- Email: [email protected]
Corrosion of hot-dipped zinc coating
Heardon the Hotline by Ted Mooney, P.E.
Dial 201/838-0113from your modem at any time.
B
etween the dial-in modem service to the Hotline, our World Wide Web version, and faxes from readers, it ‘has been one busy month! First a followup-
TREATING BLACK OXIDE A little late in the day for a reply, but-black oxide baths frequently contain chlorates and/or nitrates. I tried to use spent black oxide baths for pH adjustment and have gotten very noxious brown fumes, presumably NO, gases. This problem can be avoided if the pH is not brought too low, but it is best to add dilute acid to the spent material, with vigorous stirring, and never letting the pH get below 8 or so. David Wichern, Consultant, Calif. CORROSION OF HOT-DIPPED ZINC COATING We have a number of hot-dipped galvanized panels that were stored improperly and have developed “wet-storage stain.” Most areas are whitish in color, which I understand is zinc hydroxide, but what is the black staining? Chemical analysis reveals only zinc and oxygen. I suspected iron would be found incorporated into the black areas, but none was found. Anyone have any ideas? Bruce Liebert, University of Hawaii It is believable that you are detecting no iron. While iron-zinc compounds are formed at the interface, they do not extend all the way through the coating; rather, the surface of the hot-dipped coating is nearly pure zinc. Further, as noted in ASM International’s “Volume 5: Surface Cleaning, Finishing, & Coating,*’the white corrosion products are about 500 times as voluminous as the zinc metal, and sometimes the damage to the coating is much less severe than it looks; so it is possible that you still are seeing only zinc even after what looks like significant corrosion activity. It is desirable, and often possible, to remove wet-storage stains. Consult the above-mentioned text, the booklet “Painting Galvanized Structural Steel” by the Zinc Institute and the American Hot-Dip Galvanizers Association, or “Duplex Systems: Hot-Dip Galvanizing Plus Painting” by J.F.H. van Eijnsbergen (available from Metal Finishing). I don’t know what the black stains are, and, hopefully, a reader who is more knowledgeable than me will chime in. But meanwhile, is it possible that they are a mildew? I’m no laboratory technician, but if wiping a test piece with bleach doesn’t remove the stain, it would at least prove me wrong. Ted Mooney, Finishing Technology, N.J. 58
Studies of the corrosion of hot-dipped galvanized substrates in automotive applications have shown that typical corrosion products are zinc hydroxychloride and zinc hydroxysulfate. Now, you didn’t find any chloride or sulfate in his analysis, but then you didn’t intend to expose your panels to a corrosive environment, either. Without knowing the nature of the atmosphere where the panels were stored, it’s not easy to determine the nature of the corrosion product. George Gorecki, A&ox, Inc., Ill. GOLD PLATING I am working for West Virginia University. We have a microwave cavity that needs to be plated with gold (l-2 pm). Maybe it sounds simple for professionals but not for us. We have tried phosphate gold solution (1.3 g KAu(CN),, 120 g K,HPG,, 30 g KHaPO, per litre) and current density 1 mA/cm2. The gold layer looks matte and brownish. Is there any way to get a shiny finish without polishing? We would be glad if you could help us with advice. Alex Klymachyov, W.V.U. First, what type of waveguide/cavity material? I’ll assume copper, although aluminum is also possible, as well as ferrous based (you did say you were at a university). The next issue is that of the matte brown finish. Sounds like you’re burning the plate. I suggest a Hull cell analysis. Essentially, you plate up on a brass shim that is placed at an angle to the anode. This allows you to compare varying current density to plated appearance. I am also assuming that you are using an appropriate anode with appropriate surface area. A platinum-coated anode works best, although stainless stell can be used for short periods of time without damaging the bath. The surface area ratio of the anode to cathode should be about 1:1, although 2:l will be okay. Jeff Albom, jeffassd.loral.com RESTORING TITANIUM NITRIDE FINISH Titanium nitride (TiN) is a common hardfacing material for high-speed steel tools. It is usually a very thin coating deposited onto prc-sharpened high-speed steel. This makes tools less than half of the price of tungsten carbide tipped tools, tihich must be brazed and then sharpened. But resharpening TiN bits removes the TiN, exposing the softer steel. Does anyone out there know of a plant/company/
0 Copyright Elsevier Science Inc.
METAL FINISHING
. SEPTEMBER
1995
Dial 201/838-0113from your modem at any time.
