Question: Over the past twenty years, we have become increasingly dependent on chemical suppliers. We would now like to reverse this trend and prepare as many electrolytes as possible using raw chemicals. There are several reasons for this. First and foremost, we want to have technological sovereignty in-house again. It is also a question of cost. If we use finished products from a manufacturer whose contents we only know in part, there is hardly any alternative for us. It becomes annoying if the manufacturer is unable to deliver due to a shortage of raw materials and we don't know which ingredient is missing. The changeover will not affect all products and will also take some time. During an initial inventory, we came across ready-made stripper solutions for tin layers. These are expensive, slow and heavily pollute the waste water. We would like to start with our own approach here.
Answer: We can very well understand the motives for becoming independent of suppliers. And despite the best intentions, this can lead to considerable problems. For example, in the event of a process changeover, a complaint and subsequent legal liability. In many cases - not only in the automotive industry - the coatings are qualified and approved according to a defined procedure. As soon as a parameter changes significantly - and this includes the chemical composition in particular - the approval process must be repeated. This means that even if you stay with one manufacturer but switch from one to another, this is difficult and sometimes even impossible.
In this respect, we can only advise you to first check the processes or systems for corresponding products and customers and then contact the customers concerned. Instead of arguing on the basis of price, it is better to argue on the basis of technological sovereignty and supply capability due to a shortage of raw materials and an uncertain market situation.
We would not completely exclude stripping from these mechanisms, but in most cases it should not be affected as it is usually internal rework. In the best case, it is not even a question of galvanized goods but of rack and barrel contacts.
Electrolytic demetallization
Tin from iron
Anodic stripping in 10% NaOH at RT. The control of the anode potential during galvanostatic operation (potential jump when reaching the iron surface) can prevent the attack on the base metal.
Tin and tin-lead from copper materials
Anodic stripping in methanesulfonic acid solution, approx. 45-70 %; the concentration must not be undercut, otherwise the passive range of copper is left and copper goes into solution at a lower acid concentration; exceeding the concentration greatly reduces the stripping speed.
Chemical processes
Tin from iron
Alkaline solution with nitroaromatic compounds, e.g. 100 g/L NaOH, 100 g/L p-nitrophenol, temperature 70-90 °C
Strongly hydrochloric acid solutions with antimony (lll) compounds, tin goes into solution as tin (ll) with hydrogen evolution; however, a loose antimony coating forms which must be removed; e.g. 20 g/L antimony trioxide, 840 ml conc. HCl.
Tin from copper materials
Peroxide-containing demetallization solution with 150 g/L 35% hydrogen peroxide, 100 ml/L H2SO4, 60 g/L fluoride as NH4F, e.g. 3 g/L 8-hydroxiquinoline as stabilizer.
A solution with nitroaromatics such as 150 g/L m-nitrobenzene sulphonate (oxidizing agent) and 200 g/L tetrafluoroboric acid only attacks tin, but not a possible intermetallic Cu6Sn5 layer; if thiourea is also used, the diffusion layer is also dissolved.
Dissolve in 5-40% HNO3 when using copper inhibitors and sulphamic acid to avoid nitrous gases.
Solution with 5-20% hydrochloric acid and 2.5 g/l copper chloride in an oxygen-saturated solution (spraying process); oxygen is necessary to oxidize the monovalent copper produced by Cu-(ll) during tin dissolution; the copper content must not exceed 6 g/L, otherwise the copper base material will be too strongly attacked.
