An Overview of Stainless Steel Pickling

Steel pickling is a kind of acid pickling done for surface treatment. Strong acids are used for this purpose. This treatment changes the outer layer of steel. As a consequence, the steel becomes clean, smooth, and highly resistant to corrosion.

Pickling is a common process to achieve or restore the shine and brightness of the surface. This explains why stainless steel is composed of at least 10.5% chromium. This component forms a protective passivation layer that keeps the steel surface immune to air or moisture.

Why Steel Pickling?

Let us now study why steel or stainless steel should be pickled. Pickling is done to season the surface to make it highly corrosion-resistant. The outer surface of the material usually gets affected during the manufacturing process. As a result, the output often gets visible damage or might contain significant impurities.

The types of damages and impurities are listed below:

• Formation of scale layers during heat treatment

• Formation of annealing colors during welding, grinding or similar types of work

• Deposition of metal extraneous rust or metal oxides

• Iron abrasion due to processing with steel tools

• Formation of chromium carbide through heat influence (This happens when no cooling lubricant is used for drilling)

• Microstructural changes leading to martensite formation during cold forming

Behavioral Changes in Stainless Steel Structure During Pickling

Stainless steels having varying microstructures exhibit different reactions when pickling is done on them. Extra care is taken to pickle stainless steel made of martensite because it tends to crack in the tempered and hardened state.

Ferritic materials are prone to over-pickle because they have significantly low corrosion resistance. Therefore, milder pickling is usually done on these materials.

Preparations before Pickling

Before the pickling process starts, proper cleaning of the stainless steel surface is a must. Cleaning helps chemicals act evenly. The surface must be cleaned of oils, greases, stickers, and lubricants for an optimally satisfactory result. Usually, acid solutions are best to remove rust as well as degrease the surface.

These processes are performed at higher temperatures. If the stainless steel parts feature stubborn soiling, an electrolytic or ultrasound process is used to get the desired result.

The workpieces are rinsed properly after cleaning. Afterward, they are made acid-free and dried completely. The material is ready to undergo the next steps.

Pickling Agents

Usually, the solutions used for stainless steel pickling are based on hydrochloric or nitric acid. The following factors should be taken into consideration when choosing the right pickling solution:

• Which type of stainless steel is ideal for pickling?

• How high should the degree of scaling be?

• What are the surface requirements for stainless steel pickling?

Other important factors include further welding process and annealing. The right exposure time is equally important in the scenario.

Depending on the medium that has already undergone the pickling process, the surface features a corrosion-resistant passive layer after pickling and rising. In case, faster passivation is needed, the stainless steel is directly treated with an oxidizing medium such as hydrogen peroxide or nitric acid. This is called synthetic passivation. It takes a few seconds to minutes to complete.