46092

Strips of Phosphor Bronze turn dark red or black  

+++++++

Dear Sir,

We are a Company in India manufacturing Copper and Copper alloy Cold Rolled Flat Strips, Sheets etc.

Usually we experience blackening/ Dark Brown/ Deep Reddish Color on Copper and Phosphor Bronze Strips, these cut-to-length sheets (size 350 mm width x 1200 mm length x 0.2 mm thickness) are packed as a bundle of 50 to 60 Nos. sheets one over the other and then the bundle is wrapped by a VCI Paper from Daubert; over the VCI we have a Polythene (poly ethylene) sheet which is wrapped over and then taped from all sides. (No Desiccant like Silica Gel is being used as the bundle has no free area and in case we put the Silica Gel bag over the sheets they dent the sheets) over this Polyethylene sheet we tie a Cover of Gunny cloth before final dispatch or transferring to the finished product warehouse.

The packed bundle

Surprisingly when the bundle is opened up after, say, around 10 to 15 days we get to witness this:

1. The Top and the Bottom Sheets which are in direct contact with VCI paper have the same original Color. The other side of the sheet which was not in contact with the VCI paper turns black/ deep reddish in the center area.

Topmost sheet, in contact with VCI paper	Top sheet lifted to show underside and next sheet
Bottommost sheet, in contact with VCI paper	Another discolored sheet in the stack

2. The Rest of the Sheets in the stack turns black/ deep reddish. The sheet color is original to about 50/60 mm from all the edges in a Zig Zag pattern. We attribute this to VCI paper as the vapor could have traveled to only 50/60 mm from the edges and thereby protecting it.

We would request the experts in this forum to help us to provide some measures to prevent this discoloration and to let us know the root cause of the problem.

Process inputs:

i) We pickle the Strips with 10% Sulphuric Acid – Water Rinse – Mechanical Brushing by Nylon abrasive Brushes with lots of water – Secondary Water Spray – Hot Water Spray (De-ionised / RO water (TDS Less than 40) – Heater Blower to dry the Strip – Recoil the Strip – Coil Transferred to Cut to length line – Stack Made – Wrapped with VCI paper – Wrapped with Polyethylene sheets- Taped – Sent to customer or Stored in Our Warehouse.

Questions:

a) What could be the reason of the Strip being discolored, and why is it being discolored, is it Copper Oxide or any other thing.

b) What trials should we carry out to ascertain the actual problem?

c) Any other input which experts would like to give. We would give the feedback after taking trials as suggested by the experts.

d) Some of our staff experiences that if the sheets are allowed to be in open (not Packed) this discolouration does not happen, and if we enclose the sheet in a packing this discolouration would happen. Is this true, if yes could some one explain.

e) One more thing which we would like to share here is that if we recoil the Strip in Hot Condition i.e. above 60 Degree C, after the pickling operation the sheets tend to discolor. Could this be attributed to the condensation of moisture between the coil wraps? If any one has an idea for this it would be much appreciated.

In case you need any further clarification please pm on this forum and we would revert back immediately.

With Best Regards / Vidu

VIDU GOYAL
MANAGER - REWARI INDIA

Use a light basic solution (less than 5%). Try first with CaCO3. Finally use a ultry light oil film (paper towel impregned and passed over the metal)in order to avoid all kind of reactions.
Let me know please the results.

Jose Castellanos
- Minneapolis, MN, USA