Summary for black smut on steel Discussion
Black smut on nitride high carbon steel became the main concern when Christos Sigalas asked how to prepare a polished, nitrided Uddeholm IMPAX mold for electroless nickel plating after alkaline cathodic cleaning and a 50% HCl dip turned the surface black. He had found references to cyanide dips, anodic treatments, and periodic reverse methods, but wanted to avoid anodic treatment because the part was a polished mold. Guillermo Marrufo advised that any suggested anodic treatments or acid dips should first be tried only on a conspicuous area or a similar test piece, with an EN stripper ready in case stripping is needed. He said his shop avoids smut by using a very fine aluminum oxide blast (mesh 600) after thorough manual cleaning, then blowing off and rinsing before going directly to the Woods nickel strike. He recommended a short strike, careful inspection for coverage issues, baking right after plating, and then re-polishing.
2003
Q. Dear friends,
Having to electroless nickel plate a mold made of Uddeholm’s IMPAX high carbon steel, that has been nitride and polished, I encountered the following problem:
After alkaline cathodic cleaning and acid dipping (HCl 50% diluted with water),and before proceeding to nickel strike, the surface became black (carbon smut probably?).
Searching in bibliography I found several suggestions like cyanide dip, cyanide or alkali anodic treatment, anodic acid etch, anodic or periodic reverse treatment in a solution containing NaCN, NaOH and tetrasodium salt,etc.
Since the piece is a polished mold, I would prefer to avoid anodic treatment. Any suggestions?
Any other procedure that will not cause the production of this smut on the surface?
Thanks in advance,
CHRISTOS SIGALAS
plating shop- ATHENS, ATTICA, GREECE
A. Only attempt the suggested anodic treatments or acid dips if you can do it in a conspicuous area or better another part with equal treatment. Have ready an EN striper as you probably will need it. What we have found to provide the necessary micro-roughness and avoid smut is a blast with superfine AlO2 (mesh 600) after a thorough manual clean, blow off the sand, rinse and directly to the Woods; let stay for few seconds (only to soak it in), strike for 1-2 min, rinse, EN plate at least 0.002″ to allow for final polish. Carefully inspect the surface right after the strike. Complicated geometries are prone to incomplete coverage in areas of low current density, thus you may have smut or adhesion problems in those areas. If everything seems OK, bake the mold right after the plate. Re-polish.
Guillermo Marrufo
Monterrey, NL, Mexico
2003
Mr. Marrufo,
Thank you for your suggestion. Any particular EN stripper without toxic fumes?
CHRISTOS SIGALAS [returning]
– Athens, Greece
2003
adv.
