Optimal corrosion protection

• oxide layers, cinder and traces of tarnish
  
• even the smallest traces of other metals
  
• chloride-, bromide- and iodine ions
  
• stresses stemming from mechanical processing

surface treatment
kind of treatment	method	danger
blast	blasting with glass beads	contaminated blasting material
abrasion	mechanical wear off by means of coarse grain	new surface tension
pickling	chemical wear off by means of acid	used up acid
polishing	electrochemical wear off	to strong wear off

Table: surface treatment

	The pickling bath as the superior pickling technique

• even treatment also for hard-to-reach areas
  
• reduced emission loads
  
• reduced environmental pollution
  
• automated and free of human errors
  
• reasonable costs

• qualified and well-trained staff
  
• state-of-the-art installations
  
• waste water treatment and orderly disposal of residual wastes

• the concentration of the mixed acids (hydrofluoric acid, nitric acid),
  
• the addition of acid resistant detergent at the exact concentration,
  
• temperature and duration of the pickling bath.

Flushing and passivation:
After each pickling process it is important to flush thoroughly with water. Drinking water with a Cl content of less than 50 ppm is well suited for this purpose. In order to avoid the drying of acid residues which would prevent the formation of a passive layer, all residues of acids have to be removed.
High-pressure washing equipment is advantageous because dissolved cinder and other surface deposits can be removed without effort. To be preferred are cold water devices which avoid too fast drying which in turn would lead to the formation of stains.
Within several hours a passive layer with a thickness of about 0,005 µm (five to ten molecule layers) is forming due to the oxygen content in the air. This passive layer is the requirement for the long life of stainless steel.

techniques of pickling
kind of pickling	composition	for treatment of
spray pickling	pickling acids and detergents	bigger containers and work pieces
pickling paste	diluent and pickling acid	welding joints and local corrosion
surface pickling	detergents and orthophosphoric acid	already pickled surfaces
pickling bath	pickling acids and detergents	complete stainless steel components

Table: techniques of pickling

	Detailed description of the pickling plant of the HUBER company

Beizen und passivieren der Oberfläche The actual pickling process takes place in an pickling pool with a volume of 45 m³. This pool volume is necessary in order to pickle even the largest items with the bath method. In the pickling bath processing-related material changes and alterations (caused by edging, welding, or caused by the contact with other materials) are reversed. Especially ferritic inclusions, chromium-carbide formations and changes in the cristalline surface structures are eliminated. Thus, later-occuring types of corrosion that are associated with stainless steel, in particular intercristalline corrosion, grain border corrosion and contact corrosion are avoided. The pickling solution consists of nitric acid and hydrofluoric acid. The nitric acid serves for the oxidation of undesired materials and inclusions, while the hydrofluoric acid has the purpose of keeping the developing oxides in solution. The content of the etching bath gets constantly circulated while being kept at a constant temperature of 20 - 30° C through a heat exchanger. The bath is continously kept free of mechanical pollution by the action of a filter. The pickling bath is always kept closed. It is opened only for loading and unloading. Before the cover is removed waste air is sucked off through slots on the side. The waste air is first cleaned by passing it through filters before it is released into the atmosphere. Thus, the formation of foul smelling odours is prevented. In a room next to the pickling plant, the pickled stainless steel items are passivated and cleaned off acid residues using spray- and steam pressure installations. The flushing water is collected in a collection pool. Later, it is centrally removed from there. First the flushing water is neutralized with calcium hydroxide. The addition of the neutralisation agent takes places in portions while the pH of the flushing solution is continously monitored. Once neutralized, the flushing water is passed through a heavy metal seperator where heays metals are secreted through the action of four consecutive inclined treatment groups that have flocculation compounds (polyacrylamid, polyacrylate) added to them. The heavy metals are removed and drained in a chamber filter press. The filter cake is disposed of in a hazardous waste facility. Finally, the cleaned waste water can be taken from the neutralisation plant. Once again, the pH of the water is measured automatically and is recorded. Additional samples that are examined for their pH-values and heavy metal contents are taken discontinously from the effluent stream. In this manner, a supplementary running control is guaranteed. The bottom and the side walls of the entire pickling- and passivation bath are coated with shock-resistant and acid-proof plastic so as to avoid that water from the bath sprays outside. The residues that reach the canalisation with the effluent water correspond to the regulations: Cr <= 0,5 mg/l Cr6 <= 0,1 mg/l Ni <= 0,5 mg/l NO2 <= 5,0 mg/l F2 <= 20,0 mg/l The plant required a considerable investment of more than 300,000 ¬. However: only with this plant it is possible to achieve a professional, necessary surface treatment of stainless steel. And it is possible that the surface treatment of Huber stainless steel products which have their use in the environmental protection field, can take place in an egologically-sound manner.

Can stainless steel rust ?

What does the material stainless steel offer ?