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Why evaporators corrode

Carbon steel and stainless steel are the common materials of construction for black liquor evaporators. It is generally understood that black liquors become more corrosive as the percent solids in the liquors increases, since evaporation increases the concentrations of corrosive compounds such as hydroxide, sulphide, and thiosulphate. There has been an industry-wide demand for increasingly higher solids. Rising film evaporators can deliver up to 50% solids, falling film evaporators up to 65%, and forced-circulation concentrators can deliver liquor up to 85% solids. Corrosion environments in evaporators include immersion, splashing, and vapour.

Carbon steel predominates as the material of construction for effects handling the weakest liquors (lowest percent solids). Effects handling intermediate black liquors may be of mixed carbon steel and stainless steel construction. For example, the immersion zones of vapour bodies may be stainless steel, while the vapour zone of vapour bodies may be carbon steel. Effects handling the highest percent solids liquor may be of all-stainless steel construction. Concentrators are typically of all-stainless construction.

RAPID CORROSION OF CARBON STEEL

The phenomenon of rapid corrosion of evaporator third effects was reported by Lloyd Clay as early as 1974 in his address to the First International Symposium on Corrosion in the Pulp and Paper Industry [3]. Over 30 years later, we continue to see cases of “sudden” rapid corrosion of third-effect evaporators that had reportedly been operating for many years without problems. Figures 1 through 4 show examples of corrosion of carbon steel in third-effect evaporators.

Carbon steel can exhibit both passive and active corrosion states characterized by slow and rapid corrosion, respectively. Passive carbon steel is protected by the formation of an oxide film on the surface. Active carbon steel has no such protective layer. The suitability of carbon steel for service in black liquors depends on the ability of the passive layer to withstand those factors in the environment that are constantly acting to break it down. Carbon steel has an acceptably low corrosion rate in evaporator weak black liquors because the passive state is stable. Furthermore, the passive layer is able to withstand small changes in environmental parametres that are detrimental to corrosion, such as increased temperature, sulfidity, or velocity.

In intermediate black liquors, such as exist in evaporator third effects, the passive state is unstable and small increases in temperature, sulfidity, or velocity may remove the passive layer. Without a protective passive film, the corrosion state of the carbon steel shifts abruptly from the passive state to the active state and from corrosion at an acceptably slow rate to rapid corrosion at an unacceptably high rate. The accompanying graph is a plot of the corrosion potential for carbon steel specimens exposed in intermediate black liquor at 95C. In low sulfidity black liquor (30%), carbon steel remained passive as evidenced by the corrosion potential remaining above 0 millivolts with respect to a molybdenum reference electrode. In high sulfidity black liquor (40%), carbon steel underwent a sudden change in corrosion state from passive to active after approximately 55 hours, with the active state being characterized by significantly negative corrosion potential values.

In strong black liquors that are handled in evaporator first and second effects, carbon steel does not have stable passivity and is always susceptible to rapid corrosion. The unsuitability of carbon steel as a material of construction does not prevent evaporator manufacturers from using it as a material of construction for first and second effects. Many evaporator first and second effects are now in the process of being rebuilt or replaced. In strong black liquors, small increases in temperature, sulfidity, or velocity increase the rate of active corrosion of the carbon steel, further shortening an already short service life.

STAINLESS STEEL CAN ALSO CORRODE — AND CRACK

Stainless steels also exhibit active/passive behaviour, although the passive film is so profoundly stable that it is difficult to remove it from the surface. It is the chromium in stainless steels that stabilizes the passive film.

Type 304 stainless steel contains at least 18% chromium and has been the material of choice for corrosion protection of evaporators where carbon steels have historically been unsuitable, such as the tubes or lamellae, the first and second effects, and parts of the third effects. The tube sheets, vapour bodies, and liquor bodies are often constructed using either solid stainless steel or stainless-clad plate, or are protected using stainless steel liners that are welded to the shell.

Type 304 stainless steel is susceptible to sensitization during exposure to high temperatures (e.g., 600C) during processes such as welding or during the manufacture of roll-bonded stainless steel cladding. Sensitization is the depletion of chromium in the alloy, due to the diffusion of that element to the grain boundaries where it reacts with carbon to form chromium carbide precipitates along the grain boundaries. Selection of low-carbon grades (type 304L) is helpful, but a determined vendor can sensitize even L-grades. Type 316L stainless steel has a lower chromium content and is less-suitable than type 304L.

