Staining and Passivation: Final Surface Treatment of Alloyed Anti-Corrosion Steel

Alloyed anti-corrosion steel is more and more frequently used in our country in all the fields where it is suitable or favourable to arrange anti-corrosion characteristics without using mechanic anti-corrosion barrier only by using a suitably selected material.

In connection with the above stated there also more and more frequently appears the question of anti-corrosion resistance of the steel. Corrosion frequently appears in heat-affected zone of the weld as well as on surfaces that are not apparently damaged in the locations of attrition or abrasion of carbon material, external occlusions in the surface and in spots, where – due to thermal, chemical or mechanical loads – changed the chemical composition or structure on the surface of the material.

What is it caused by?

Usually by damaged or poor-quality passive layer. The resistance of alloyed anti-corrosion steels against corrosion occurrence is set by the microscopically thin layer of the alloying elements layer, so called passive layer. It is usually breached:

• thermally in the weld location and in its affected zone or different thermal load
• mechanically by abrasion, solder up and pressing foreign material in
• chemically

In case of the passive layer to be breached in any way, it is necessary to arrange creation of a new high-quality passive layer. A necessary pre-assumption for creation of fault-less and efficient passive layer is metal-clean surface free of any flakes and lead-on paints after welding and heat processing, occlusions, abrasions and sold-ins of carbon and other foreign bodies, mechanical impurities including paint inscriptions and old damaged passive layer.

How to reach metal-clean surface?

The most suitable and efficient way is mostly the chemical cleaning – so called staining.

In case of mechanic processing, the surface is not perfectly cleaned. In case of grinding, a part of flakes, lead-on paints and carbon material is removed, but simultaneously, the remaining part is ground and blurred on the whole surface and corrosion may appear even in case of slightly increased humidity. Grinding with a dull tool, coarse grit and with high removal., there may appear locations with high stress on the surface and these are prone to pitting corrosion in the environment with high contents of chlorides. In the course of blasting, a part of the abrasive material remains cut into the surface and the impurities remain below it together with the breached passive layer that we originally wanted to remove. So again, we will not reach the metal-clean surface and more, blasting increases several times the surface and accordingly even the risk of corrosion occurrence.

On the other hand, staining may be used – thanks to extensive variability – for reaching perfect metal-clean surface practically on any product or equipment under acceptable price conditions. Other benefits of staining include unification of surface appearance after correctly performed staining.

How the passive layer is created?

The passive layer is created on metal-clean stained surface in two different ways:

 

1. Reaction of metal-clean surface with air oxygen causes establishment of passive layer in the course of a few days (so called auto passivation). This method is sufficient for standard use of alloyed anti-corrosion steels.
2. The use of passivation means will immediately cause establishment of a passive layer that is several times stronger than in case of auto-passivation. This is performed in case of special use of alloyed anti-corrosion steels in power industry, chemical industry, etc. The work procedure in case of passivation solution use is identical with staining by spraying, respectively in a bath (see below).

What is the correct work procedure for staining?

Staining – regardless the method of staining means application – must be performed in compliance with the following scheme:

1. Surface de-greasing and removal of mechanic impurities.
2. Staining means application.
3. Action of the staining means (the time of action is dependent mainly on the type of alloyed anti-corrosion steel, degree of surface pollution and the passivation means used).
4. Rinsing with water with minimal pressure of 12 MPa (the pressure is important for perfect rinsing of the staining means and impurities even from poorly accessible locations). In case of staining of small and simple parts it is possible to use – instead of pressure water – thorough mechanic cleaning with a clout or brush under water.

What does the surface look after staining like?

The surface of alloyed anti-corrosion steel after staining must be metal-clean, without any residuals of impurities of any type (rust, residuals of carbon material, foreign occlusions, lead-in paints, flakes after welding and thermal loads, mechanical impurities). Staining makes the surface matt and simultaneously unified.

Which staining and passivation means are to be used?

The most popular and most frequently used staining means include the products of the ANTOX brand, characterised by high quality, efficiency and spreading rate. Our company FK system Brno, s.r.o. is an exclusive representative of the brand.
Depending on the way of the staining means application, staining may be classified into three basic methods:

• staining by submersion in a bath
• staining by pasta
• staining by spraying

Under condition of correct application, all the three methods are equal in fact as far as it is concerned the quality. But staining in a bath is usually significantly cheaper and the surface has maximally unified appearance.

• Staining in a bath is performed by submersion of parts into the staining bath. This is used for staining of parts of various sizes and shapes, the limitations include size of the tub and handling equipment, sometimes also the shape of the part. ANTOX 80 E is used for staining in a bath.
• In case of staining with pasta, the staining pasta is applied with a brush. This is suitable for staining of welds and small parts. This method of staining uses staining pasta ANTOX 71 E and ANTOX 71 E EXTRA, for polished parts it is ANTOX 3d.
• In case of staining by spraying, the staining gel is applied to the product by spraying, using a special application device or a simple spraying bottle with a pump (see figure 2). It is used for staining of large surfaces. It is possible to add colour indicator to the staining means so as to make easier distinguishing of sprayed and non-sprayed surface. Staining by spraying is performed using the means ANTOX 73 E, ANTOX 73 E PLUS and ANTOX 73 E EXTRA.

As a special feature, it is possible to perform staining of internal surfaces of piping by rinsing it with staining bath including de-greasing and staining to oxygen purity and purity for pharmaceutical industry.

Staining means are graded depending on their chemical composition from those least concentrated (E) for staining of low-alloyed chrome steels through medium-concentrated (E PLUS) for chrome-nickel austenitic steel up to high-concentrated means (E EXTRA) for staining of high-alloyed steels.

Work Safety and Environmental Protection during Staining

Staining means contain nitric acid HNO3 and hydrofluoric acid HF. That is why they are classified as caustics and poisons. The employees must undergo the safety and health protection training. It is necessary to use special protection aids. In case pasta staining, it is enough to use protective face shield, rubber overall and gloves. In case of staining by spraying or in a bath, it is necessary to use protective mask with a filter against acid emissions and all-rubber suit.

Waste water from staining and passivation is acid and it contains metals diluted from basic material. That is why it is necessary to trap it and liquidate it in environment-friendly way.

Is it possible for individual companies to perform staining and passivation separately?

The final result of staining depends on setting the right technological and work procedure, experience of employees, high-quality technical equipment, use of correct and high-quality staining means and other factors. Simultaneously, it is necessary to meet regulations on health protection and work safety. It is necessary to satisfy many legislative regulations and regulations on environmental protection. Due to these reasons, staining and passivation of large parts by spraying or staining in a bath may be performed only by companies with their own and approved special staining plant. We recommend the companies that do not have their own staining plant to perform staining of small party only, using pasta or gel.

Our company provides staining of large parts and equipment by spraying and staining in a bath. The staining baths are located in Brno, while we are able to provide staining of large parts by spraying directly at the client or on assemblage site (see figure 3). Simultaneously, we provide complete technical and consulting services referring the matters of staining and passivation. We perform the works including waste water trapping and liquidation. We hold all the licences and approvals for performing staining t the client.

We have performed staining and passivation of the most prestigious orders in the CR and SR including complete staining of technological equipment and tanks in the Temelín NPP, Dukovany NPP a Mochovce NPP.

Conclusion

Finally, it is possible to say that high-quality performance of staining and passivation provides – in case of correct selection of alloyed anti-corrosion steel – a guarantee of anti-corrosion functionality and unified metal-clear appearance of products.