Pulp & Paper Industry
40 Years of Cooperation
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.
Process development has changed the corrosion 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. Savcor has tailor-made proven Electrochemical Protection Systems for a wide range of equipment in the pulp and paper industry.
Savcor Electrochemical Protection System provide long term protection extending significantly the life time of the equipment
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.
The bleaching filters in the pulp industry are typical examples of the equipment where corrosion can be prevented by Cathodic Protection. The rotating filter drum is a very expensive part of the filter that greatly benefits from CP. Naturally also the inlet vat and the filter vat will be protected to prevent leaks.
References in Pulp & Paper
Anodic Protection for Pulp Digester, Finland
In early 80’s, then new, Metsä-Botnia Kaskinen Pulp Mill had cracks in their impregnation vessel caused by stress corrosion. There was a threat of immediate close-down based on the pressure vessel code. Instead, Savcor suggested a new electrochemical solution, Anodic Protection, for the remedy first time to be tested in a Finnish pulp mill. The protected vessel remained operative until the mill was closed in 2009, a healthy 25-year lifetime extension with minimal maintenance costs.
Anodic Protection for Pulp Digester, Portugal
In late 80’s, the Soporcel, Figueira da Foz Pulp Mill (now Navigator Co.) in Portugal had severe cracking in their impregnation vessel of the pulp digesting process caused by stress corrosion. Again, there was an imminent threat of banning the vessel from use. This time Savcor had a ready and tested solution, Anodic Protection. The system had its third control system update in 2017, 30 years after the AP installation.
Anodic Protection for Pulp Digester, Finland
In 2002 Oy Metsä-Botnia Ab (now Metsä Fibre Oy) Kemi’s pulp mill decided to change its pulping process to the modified cooking. Prior to the process change, it was recognized that the modified process conditions pose a very significant corrosion risk. Because of this it was decided to install the Savcor Anodic Protection System to the digester to prevent corrosion. In 2018, Anodic Protection with Savcor’s monitoring service has been in operation for 16 years. It has protected the digester against corrosion and significantly reduced the maintenance costs.
Cathodic Protection for Bleaching Filters, Japan
Daishowa Paper Co. ordered Savcor CP to protect their bleaching filters in 1991. After the installation, more than 90% reduction in the corrosion rates was measured by Daishowa. The main savings came from the cutting of the maintenance costs and especially due to the related down-time.
Anodic Protection for Pulp Digester, Brazil
In 1994 Bahia Sul in Brazil (now Suzano Mucuri) installed Savcor Anodic Protection in their digester, then one of the largest in the world, to counteract the measured corrosion rate as high as 2.5 mm/year. The installation was successfully completed in only 38 hours, a significant reduction from two weeks required by other repair methods. This meant vast gains in production when the down-time was minimized. Afterwards a 95% reduction in corrosion rate was reported by the mill.
Anodic Protection for pulp digester, Brazil
Cenibra digester was started up in 1975. In 2000 the new cooking process increased the production up to 1400 t/d. The new process activated corrosion which reached a speed of up to 5 mm/a. Corrosion took place as general thinning and stress corrosion cracking. In 2003 the Savcor Anodic Protection was installed and corrosion stopped. Currently the mill continues with the modern process and with the Savcor AP without any corrosion risks.