Surface treatment

Technical Details

Treatment of the stainless steel surface

The fasteners supplied by Modersohn are made from high-grade stainless steel in accordance with the permit for stainless steel issued by the Construction Technology Institute (material number 1.4571 or 1.4401). A manufacturer‘s certificate (version 3.1 B) can be sent to you on request at any time with further information on the mechanical properties and chemical composition of the steel supplied.

The parts are finally pickled and passivated in our production so that the passive layer reforms immediately after machining and normal steel abrasion is excluded on the surface of the stainless steel parts.

If stains of any kind nevertheless appear on the surface after delivery to the building site, they are in no way related to the actual resistance and load-bearing capacity of the stainless steel supplied by our company.

Some of the commonest causes of stains and discolorations on stainless steel parts are outlined below.

1. During transport / unloading:
When the parts are unloaded, the haulage company‘s driver removes the parts from their packaging and pulls them over the unprotected loading edge which is made of St 37 (normal steel). Even slight
pressure is sufficient for normal steel to be transferred to the surface of the harder stainless steel. Only this abraded material subsequently corrodes during storage in damp conditions (e.g. outdoors).

This problem naturally also arises when handling or hoisting gear made of normal steel comes into direct contact with the stainless steel surface. For instance, if the parts are placed in a crane chain without any protective layer between the normal steel chains on the crane and the stainless steel parts. A brown abrasion stain forms on the stainless steel parts at the points of contact, particularly in combination with moisture.

2. During interim storage:
The unprotected stainless steel parts must not be stored directly beside one another or worse still on top of one another with normal steel parts. As already mentioned, the normal steel abrades onto the stainless steel parts with which it is in direct contact. When stored side by side out of doors, exposed to wind and rain, a so-called rust film forms and is precipitated onto the stainless steel parts too. It cannot simply be wiped off. If stainless steel and normal steel must be stored together in direct vicinity of one another, then only if separated by a protective plastic sheet.

It is also important to ensure that parts containing normal steel are not worked in any way nearby during storage. Cutting normal steel sections, for instance, is a common cause of staining on stainless steel parts, as the glowing sparks can fly over great distances. The greatest problem with this type of impurity is that the normal steel particles burn into the surface of the stainless steel..

3. During installation:
According to our installation instructions for supporting the brick facework „The brackets must be fully supported until the brick mortar has set hard.“ Under no circumstances, however, may a normal steel support be used for this purpose! Here too the normal steel abrades and will spoil the entire appearance, particularly in the case of visible supports over windows. Always use a wooden beam which has not been in contact with normal steel beforehand when supporting the brackets.

It is also important during installation to ensure that tools or splashes of normal steel do not come into contact with the stainless steel surface, thus causing abrasion or burn marks on the stainless steel parts. A light tap with a hammer made of common tool steel or pliers will suffice to produce such marks. The tools used should at least be chrome-plated; they should preferably not be made of normal steel at all.

4. During subsequent work on the façade:
Das Absäuern der Klinkerwand - vielerorts üblich mit verdünnter Acid-treatment of the facing wall is an extremely problematical case for stainless steel. It is not uncommonly carried out using dilute hydrochloric acid or an agent containing hydrochloric acid. Hydrochloric acid is one of the most corrosive chemicals for all metallic materials. This substance should therefore not be used on any façade parts in which it could come into direct contact with stainless steel or which it could reach either as droplets carried by the wind or through spraying! If acid-treatment of the complete façade is absolutely essential, all stainless steel parts should be protected with plastic film before starting the treatment! Any stainless steel parts which nevertheless come into contact with the acid-treatment agent must immediately be thoroughly cleaned with fresh water and a sponge. Never wait before cleaning the parts and let the acid work its way into the stainless steel!

However, any stains which form due to acid residues remaining
on the stainless steel parts for some time merely constitute a visual
impairment. The brown discoloration can be removed with a special
passivating agent, particularly on visible window lintels. Once it
has been allowed to take effect, the passivating agent must also be
thoroughly removed by rinsing with fresh water. The passivating agent
is described in more detail below.

Action to remove stains due to abrasion, rust film or acidtreatment:

Impurities which have formed on stainless steel surfaces cannot simply
be wiped off with a rag, not even if they are merely due to a film of
rust. The minute particles are wedged into the pores on the metal
surface or are forced into the surface by the rubbing or brushing
action. Such pores are even to be found in seemingly smooth, coldrolled
metal surfaces, with the result that the brown discoloration due
to normal steel impurities constantly reappears after being wiped or
brushed off.

These stains can only be removed with a passivating agent containing a weak nitric acid. Nitric acid is an oxidizing acid. It therefore speeds up the formation of a passive layer on the surface of the stainless steel. In addition to removing undesirable normal steel and dirt residues, the composition of the nitric acid used also enhances the mechanical strength of the stainless steel without attacking its surface. After a corresponding exposure time, the passivating agent must be removed by thoroughly rinsing with fresh water.

Coarse dirt can naturally first be removed with a stainless steel brush or a pickling paste for stainless steel. The paste is applied with a brush and must subsequently be washed off again with fresh water. The pickling paste is highly corrosive and may only be applied with suitable personal protective equipment, such as gloves, apron and goggles, in morder to avoid caustic burns.

Surface damage on the stainless steel parts can be removed with special roughing or flap wheel attachments for your electric drill. However, it should be remembered that the surface of the stainless steel will be damaged in the process and that the protective passive layer only re-forms automatically in the course of several days. Here too, the passivation process can of course be speeded up considerably with the aid of the passivating agent described above.