Shop Problems Plating and Anodizing Use the Shop Problem Card in this issue for free expert advice.

READERRESPONSE

We repeat the question and answer originally published for convenience.

Rust Formation After Polishing

Q . I have a marine fabrication and ?? welding shop. The work con-

sists of mostly stainless steel, which is often polished to a mirror shine. One problem is the development of surface rust, another problem is polishing hard to get to spots, which will still show color from the heliarc weld. Is it possible to use electropolishing to solve these problems?

J.H.

A . Passivation of the stainless steel ?? after polishing will certainly

help to eliminate the surface rust. This can be accomplished by electropolish- ing or by immersion in a room- temperature or heated passivation bath, consisting of 40% by volume nitric acid, for about one hour.

R . I have a concern with your ?? answer to the question entitled,

“Rust Formation After Polishing,” which appeared on page 62 of the March 1995 issue.

Heliarc welding of nonstabilized stainless steel will result in “sensitiza- tion,” also described as chromium carbide precipitation in the weld-heat- affected zone. Passivation will not correct the problem, which requires reannealing after welding or use of stabilized stainless steel grades, such as 321 or 347.

What is Sherardizing?

Q ?? Can you tell me what is meant ?? by the term “Sherardizing?”

R.H.

A . At the turn of the century, ?? Sherard Cowper-Coles found

that when iron and steel surfaces were packed in zinc powder and heated, a rust-proof surface was produced. He

patented the method for protection of steel under the name “Sherardizing.”

The process uses finely divided zinc dust, a product of zinc distillation. When the iron or steel is heated in the zinc dust, an amalgamation of the two metals occurs with the formation of a corrosion-resistant iron-zinc alloy em- bedded into the surface.

Getting the Lead Out

Q . We plate 63% tin/37% lead ?? alloys on the terminals of pas-

sive components (SMD). Is it possible to plate pure tin and eliminate the lead? In what ways might this be problema- tic? What is the current trend in the telecommunications industry?

E.E.

A . If you plate pure tin, there is the 9 possibility of tin whisker for-

mation, which can be detrimental to low voltage devices by causing short circuits. The industry trend is toward lower lead alloys, such as 93% tin/7% lead or 90% tin/lo% lead. Other alloys including tin-bismuth and tin-indium have been tested but no universally acceptable replacement for tin-lead has been found.

Black Pits in Brass Castings

Q . We get very tiny black pits in ?? our solid brass sand castings,

which cannot be removed by polishing or vapor degreasing. These become more pronounced after lacquering.

Will you kindly advise how to handle these?

J.V.

A . The problem is likely due to ?? porosity in the casting. A more

vigorous cleaning step is required. You might try ultrasonic cleaning in a mild alkali or else an acid pickle. A simple pickle would be 10% by volume of sulfuric acid operated at room tempera- ture up to 125°F.

If these procedures aren’t sufficient, you could try bright dipping in a solution consisting of 55% by volume phosphoric acid, 20% by volume nitric acid, and 25% by volume acetic acid at a temperature in the range of 130- 175°F.

Precipitate in Activator Bath

Q . I have recently been preparing ?? my own stannous chloride acti-

vator bath using 70 g/L stannous chloride and 40 ml/L concentrated hydrochloric acid. This bath forms significant amounts of white precipi- tate after two or three days. Is there a method to improve stability and/or a commercially available product?

K.W.

A . The stannous chloride is being ?? oxidized to stannic chloride by

oxygen in the atmosphere. A number of vendors offer proprietary additives to improve the stability of the solution.

Coated Anodes in Copper Strike

Q . In our copper strike, we use 316 ?? stainless steel anodes and fre-

quently find a dark film coated on them. In addition, the copper strike solution turns to an intense yellowish color even after a few days. We maintain the bath at 30 g/L copper, 15 g/L free cyanide at a pH of 12-13. We also find that the silver deposit be- comes rough as the copper bath turns yellow.

Y.K.

A . You cannot use stainless steel ?? anodes in the copper strike. The

anodic electrolytic attack of the alka- line solution will strip chromium from the stainless steel. Switch the anodes to cold-rolled steel, which is not attacked by the solution.

60 METAL FINISHING . MAY 1995