Unexpected corrosion of stainless steel in high solids black liquor service: three recent papers explore the complex causes of stress corrosion cracking.

Corrosion problems, including stress corrosion cracking (SCC) of stainless steels in black liquor evaporators, were the subject of three papers presented last year at the 10th International Symposium on Pulp & Paper Industry Corrosion Problems (ISCPPI) in Helsinki, Finland. (The proceedings are published by VTT-Technical Research Center of Finland.) The papers all describe a growing incidence of both general corrosion and SCC of types 304L and 316L in evaporators and concentrators handling the highest solids liquors in Scandinavian mills. Similar problems are likely in North American mills. (To more accurately assess the extent of the problem in North America, the TAPPI Corrosion and Materials Engineering Committee is actively seeking reports on mill experiences with these types of corrosion. Questions or information can be sent to the authors.)

SCC and general corrosion of Types 304L and 316L stainless steel are more likely to occur in heavy black liquors with solids contents above 70%. This is due to the high process temperatures required, both to concentrate the liquor solids to such high levels and to keep the viscosity low enough for pumping. This should be expected: 304 and 304L stainless steel heat-exchanger tubes--for decades the "standard" metallurgy in liquor heaters for kraft pulping--are known to experience SCC and to corrode away when service temperatures are too high. (Acid cleaning is sometimes a culprit, too.)

Mill engineers should keep in mind the vulnerabilities of 300 series stainless steels when setting the inspection scope and frequency for existing liquor heaters or liquor concentrators. The same is true when selecting materials for new equipment. Duplex stainless steels and high chromium ferritic stainless steels provide superior resistance to corrosion and to SCC in liquors.

UNDERSTANDING SCC

SCC of stainless steel occurs when the stressed part is in a potential range in which a stable, self-protective, oxide-rich, "passive" film does not immediately reform when it is damaged (Fig. 1). Cracking may be transgranular or intergranular. This depends mostly on the stress level, but also on the specific corrodent that undermines the stability of the passive film. Stainless steels experience SCC both in neutral and acidic solutions containing chlorides; and in hot, strong caustic solutions. The latter applies here, although, as mentioned earlier, damage may also be done by acid cleaning, especially with hydrochloric acid.

[FIGURE 1 OMITTED]

SCC of stainless steels in caustic environments can occur if the alkalinity and temperature are high enough to enter the "Risk of SCC Zone" (shown in Fig. 2). These conditions occur as black liquor is concentrated to higher and higher solids content because removal of the water significantly concentrates the alkalinity, which includes sodium hydroxide.

Kottila and Rauscher, in their paper at the 10th 1SCPPI titled "Selecting the proper material for black liquor concentrators," show how total alkalinity (NaOH + [Na.sub.2]S) increases dramatically as the solids level of real liquors increases. The total alkalinity can be about 300 g/kg of free water at 65% solids in some black liquors. Depending on the stress levels and NaOH proportion of the total alkalinity, this could put it in the SCC zone for 304L and 316L at around 160[degrees]C (320[degrees]F), which is the temperature of 90 psi saturated steam. In addition, liquors concentrated to more than 70% solids often require additional heating to keep the viscosity low enough for convenient pumping.

Another contributing mechanism to SCC at higher solids concentrations is generation of organic sulfur gases that promote SCC as the solubility of these gases in the remaining water solution decreases. Presence of harmful organic sulfur species like methyl mercaptan and di-methyl sulfide effectively increases the size of the SCC zone shown in Fig. 2.

Documented North American experience has involved SCC of 304 SS in the high solids (73%) concentrators in two mills. SCC of 304 has also occurred in the vapor phase of the concentrators in two mills and, as shown in Fig. 1, in the steam chest of the associated, first-effect evaporator at the same mill.

UNDERSTANDING GENERAL CORROSION

Resistance to general corrosion depends on fine continued presence of the passive film. The tenacity of this film decreases at higher temperatures and under harsher flow conditions. Figure 3, taken from a 1996 TAPPI Engineering Conference paper titled "Caustic corrosion in black liquor evaporators" by Anja Klarin and Mika Kottila, shows how the corrosion rates of 304L and 316L stainless steel in heavy black liquor vary with temperature and the NaOH content in the liquor. Test time was 14 days. At 150[degrees]C, 304L is superior to 316L up to a NaOH concentration of 6%. The authors attribute this to the higher molybdenum content in the 316L. Tests show higher corrosion rates of both 304L and 316L at 6% NaOH at 170[degrees]C vs. 150[degrees]C. This indicates that these test conditions approach critical temperatures for overall loss of stable passivity for both alloys. Klarin provides more discussion on this in her paper presented at the 10th ISCPPI, titled "Corrosion phenomena in black liquor evaporators."

Driving home the point about the cost-effective performance of duplex stainless steel, Miko Kottila and John Rauscher showed that 2205 duplex SS corroded only half as rapidly as 304L SS after one-year exposure in 75%-80% solids black liquor. Their paper, presented at the 10th ISCPPI, was titled "Selecting the proper material for black liquor concentrators." Table 1 summarizes their test results. The 2205 duplex coupon also did not experience SCC, which reportedly affected most of the other stainless steels, all of which are austenitic types.

Table 1. Relative corrosion rates of of stainless steels in
75%-80% solids black liquor.

Alloy      Relative corrosion
          rate (304L rate = 1)

654SMO            0.3
2205              0.5
304               0.9
304L              1
AL-6XN            2.1
904L              3.4
254SMO            3.6
316L              3.7
317L              3.9

David C. Bennett is principal engineer at M&M Engineering, Ramsey, New Jersey. He can be reached at dave_bennett@mme ngineering.com. Craig Reid is a metallurgical engineer for Bacon-Donaldson, Richmond, British Columbia, Canada. His email address is [e-mail]