Soon 40 years, Savcor´s innovative technology has been used in Pulp and Paper Industry to prevent stress corrosion cracking, to stop corrosion or to slow down thinning in numerous digesters around the world. It is also used to prevent pitting, general corrosion or stress corrosion cracking in stainless steel equipment in bleaching plants, in liquor tanks or in water circulation systems of a paper machine – whatever material has been used.
Savcor knows that the life time of the structures can be extended, even multiplied, with a well-planned and perfectly executed Electrochemical Protection Systems. Further, structural risks can be detected and alerted with Savcor’s advanced monitoring systems.
Savcor´s solutions in Pulp and Paper Industry increase production and safety while saving resources, maintenance cost and environment.
Digesters, impregnation vessels, storage tanks and liquor clarifiers in the pulp industry were traditionally made of carbon steel which is reasonably corrosion resistant in liquor. The corrosion resistance in alkaline environments is based on a protective oxide film that forms on the surface of the metal.
Process development for improved productivity has changed the situation. Chemical dosing and feeding points have been altered. Chemical circulations have become more closed for environmental reasons. The natural protective oxide film is not resistant enough anymore: Low alkali in liquid dissolves oxide and results in general corrosion or erosion corrosion. Repair welds and welded seam are prone to further corrosion failures. In impregnation vessels and digesters, stress corrosion and corrosion fatigue are the most severe and dangerous causes of failures.
In Anodic Protection (AP), the metal surface is polarized with rectifiers feeding direct current through cathodes. Anodic current forms a strong passive oxide film on carbon steel and makes it corrosion resistant, even in the harshest environments. Erosion corrosion or pitting corrosion cannot cause failures on surfaces with anodic protection. Dangerous cracks by stress corrosion are also avoided. When installed in old equipment, anodic protection prevents corrosion already in action. Savcor Anodic Protection has proven very effective in digesters and impregnation vessels.
A Cathodic Protection (CP) operates with opposite direction of the current. Compared with an Anodic Protection current is fed through the anodes and the protected vessel is the cathode. In mild steel structures, the steel will be driven by the external current of an EPS system to a status of immunity against corrosion whereas stainless steel will be maintained in a region of perfect passivity.
The key for protection control is measuring the electrochemical potential of the protected structure. The required protection current is adjusted based on the potential values measured. In an Anodic Protection, the target potential is more positive than the natural potential. With a CP, the protection current will change the potential to a more negative value.
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