Hardened material becomes black after acid pickling

A. Good afternoon:

There is a good chance that the parts were overpickled and developed smut. Especially if the parts were carburized.

A. Hello,

So, in order to shine up again dip your parts in permanganate of potassium (it will dissolve the smut).

Let me know the results Please.

A. Sir:
In the case of hydrochloric acid (17% and 20 °C to 30 °C) and sulfuric acid (10% and 60 °C to 80 °C) for pickling; it is very necessary to use acid inhibitors. If you do not, then you may over pickle the steel and black carbon will appear and pits will appear on threads (They will be sharp to the touch.). Sulfuric acid is much worse for over pickling and higher temperatures (for both acids) cause more over pickling ("burning"). 35 years ago I did much testing of acid inhibitors and out of about 100 inhibitors I only found one for each acid that I liked. The best ones protected the base steel after the mill scale was removed but did not slow pickling. The bad inhibitors did not work, slowed pickling, evaporated out, fouled the tanks, were very toxic and smelled bad. I published the results in the journal, Metal Finishing. I also developed testing to determine how much inhibitor is in the acid baths. Properly inhibited the steel comes out bright and the acid fumes in the plant are greatly reduced (99.9%+). In addition, the acid baths last 2 to 4 times longer using the right inhibitors at the right concentrations.
Regards,

A. Hi Snehal. Unless I misunderstood, Dr. Cook has already answered that question immediately above: "sulfuric acid (10% and 60 °C to 80 °C) for pickling; it is very necessary to use acid inhibitors"

Dr. Cook notes on thread 58440 that his article was in Metal Finishing, Oct. 1982, pages 15-18. Hopefully a library can get you a copy.

opinion! Elsevier shut down Metal Finishing magazine, and their Science Direct paywall doesn't seem to offer that article. The advantages & disadvantages of "Open Access" to research are very much in the news these days ... proponents should probably note the prevalence of situations like this one where a company doesn't find it financially worthwhile to sell copies of an article on their site, but retains copyright anyway so that everyone loses by being denied access to the research :-(

Regards,