Technical information

Chemical Surface Treatments

Successful use of stainless steel depends on environmental conditions, quality selected, surface quality, customer expectations and defined maintenance conditions.

 Stainless steels offer precise solutions, but working and environmental conditions at the corrosive boundary will have significant effects on surface quality with high durability expectations. Surface qualities are obtained mechanically with abrasives and chemically. Understanding mechanical and chemical processes, understanding surface characteristics and how it affects ensures the best possible result for customers and their structures. Chemical treatment improves the corrosion performance of the steel and, of course, its appearance during use. Stainless steels are more resistant to corrosion if their surfaces are clean and smooth.

The meaning of clean means that it does not contain a pollutant layer and adhesive on the surface, or does not contain the surface itself with the steel itself (as in carbon steel or in salt), or factors such as a starting point or a gap or crack that can form a corrosion starting point.

Uniform means having a minimum area of ​​"micro" level. Abrading the surface mechanically can make the surface of the steel rough or streaked and bury unwanted small particles on the surface.

The general feature of chemical processes is that they clean the entire surface of the steel. Chemical treatments can also smooth or roughen the surface of the steel, depending on the chosen process, or leave the surface unchanged. But if they are properly selected and applied correctly, all chemical processes increase corrosion resistance.

Stainless Steel Products

During the production of steel, the sulfur content of the steel is kept under strict control. Even under these conditions, some sulfur particles remain in the steel and form corrosion start points. In these conditions, chemical surface treatment increases the surface quality by taking these particles.

Many stick-shaped products contain slightly higher sulfur content when manufactured. Therefore, chemical surface treatments are even more important for such products. In general, the flatness of flat products, especially cold rolled flat products (such as sheet, plate, roll) decreases and their surface flatness increases.

A typical No.1 surface has a thick plate with minor surface defects, invisibly small indentations and protrusions, and a 5-6 micron surface roughness (Ra).

A typical 2D surface cold mill 1.7 mm sheet has a slightly better appearance than a 0.2 micron surface roughness or hot roll outlet.

            It will be created on new surfaces during production processes (cutting, bending, welding and polishing). The corrosion performance of the newly formed surface will generally be lower than it is supplied because the surface will be more rough, or sulfur inclusions just below the surface will be exposed to the surface or contamination after treatment. Properly applied chemical treatments will keep the surface clean as well as guarantee optimum corrosion performance.

Chemical processes can be examined in four groups:

Immersion in Acid Bath: The dipping process in the controlled acid bath will remove all the dirt and debris on the surface (also those that depend on high temperature such as heat treatment and welding). The process of dipping into the acid bath also means that a small portion of the chromium oxide layer formed on the surface is removed.

Passivation: It is the oxidation of acid or chemicals that take the residue and increase the chromium oxide layer on the surface.

Clamping Chemicals: Clamping chemicals are chemicals used to remove residues that cause surface contamination.

Electro-Chemical Processes: It is a chemical process that removes the residues on the surface and provides the surface to be polished and smoothed.

Immersion in Acid Bath

Dipping into nitric and hydrofluoric (HF) acid mixtures is the most used and effective process. Acids can be found as baths, gels and pastes. These mixtures contain up to 25% nitric acid and 8% hydrofluoric acid commercially. These chemicals penetrate the surface of the stainless steel so as to make the surface dull and rough.

Great care should be taken when working with these chemicals for health, the environment and safety. There are laws restricting the use of HF in many countries and calling it poison number 7.

Pasivation

Clamping Chemicals

Clamping chemicals are chemicals that are used to selectively clean certain parts of the surface. "COOH" Carboxylic acid group is the basis for the clamping chemicals used as cleanser, water softener and lubricant. Correct application of temperature and pH value is mandatory in order to perform its function fully and accurately. It is important to shake the pipes and vessels with turbulence after the process.

Cleaning with chelating chemicals is mostly dependent on the knowledge of the system and the commercial owner of the chemical and is not standardized like other described methods.

Successful use of this system is with the user manual or instructions created separately for each application.

Warnings

For chemical processes penetrating the surface in stainless steels, the reaction time increases with increasing quality. More attention should be given to free processing qualities, and the processes of these qualities are generally carried out with less effective chemicals. The addition of sulfur to steels causes chemicals to work more effectively during such chemical processes. Care should be taken when processing martensitic or low chromium ferritic stainless steels.

Detailed recommendations for each stainless steel grade are given below:

4 types of transaction categories are defined in the standards, but the most widely used are:

• ASTM A380 - Cleaning, peel and passivation of stainless steel parts, tools and systems

• ASTM A967 - Chemical passivation processes for stainless steel parts

• ASTM B912 - Passivation of stainless steels using electro-chemical methods

These invaluable standards provide detailed advice on the selection, implementation and evaluation of these processes. It is recommended to read.

Impurities and oils adhering to the surface prevent the surface from being subjected to the processes mentioned above. Therefore, the steel surface must be free of such obstacles before applying chemical treatments.

Many chemical processes described above contain strong acids. In order to remove such acids at the end of use, it must be deactivated. Permission is required from local municipalities and / or relevant ministries to deactivate and release them as waste. Most of the chemicals described above are in the class of harmful and toxic substances according to state health and cleaning laws, and their purchase, transportation, storage and use can be made within the limits specified by law.