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Treatment of stainless steel surfaces

Some of the commonest causes of stains and discoloration on stainless steel surfaces are outlined below.

1. During transport / unloading:

When unloading the material, the haulage company's driver unwraps the parts and pulls them over an unprotected loading edge made of St37 (standard steel). In the process, standard steel is transferred to the surface of the stronger stainless steel, even if only slight pressure is applied.

The corrosion which then forms when these parts are stored in a damp environment (e.g. outdoors) merely affects this abrasion. The problem naturally also arises when conveyors or hoisting gear of standard steel come into direct contact with the stainless steel surface. For instance, when parts are attached to a chain sling without applying a protective layer between the standard steel chain on the crane and the stainless steel parts. In the presence of moisture, a brown abrasion stain will form on the stainless steel parts at the point of contact.

2. During interim storage:

The unprotected stainless steel parts must not be stored directly side by side – or worse still directly on top of one another – together with standard steel parts. Direct contact will naturally also produce the abrasion of standard steel on the stainless steel parts as described above.

When stored outdoors side by side, exposed to the effects of wind and rain, a so-called rust film forms on the stainless steel parts and cannot simply be wiped off. If stainless steel and standard steel have to be stored together, in direct vicinity of one another, then only when separated by a protective tarpaulin.

During storage, care must be taken to ensure that no work of any kind is performed nearby on parts containing standard steel. Machine-cutting standard steel sections, for instance, is a common cause of subsequent staining on stainless steel parts, as the glowing sparks produced when cutting can fly over great distances. The particular problem with this type of contamination is that the standard steel particles literally burn into the surface of the stainless steel.

3.During assembly or installation:

The instructions for our masonry support specify that the "supporting brackets must be supported until the mortar has hardened completely". Under no circumstances may they be supported by an unprotected standard steel girder! This would naturally give rise to abrasion of the standard steel, spoiling the entire appearance, especially of visible supports on windows. A wooden beam should therefore be used as support, taking care to ensure that it has not come into contact with standard steel in the past.

During assembly or installation, care must also be taken to ensure that implements of standard steel and splashes of standard steel do not come into contact with the stainless steel surface during processing operations, as this would cause abrasion or burn-marks on the stainless steel parts. Even tapping lightly with a hammer of common tool steel or with pliers would suffice to produce such an effect. Chrome-plated tools should be used as minimum – or better still, tools not made of standard steel.

4.During subsequent work on the façade:

Treating engineering brick walls with acid – often using hydrochloric acid or an agent containing hydrochloric acid – is a highly problematical process for stainless steel. Hydrochloric acid is one of the most corrosive chemicals for all metal materials.

For this reason, this chemical should preferably not be used near the façade where it can come into direct contact with stainless steel or where it could be carried away by the wind or by splashing. If it is absolutely essential to treat the entire façade with acid, all stainless steel parts should be protected by plastic sheeting before starting work. Any stainless steel parts which come into contact with the acid treatment agent despite these precautions must immediately be thoroughly cleaned with fresh water and a sponge.

Do not delay and let the acid take effect on the stainless steel parts first! However, if acid residues have occasionally been allowed to remain on the stainless steel parts for longer, the resultant stains are merely a visual impairment. The brown discoloration can then be removed with a special passivation agent, especially on visible window lintels. The passivation agent must also be thoroughly removed by rinsing with fresh water after being allowed to take effect. The passivating agent is described in more detail below.

Measures to remove stains due to abrasion, airborne impurities or acid treatment:

 Impurities which have formed on stainless steel surfaces cannot simply be wiped off with a cloth, even if they were caused by airborne impurities. The small particles have settled in the pores in the metal surface or are positively forced into the surface by rubbing or brushing action. These pores are to be found even in seemingly smooth metal surfaces, with the result that the brown stain due to standard steel impurities will always reappear after wiping or brushing the surface.

A passivating agent containing a light nitric acid is needed to remove the stains. Nitric acid is an oxidizing acid. It consequently speeds up the formation of a passive layer on the surface of the stainless steel. In addition to cleaning the surface by removing undesirable residues of standard steel and dirt, the agent's combination with nitric acid improves the durability of the stainless steel without attacking its surface. After being allowed to take effect for a corresponding length of time, the passivating agent must be thoroughly washed off with fresh water.

Coarse impurities can naturally first be removed with a stainless steel brush or with a pickling paste for stainless steel. This paste must be applied with a fine brush and subsequently washed off again with fresh water. The pickling paste is highly aggressive and may only be applied when wearing corresponding protective equipment, such as gloves, aprons and goggles, in order to prevent caustic burns.

Surface damage on stainless steel parts can be removed with special rough grinders or flap wheels for the electric drill. However, it is important not to damage the surface of the stainless steel part and to remember that the protective passive layer only forms naturally after a few days. In this case, too, the passivation process can naturally be speeded up considerably by using the passivating agent described above.

Surface treatment products and protective clothing

Surface treatment products and protective clothing can be ordered from us.
Please also always follow the instructions on the packaging!