B
etween the dial-in modem service to the Hotline, our World Wide Web version, and faxes from readers, it ‘has been one busy month! First a followup-
TREATING BLACK OXIDE A little late in the day for a reply, but-black oxide baths frequently contain chlorates and/or nitrates. I tried to use spent black oxide baths for pH adjustment and have gotten very noxious brown fumes, presumably NO, gases. This problem can be avoided if the pH is not brought too low, but it is best to add dilute acid to the spent material, with vigorous stirring, and never letting the pH get below 8 or so. David Wichern, Consultant, Calif. CORROSION OF HOT-DIPPED ZINC COATING We have a number of hot-dipped galvanized panels that were stored improperly and have developed “wet-storage stain.” Most areas are whitish in color, which I understand is zinc hydroxide, but what is the black staining? Chemical analysis reveals only zinc and oxygen. I suspected iron would be found incorporated into the black areas, but none was found. Anyone have any ideas? Bruce Liebert, University of Hawaii It is believable that you are detecting no iron. While iron-zinc compounds are formed at the interface, they do not extend all the way through the coating; rather, the surface of the hot-dipped coating is nearly pure zinc. Further, as noted in ASM International’s “Volume 5: Surface Cleaning, Finishing, & Coating,*’the white corrosion products are about 500 times as voluminous as the zinc metal, and sometimes the damage to the coating is much less severe than it looks; so it is possible that you still are seeing only zinc even after what looks like significant corrosion activity. It is desirable, and often possible, to remove wet-storage stains. Consult the above-mentioned text, the booklet “Painting Galvanized Structural Steel” by the Zinc Institute and the American Hot-Dip Galvanizers Association, or “Duplex Systems: Hot-Dip Galvanizing Plus Painting” by J.F.H. van Eijnsbergen (available from Metal Finishing). I don’t know what the black stains are, and, hopefully, a reader who is more knowledgeable than me will chime in. But meanwhile, is it possible that they are a mildew? I’m no laboratory technician, but if wiping a test piece with bleach doesn’t remove the stain, it would at least prove me wrong. Ted Mooney, Finishing Technology, N.J. 58
Studies of the corrosion of hot-dipped galvanized substrates in automotive applications have shown that typical corrosion products are zinc hydroxychloride and zinc hydroxysulfate. Now, you didn’t find any chloride or sulfate in his analysis, but then you didn’t intend to expose your panels to a corrosive environment, either. Without knowing the nature of the atmosphere where the panels were stored, it’s not easy to determine the nature of the corrosion product. George Gorecki, A&ox, Inc., Ill. GOLD PLATING I am working for West Virginia University. We have a microwave cavity that needs to be plated with gold (l-2 pm). Maybe it sounds simple for professionals but not for us. We have tried phosphate gold solution (1.3 g KAu(CN),, 120 g K,HPG,, 30 g KHaPO, per litre) and current density 1 mA/cm2. The gold layer looks matte and brownish. Is there any way to get a shiny finish without polishing? We would be glad if you could help us with advice. Alex Klymachyov, W.V.U. First, what type of waveguide/cavity material? I’ll assume copper, although aluminum is also possible, as well as ferrous based (you did say you were at a university). The next issue is that of the matte brown finish. Sounds like you’re burning the plate. I suggest a Hull cell analysis. Essentially, you plate up on a brass shim that is placed at an angle to the anode. This allows you to compare varying current density to plated appearance. I am also assuming that you are using an appropriate anode with appropriate surface area. A platinum-coated anode works best, although stainless stell can be used for short periods of time without damaging the bath. The surface area ratio of the anode to cathode should be about 1:1, although 2:l will be okay. Jeff Albom, jeffassd.loral.com RESTORING TITANIUM NITRIDE FINISH Titanium nitride (TiN) is a common hardfacing material for high-speed steel tools. It is usually a very thin coating deposited onto prc-sharpened high-speed steel. This makes tools less than half of the price of tungsten carbide tipped tools, tihich must be brazed and then sharpened. But resharpening TiN bits removes the TiN, exposing the softer steel. Does anyone out there know of a plant/company/
0 Copyright Elsevier Science Inc.
METAL FINISHING
. SEPTEMBER
1995