Types 304 and 304L stainless steels are essentially immune to corrosion in weak and intermediate solids black liquors. Passive stainless steel can experience two forms of corrosion in strong and heavy (high solids content) black liquors: (1) intergranular attack (IGA), and (2) stress corrosion cracking (SCC). IGA affects sensitized stainless steel with the attack progressing along the grain boundaries until the individual grains are either consumed by corrosion, or they fall out of the microstructure, resulting in thinning at a perceptible rate. IGA is often seen as “frosting” of stainless steel welds and their adjacent heat-affected zones, as can be seen in Figure 5. IGA can also manifest as widespread uniform thinning of lamellae or of stainless-clad walls.

SCC requires the simultaneous presence of a susceptible material (austenitic stainless steels such as types 304 or 304L), tensile stresses (typically from welding and forming), and a stress corrosion agent. While hydroxide is the most obvious candidate as the SCC agent, sulfur-species may act as SCC agents [Ref. 4]. SCC can propagate completely through stainless steel components and lead to failure. SCC can be intergranular and follow the grain boundaries (particularly if the stainless steel is sensitized) or it can be transgranular and propagate through the grains, rather than around them.

During the 10th and 11th International Symposia on Corrosion in the Pulp and Paper Industry, there were a number of papers presented describing IGA and SCC of type 304 stainless steel in high-solids (>70%) evaporators and concentrators. A summary of these “unexpected” phenomena can be found in Ref. 5. Severe SCC can occur under conditions of splashing on the hottest surfaces such as steam headers. SCC has also been observed in splash and vapour zones that are not directly heated. SCC does not readily occur under immersion conditions, but it is predicted to occur at the hydroxide concentrations that are known to exist in most high-solids evaporators and concentrators.

DISCUSSION

Carbon steels have been used successfully for many years as a material of construction for evaporators, although their use is not without risk. Carbon steels can switch abruptly from the passive to the active state as the result of small increases in temperature or sulfidity. When the corrosion state switches to the active state, it is unlikely that it will ever return to passive state. Left unchecked, rapid active corrosion can lead to perforation of the walls, tubesheets, or heater boxes. Repairs such as re-plating or sta
inless steel lining may buy some time, but the ultimate solution is partial or complete replacement using stainless steel.

Type 304 stainless steel has also been used successfully for many years in evaporators and concentrators, but is susceptible to IGA and SCC in higher-solids black liquors, particularly if the stainless steel is sensitized. Small increases in temperature, sulfidity, or solids content may be enough to initiate IGA or SCC in stainless steels that had previously resisted corrosion.

As always, the key to avoiding corrosion-related failures is inspection. Visual inspection is often sufficient to determine if carbon steel is corroding or if stainless steels are experiencing IGA. Thickness testing can be used to assess the severity of thinning of stainless steel due to IGA. Penetrant testing is usually required to detect SCC of stainless steels. It is important to use proper surface preparation such as light grinding or polishing using flap discs prior to penetrant testing, or else cracks may not be revealed.

For new evaporators or concentrators, duplex stainless steels are a better choice of material of construction. Duplex stainless steels have a higher content of chromium and offer greater resistance to corrosion, SCC, and erosion than do conventional austenitic stainless steels. The predominant grade used for new evaporators or for rebuilds has been type 2205 (with a chromium content of at least 22%). Other duplex grades such as type 2304 and the newer “lean” duplex stainless steels are also candidate materials of construction.

REFERENCES

1. Wensley, A., and Christie, D., Corrosion of Evaporators, Paper presented at the TAPPI Engineering, Environmental, and Pulping Conference, Philadelphia (2005).

2. Wensley, A., Corrosion Testing in Evaporator Liquors, Paper No, 06242 presented at the NACE Corrosion 2006 Conference, San Diego (2006).

3. Clay, L., Summary of Key Pulp and Paper Industry Corrosion Problems, Proc. 1st Intl. Conf. on Corrosion in the Pulp and Paper Industry, Chicago, pp. 7-8 (1974).

4. Klarin, A., Corrosion Phenomena in Black Liquor Evaporators, Proc. 10th Intl. Conf. on Corrosion in the Pulp and Paper Industry, Helsinki, pp. 469-483 (2001).

5. Bennett, D., and Reid, C., Unexpected Corrosion of Stainless Steel in High Solids Black Liquor Service, TAPPI Solutions Mag, pp. 57-58, Sept. (2002).