Heat Exchanger World Magazine September 2023 | Page 36

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Materials

Corrosion rate mm / year

2,5

2

1,5

1

0,5

0 20

316L

304L

25 30 35 40 45 50 55

Concentration NaOH /%

2304 2205 2906 2507

Fig . 1 . Corrosion rates of duplex grades 2304 , 2205 , 2507 , and 2906 in boiling NaOH solutions . Data points for 304L and 316L are also shown for boiling 30 % NaOH .

the electrolyzer system can result in the accumulation of corrosion products that can block small-diameter tubes , lead to scaling , and the presence of metal ions in the electrolyte , which can be detrimental to the electrodes .

Corrosion resistance of stainless steel in strong caustic solutions Below is a non-exhaustive summary of the corrosion resistance of some austenitic and duplex stainless steels used in caustic industrial applications . Due to the strong oxidation properties of oxygen , the high flammability of hydrogen , and the diffusivity of hydrogen through polymers , plastic and fiberglass parts are generally unsuitable as MOC for hydrogen electrolyzer systems .

Austenitic stainless steel in caustic processes In caustic water solutions , such as sodium hydroxide ( NaOH ), chromium plays a crucial role in maintaining the passivity of stainless steels . 1 Nitrogen is another beneficial alloying element for corrosion resistance , while a high nickel content can help reduce the corrosion rate in cases of active corrosion . 1 The pulp and paper industry handles a lot of caustic solutions , and welded 304L , with approximately 18 % chromium and 10 % nickel , offers a good service life for piping . In such piping applications , the caustic concentration is seldom higher than 30 % NaOH , and the temperature is usually below 80 ° C . However , as shown in Table 1 , when exposed to higher temperatures , 304L can start to corrode , and there is also a risk of stress corrosion cracking ( SCC ) at atmospheric boiling points . 2 Since 316L is usually slightly lower in chromium content than 304L , and also suffers from SCC at similar temperatures , 316L does not offer any advantages compared to 304L . Additionally , the higher-alloyed austenitic stainless steel grade 904L is susceptible to SCC when exposed to boiling 30 % NaOH solution . The typical chemical compositions of 304L , 316L , and 904L stainless steel are listed in Table 2 .

Duplex stainless steel in caustic processes For more demanding or critical applications , it is common to use a duplex grade such as duplex stainless steel ( DSS ) 2304 or 2205 instead of austenitic grades . The duplex grades have higher chromium and nitrogen content compared to 304L , 316L , or 904L , resulting in an inherently higher corrosion resistance in caustic media . They can provide a good service life in both more concentrated solutions and at higher operating temperatures . Duplex grades are also lower in nickel content than austenitic ones , making them more costefficient . Finally , duplex stainless steels have higher mechanical strength and better thermal conductivity than standard austenitic grades . They can be utilized to achieve more efficient heat exchangers or reduce the weight of an installation . The typical chemical compositions of some duplex grades are shown in Table 3 . Figure 1 shows a graph depicting the measured corrosion rates of various stainless-steel grades in boiling NaOH solutions as a function of NaOH concentration . 3 The duplex grades exhibit superior corrosion resistance compared to the standard austenitic grades . It is also evident that DSS 2304 and DSS 2205 demonstrate very similar levels of corrosion resistance , DSS 2507 is better , while DSS 2906 exhibits the best corrosion resistance of the tested grades . This further supports the notion that chromium is the most important alloying element for materials used in caustic solutions .

Converting NaOH data to KOH Since NaOH is a very common chemical in the process industry , almost all published corrosion data for highconcentration caustic media is from tests conducted in NaOH aqueous solutions . By contrast , the caustic solution commonly used in alkaline electrolyzers is KOH , for which published data is very scarce . Moreover , most published data for NaOH solutions is expressed with a concentration in weight- %. However , due to the difference in weight between Na + and K + , 22.99 g / mole and 39.10 g / mole , respectively , mass percentages of NaOH and KOH are not equivalent . However , in a caustic solution containing NaOH or KOH , the Na + and K + ions are inactive in the corrosion process . Therefore , the corrosivity of a caustic solution at a certain temperature is a function of the molar concentration of hydroxide ions ( OH- ). Thus , by calculating the molar concentration of a NaOH solution , the available corrosion data for that concentration of NaOH can be recalculated for the corresponding concentration of KOH . This is illustrated in Figure 2 . It can be observed that a KOH solution with a certain weight- % concentration will always have a lower molar concentration of OH- and therefore be less corrosive than a NaOH solution with

Table 2 . Typical chemical composition for three common austenitic grades used in caustic service conditions

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