Stainless steels and other nickel-containing alloys are used extensively in the pulp and paper industry, providing excellent corrosion resistance against the process liquors and offering valuable mechanical properties.

Stainless steels and nickel-containing alloys have a long and successful history in the pulp and paper industry, finding applications in all aspects of the industry.


There are two types of chemical pulping processes; batch and continuous cooking. Mills that use batch cooking typically have several batch digesters, while mills that use continuous cooking may only have one or two continuous digesters; these are larger than batch digesters. Materials of construction for both types have followed parallel evolutions, from carbon steel to austenitic stainless steel and, most recently, duplex stainless steels.

Oxygen delignification and brown stock washing

Early oxygen delignification reactors were made from Type 316L (UNS S31603) stainless steel, showing that the process conditions are relatively benign. However, following external chloride stress corrosion cracking under the insulation in some 316L reactors, the industry migrated to standard and lean duplex grades.

Chemical recovery

This process recovers inorganic alkaline chemicals used to cook the wood chips. The high temperatures associated with evaporating black liquor above 40% solids make stainless steel essential for liquor-wetted surfaces. Stainless steels with higher chromium and no molybdenum, have the highest corrosion resistance to the strongest/hottest black liquors.
Lime mud from white liquor clarifiers is washed and fed to the lime kiln, where it is dried and calcined to produce lime. The chains in the hotter sections of the kiln are made from Type 304 (UNS S30400), Type 310 (UNS S31000) and 330 (UNS N03300) stainless steels, as they provide higher resistance to scaling than carbon steel.

Tall oil manufacture

Many older batch reactors were made using carbon steel with an acid-resistant brick lining. In the 1990s, batch tanks made from Alloy 20 (UNS N08020), less expensive than brick-lined steel, were constructed. Alloy 20 resisted corrosion by the alternating conditions of an alkaline environment when the soap enters the tank and the hot, acidic environment created when acid and steam are added to produce the tall oil. Comparable alloys, such as 904L (UNS N08904) provide acceptable resistance as does the nickel-based Alloy G-30 (UNS N06030). Super duplex grade 2507 (UNS S32750) is acceptable for tall oil batch tanks and for storing the stratified tall oil products from continuous acidulation.

Sulphite process

The sulphite process was once the principal method for producing chemical pulp, but has lost ground to the kraft and chemical-mechanical pulping methods. However, sulphite pulping remains important.
Stainless steels and sulphite pulping share a mutual heritage; many new alloys were tested in acid sulphite mills. Type 316L stainless steel became the principal material of construction for sulphite mills, although more highly-alloyed materials are required in a number of specific applications. Duplex stainless steels have been specified, due to their excellent resistance to pitting corrosion and stress corrosion cracking, by chloride ion. There are also several applications where specialty alloys, such as Alloy 20, 6Mo alloys, G30 and C276 (UNS N10276) prove advantageous.

Neutral sulphite semi-chemical pulping

As NSSC (Neutral Sulphite Semi-Chemical) spent liquors are more corrosive than kraft liquors, all equipment relies on stainless steels. While Type 304 has been used successfully, Type 316 (UNS S31600) is generally preferred given its greater corrosion resistance.

High yield mechanical pulping

This is the generic term for producing pulps mechanically using disc refiners. Newsprint typically has high levels of refiner pulps, sometimes as much as 100%.
Type 316L is the standard alloy used for the major components of refiner systems. However, Type 316L can be susceptible to attack by chlorides at coastal mills and by the concentration of chloride by recirculation of water used in the process. In these cases, higher-alloyed materials such as 2205 (UNS S32205), 904L and C-276 are used.

Fibre recycling

Paper recycling is a major part of the pulp and paper industry, with the process and equipment constantly undergoing change. Type 316L and its cast counterpart CF3M are the principal materials for constructing the equipment.

