around passivated 430 (?) ss rivets and I have no idea of the process other than it was an acid. (you are eating away the iron from the surface and leaving the chrome/nickel. The parts I dealt with were not polished, is it possible to polish after passivating process and not bring new iron to the surface?
From what I've been reading passivating will preserve a polished surface, so I'm assuming you polish first then you passivate. I want to polish the front face of the bars on this grill.
Test pieces for sure. I'm sure I can get some scrap from the place that builds the food processing machines. I'll ask around there also to find out what they do but their primary work isn't treating metal, it's bending and welding. They're more in the rough side of the work, not the finesse side. Their products have to be more functional than cosmetically nice.
You can probably just get a local plating shop to do it for you. They should have acid dip tanks for passivating. For a one-off job it may cost less than buying the materials and trying to do it at home. The reason passivation works is that it changes the surface potential to be a lower value (more passive). The surface potential is compared to the galvanic series scale. It is low cost way to improve the surface corrosion protection for stainless steel.
Bob , if your near Racine or Delevan , try a tanker truck repair shop or semi tanker wash to see if they can do it for you as we pasivate tankers and fittings all the time to keep corrosion down and make cleaning easier . a food service tankerwash should be able to help you better as they use them more than chemical trucks . we use the nitric acid vapor solution as it makes a better finish and holds up better . all polishing must be done prior to passivating and darkening of the metal ( burning ) sometimes happens . only place I know it that does parts seperately is up in Manitowac . and the name slips me at the moment . but it is a big commercial anodization/plating works By Manitowac crane
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