PDA

View Full Version : Black stain on stainless



Gbbc
10-04-2016, 09:43 PM
Hi there,

I've recently noticed a black stain on some part of my mash tun. I clean after a brew with 3% caustic, hot water rinse, nitric/phosphoric 2.5% and rinse with cold water.

This is only a recent thing. Any help would be appreciated

Dean of Clean
10-14-2016, 01:37 PM
I have some ideas for you, sorry about the delay, had to reset my password.
It looks like the passive coating on your tank parts has been destroyed. I have destained many pieces of equipment in forty plus years, and most of it has been because of excessive concentrations of chemicals, and too high temperatures used. It particularly shows up at stress points, weld lines, and heat sections. I don't know how often you Clean and temperatures used, but they look excessive and should be lowered. That said, the damage has been done and you can either use special polishing pads that are used to buff out welds(welding supply outlet), or hook up with a knowledgeable chemical supplier to destain equipment with caustic, potassium permanganate, neutralized with gluconic acid before dumping down the drain, and finishing with a nitric acid solution solution of 4% strength circulated for 1 hr, and thoroughly rinsed. Hope this helps, good luck ! Dean of Clean

Gbbc
10-18-2016, 07:28 PM
Hi Dean of Clean, thank you for getting back to me and for the advice, cheers!

Gbbc
10-19-2016, 02:42 PM
Hi Dean,

Could you please be more specific about the chemicals and procedure for de staining equipment? concentrations etc?

Cheers

Dean of Clean
10-21-2016, 11:39 AM
Hi Dean,

Could you please be more specific about the chemicals and procedure for de staining equipment? concentrations etc?

Cheers

The procedure is tricky, especially with geographic disadvantages. Wish I was nearer to help you. I will dig the
Procedure out, and if you give me a written disclaimer, I will send it to you. I would try grinding it out, buffing it, repassivate with standard caustic, nitric acid treatment, and final acidified nitric acid rinse. Air drying after final rinse should restore it. If not give me a note back and I will advise. You are not the first to experience this, and need to cut back your chemical values and heat, or you will fighting it again. Regards, Dean of Clean