Bleach plant and pulp/paper stock preparation

Bleaching typically begins with an oxidising, acidic stage using chlorine dioxide (ClO2) bleachant. This is followed by an alkaline extraction stage to remove soluble lignin and at least one more acidic, oxidising stage.
Acidic, D-stage (chlorine dioxide bleaching): Comprehensive industry research programmes in the 1970s and 80s showed the controlling parameters for chloride-related, localised corrosion of an alloy in D-stage environments are, in order of decreasing effect:

  • Maximum residual chlorine dioxide concentration
  • Maximum solution temperature
  • Maximum chloride concentration
  • Minimum pH.

Stainless steels resist localised corrosion based on their alloying content. Pitting resistance correlates with Cr, Mo and N contents according to the following formula:
Pitting resistance equivalent (PREN) = Cr + 3.3Mo + 16N
Generally, alloys with PRENs of 35 or above resist localised corrosion in D-stage environments below 75°C with maximum residual chlorine dioxide content of less than 50 ppm. Stainless steels with PRENs of 42 or above, including 6Mo, super austenitic grades and super duplex grades, resist localised corrosion in more highly-corrosive conditions.
Alkaline, E-stage (extraction stage): Brown stock coming to the bleach plant typically has a pH of over 9.5, while extraction (E) stages operate at pH above 10. These are benign environments for carbon steel and for regular stainless steel grades, including Type 304L (UNS S30403) and Type 316L as well as lean duplex grades, including 2101 (UNS S32101), 2304 (UNS S32304) and 2202 (UNS 32202), even at process temperatures above 70°C. The notion of PREN does not apply for alkaline bleaching stages; in fact, higher molybdenum reduces the corrosion resistance of stainless steels in hot alkaline environments.

Paper machine

Wet environments such as the stock approach piping, headbox, and forming section are usually manufactured from Types 316 and 316L stainless steel. Heavy-strength components are fabricated from mild steel and clad or wrapped in Type 316L stainless steel.
The press section of the machine is primarily fabricated out of steel and is either painted or clad in Type 316L stainless steel for corrosion protection.

Suction rolls

Usually drilled to about 20% open area, suction rolls are used to remove water from paper at the wet end of the paper machine. Prior to 1950, suction shells were cast from copper alloy C83600. As paper machine widths, speeds and nip pressures increased, stronger alloys were introduced; nickel aluminium bronze, forged Type 410 (UNS S41000) along with centrifugally cast stainless steels Type CA15 (UNS J91150), CF3M and CF8M (UNS J92900).
For greater detail, review the following publication - Stainless Steel and specialty alloys for pulp, paper and biomass conversion.
For more information on this topic please see Stainless Steels and Specialty Alloys for Pulp, Paper and Biomass Conversion, a fully revised second edition of a popular and practical guide for mill and project engineers within the pulp, paper and biomass sector.

Alloy Type UNS %Cr %Ni %Mo %Cu %Fe  
Austenitic stainless steels and nickel-base alloys





18 8 - - bal "L" is the low carbon version that maintains corrosion resistance after welding.





16 10 2.1 - bal "L" is the low carbon version that maintains corrosion resistance after welding.
904L N08904 20 24 4.1 1.5 bal  
Alloy 20 N08020 20 33 2.1 3.2 bal  













These are two well-known examples, there are others.
310 S31000 25 20 - - bal High chromium stainless steel with excellent resistance to high temperature oxidation.
330 N03300 20 32 - - bal Stainless steel with excellent high temperature strength and resistance to high temperature oxidation.
C-276 N10276 15 bal 16 - 5 One of the best-known nickel alloys with excellent corrosion resistance to reducing acids.
Duplex stainless steel
2101 S32101 21 1.5 0.5 0.5 bal  
2202 S32202 22 2 - - bal  
2304 S32304 23 4 0.3 0.3 bal  
2205 S32205 22 5.7 3.1 - bal Most common duplex.
2507 S32750 25 7 4 - bal Greater corrosion resistance than 2205.
Martensitic stainless steel
410 S41000 12 - - - bal  
Castings - stainless steel
CA15 J91150 12 - - - bal Cast equivalent of 410.















CF3M is enssentially the low carbon version of CF8M. These are considered the cast versions of Type 316L and 316.
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