How to Process and Store High-Performance Austenitic Stainless Steel Safely?

How to Process and Store High-Performance Austenitic Stainless Steel Safely?

Maintaining a smooth and clean stainless steel surface is the most important rule that stainless steel fabricators need to follow. If care is not taken during handling and fabrication, the passive film on the stainless steel surface will be destroyed, reducing the corrosion resistance of the stainless steel. Surface contamination during transportation or at the work site, especially carbon steel particles and iron scale, as well as surface roughness or scratches, are common causes of passive film destruction. If the roughness Ra>0.5 microns of the stainless steel surface after grinding, the pitting resistance will be reduced. The loss of corrosion resistance caused by rough grinding can be minimized by acid passivation.

The fabricator should develop workshop operating procedures to protect the stainless steel surface, including raw material receipt and storage, all handling and processing operations, and finished product storage, shipment and pre-assembly. The procedures should also list the raw material factory information to ensure traceability from raw material receipt, storage to finished product shipment.

Initial delivery and storage

When ordering stainless steel, waterproof packaging should be requested and transported by van as much as possible. This is especially important if the stainless steel may be contaminated by halides (salts) during transportation. For example, exposure to de-icing salt during sea transport or winter transport. Stainless steel transported by ordinary trucks should be inspected and if there is a possibility of water ingress to the outer packaging, the packaging should be opened and dried. If there is salt on the surface of the stainless steel, the packaging should be removed and the surface should be thoroughly rinsed. Packaged stainless steel should be stored in a clean and dry place indoors.

Stainless steel plates, tubes and other large products can be stored outdoors and, if necessary, in a covered warehouse. Avoid contact with sea salt, de-icing salt, and air pollutants such as acid mist and grinding dust. The design and construction of the shelves should prevent carbon steel contamination and avoid contact with stagnant water. Stainless steel should not be protected with oil, paint or other coatings during storage or processing.

Stainless steel should be stored in a clean area, isolated from carbon steel, and away from carbon steel processing areas. If stainless steel and carbon steel are processed in the same workshop, carbon steel dust will fall on the surface of the stainless steel, which can easily cause iron contamination on the stainless steel surface. This is very important because the surface of stainless steel contaminated by carbon steel particles (i.e. free iron) is prone to rust and pitting during use.

The stainless steel elbow on the left is contaminated by carbon steel particles because sparks from nearby grinding of carbon steel splashed onto the stainless steel surface

Thin steel plates cut to length should be placed horizontally in wooden boxes to maintain straightness and avoid edge damage. To ensure surface quality and finish, paper or plastic film should be placed between the steel plates. Cardboard or other materials that may leave marks on the surface of the steel plates should not be used for highly polished surfaces. The packaging box must have a lid to prevent atmospheric contamination. Thicker steel plates are placed upright or nearly upright on dry, covered shelves to minimize atmospheric contamination and avoid footprints from contaminating the steel surface.

Stainless steel should not be in contact with galvanized steel or copper, nor should they be stored in the same area. Surface contamination caused by copper, zinc or similar low-melting-point metals can cause stainless steel to become embrittled (liquid metal embrittlement) during welding or heat treatment.

Stainless steel coils and cut parts are stored in a clean distribution center ©Vogel-Bauer

Peelable protective film

Stainless steel sheets can be ordered with a plastic protective film, which is specially designed for subsequent laser cutting, bending, forming, marking and transportation to the site. The protective film reduces or prevents the surface from being scratched or contaminated by other metal particles throughout the entire processing process. Applications that require strict appearance or surface quality, such as building components, household appliances and surfaces that come into contact with food, cannot do without peelable protective films. For short-term storage and processing, the protective film does not need to be removed.

Such protective films used in the processing process usually cannot withstand ultraviolet (UV) radiation, such as exposure to sunlight. If UV radiation cannot be avoided, it is necessary to remove and replace the protective film before the finished product is installed and shipped. Protective films that are not suitable for UV environments will cause the adhesive to deteriorate even when exposed to UV radiation for a short time. This makes the protective film brittle and almost impossible to remove from the metal surface without leaving a mark. UV-resistant protective films can generally withstand extreme temperature changes. UV-rated protective films are suitable for parts that will be exposed to sunlight on the construction site.

All protective films have an expiration date, which is determined by the date of manufacture and the environment in which they are placed, including the maximum storage temperature and other factors that limit the service life. Depending on the environment, the shelf life of the protective film varies from 3 to 9 months. Depending on the use period of the protective film, the protective film should be removed before it expires. When purchasing products with removable protective films, you should ask for information on the use period, storage temperature and UV exposure.

If the removal method is not appropriate, the adhesive on the protective film will remain on the stainless steel surface and will be difficult to clean. There are many types of adhesives, and you should consult the protective film manufacturer before cleaning the residual adhesive. Some can be removed with household detergents and water, while others require special debonding agents. Residual adhesive should be cleaned as soon as possible.

Stainless steel with protective films should also be stored in a temperature-controlled, low-humidity environment. All protective films are not moisture-proof, and rain, fog and moisture, as well as dissolved salts and other contaminants, can penetrate the protective film. This can cause crevice corrosion, which is more likely to occur in coastal areas or areas where deicing salts are used, where the chloride content is high. Therefore, if stainless steel is to be installed outdoors where it may be contaminated by chloride ions, it should be stored off-site before being transported to the site for installation. Once the panels are in place, the protective film should be removed.

Peel-off films protect the stainless steel surface from scratches and stains during manufacturing and transportation ©IMOA

Avoid iron contamination

Avoid contact with carbon steel during manufacturing and transportation. Shipping clamps, straps, chains, hooks and other devices should be stainless steel or protected with plastic or cardboard to prevent carbon steel contamination. Special tools should be stainless steel-specific. Different colors can help distinguish carbon steel tools from stainless steel tools.

Only clean, iron-free grinding wheels, belts and abrasives (glass or ceramic beads, corundum and stainless steel shot) should be used on stainless steel. Abrasives that have been used on carbon steel should not be used on stainless steel. Iron particles in unclean abrasives will embed in the stainless steel surface and cause rust. Carbon steel beads should not be used to clean stainless steel. For the same reason, only stainless steel wire brushes and stainless steel wool should be used, and they must never be used on carbon steel.

Carbon steel strapping leaves rust on stainless steel pipes

Using a carbon steel wire brush to "clean" stainless steel welds causes discoloration and rusting. Stainless steel wire brushes must be used © Outokumpu Stainless

Rollers, punches and dies that have been used on carbon steel must be thoroughly cleaned to remove any particles or scale before being used on stainless steel. Lining paper or plastic film can prevent contamination of carbon steel particles on forming tools.

If stainless steel is contaminated with carbon steel, it must be acid passivated to remove the contaminants and restore the surface. If a large number of carbon steel particles are embedded in the stainless steel surface, it may be necessary to pickle with stronger acids to remove all iron contaminants in time. However, the most cost-effective solution is to completely avoid carbon steel contamination.