During chemical polishing, the effect of solution and galvanic couples on a metal and its surface causes the formation of a passivation layer. The direct result of chemical polishing is microroughnesses smoothing and polish formation along with parallel dissolution of an upper layer. Polish enhancement is connected with the preventing of metal etching, resulting from the formation of a passivating film on a metal surface. However, electrochemical polishing leads to better polish rates, as opposed to chemical one.
The polished surface quality depends on the relation between the rates of layer formation process and its dissolution. The predominance of the first one leads to metal oxidation, while that of the second one leads to metal etching. The highest polish rate can be provided by the passivating film thickness being minimum, but sufficient to prevent etching solution effect on a metal. It is only possible when the layer formation process rate is equal to the rate of its chemical dissolution. The film thickness by chemical polishing is lesser than by electrochemical one. It can explain the lower microroughnesses smoothing efficiency and the increase of metal polish rate.
The positive results of the 12X18H10T type stainless steel polishing can be achieved with the use of solution in following ratio: Sulfuric Acid - 34%, Hydrochloric Acid - 6.5%, Nitric Acid - 4.5%, Sodium Chloride - 0.5%, Water - 54%, ZM dye Acid Black - 0.5%. The solution correction consists in periodic addition of water and nitric acid. The processing is carried out for 3-10 minutes at temperature of 70–75оC.
With the increasing of the iron salts content in the solution the processing time increases to 15-20 minutes. By chemical polishing, the surface quality depends on the density of workpiece loading into a bath. Overloading can cause uneven surface treatment and sometimes, as a result of insufficient access for the solution to treat workpiece surfaces, surface etching and formation of other defects.
After chemical polishing, the workpieces are to be cleaned in the running water and dried. It is advisable to carry out chemical passivation of polished products. And it should be noted that in comparison with mechanical polishing chemical one requires less labour and energy intensity. However, the chemical polishing process is closely associated with the following shortcomings:
- Processing complexity
- High rejection rate
- Toxicity and fire risk
- Equipment corrosion
- Expensive materials recycling