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Materials the same weight- % concentration . For example , as shown in Figure 2 , 30 % KOH corresponds to 23 % NaOH . Therefore , applying data for 30 % NaOH when selecting MOC for a system using 30 % KOH would yield a conservative result .
Material selection criteria for alkaline electrolyzers Different materials of construction can be used depending on the operating and design criteria of the alkaline electrolyzer . As mentioned earlier , non-metallic materials are generally unsuitable for alkaline electrolyzers due to flammability and hydrogen permeability , and metallic materials are used . Table 4 shows some typical operation and design values for an alkaline electrolyzer . It should be noted that the caustic brine after the electrolyzer will have a higher concentration of KOH as water has been consumed . Therefore , these parts may show higher corrosion rates than other parts .
Standard MOC As already mentioned , many alkaline electrolyzers are constructed using 304L or 316L austenitic stainless steel . These austenitic grades can withstand certain upset process conditions and temperatures with little corrosion damage and a limited risk of material failure . The MOC will often provide a suitable service life for the application , although part replacement is to be expected . DSS 2304 or 2205 can be used with a limited increase in material cost to achieve reduced corrosion and longer service life .
Advanced MOC In some cases , there is a requirement for the electrolyzer to operate without any part replacements for up to 30 years . This also means the MOC should withstand upset operating conditions without corrosion or cracking . It is important to keep in mind that even a low corrosion rate of 0.05 mm / year will result in a significant material loss of 1.5 mm over a period of 30 years . Therefore , an adequate corrosion allowance must be applied to the installed components , or a MOC must be chosen to minimize any corrosion . Furthermore , from a fouling perspective , extremely low corrosion rates will be essential to keep the electrolyzer clean throughout its 30-year service life . For these applications , the MOC must be immune to corrosion at the design criteria specified in Table 4 . By consulting the information in Table 1 and Figure 1 , along with the conversion diagram in Figure 2 , it becomes clear that DSS 2304 and DSS 2205 do not provide sufficient corrosion resistance . On the other hand , DSS 2906 shows zero corrosion in boiling 30 % NaOH ( approximately 39 % KOH ), making it a suitable choice as an MOC in these extreme cases , even during upset conditions . DSS 2507 could also be used , but this grade may not offer complete corrosion resistance during operational upsets .
Molar concentration / KOH or NaOH
16 14 12 10 8 6 4 2
Molar concentration vs . weight concentration
Same molar concentration
Weight- % NaOH
0
0 10 20 30 40 50 60 Weight concentration / KOH or NaOH
Molar conc . KOH
Fig . 2 . Graph for conversion between the weight concentration of NaOH and KOH . The example shows that 30 weight- % KOH corresponds to ~ 23 weight- % NaOH .
Weight- % KOH
Additional considerations for parts after electrolyzer stack and gas separation After the electrolyzer stack , parts of the alkaline electrolyzer system will be exposed to highly pressurized hydrogen and oxygen . Hydrogen has been shown to cause loss of ductility and embrittlement of austenitic stainless steels with low nickel content , as these grades are prone to forming deformation martensite upon cold working . 4 Also , due to containing ~ 50 % ferrite , duplex grades will likely lose mechanical properties when exposed to hydrogen gas under high pressure . 5 Therefore , higher-alloyed austenitic grades should be considered for these specific parts of the electrolyzer plant . It has been established that highly oxidizing environments can increase the susceptibility of stainless steel to SCC . 6 Consequently , the highly oxidizing power of oxygen gas may increase the susceptibility of stainless steel to caustic SCC . However , this has not been thoroughly investigated , so further experiments are required to verify if this can inhibit the use of certain stainless steel grades in these parts of an electrolyzer .
Cooling system The complete alkaline electrolyzer plant also includes auxiliary cooling systems , which can be either of a direct type or an indirect type . In the direct type , the caustic electrolyte is cooled directly using cooling water in a heat exchanger , while in the indirect type , a closed loop system is used , using , for example , a glycol solution . Due to the cooling effect on the heat exchanger tubes for direct cooling systems , the corrosion rate will be lower than experienced in the electrolyzer . Depending on the cooling media employed , DSS 2205 can be a suitable material for sweet water systems if some corrosion
Molar conc . NaOH
Alloy |
UNS |
C |
Cr |
Ni |
Mo |
N |
2304 |
S32304 |
< 0.03 |
22.5 |
4.5 |
0.3 |
0.1 |
2205 |
S32205 |
< 0.03 |
22 |
5 |
3.2 |
0.18 |
2507 |
S32750 |
< 0.03 |
25 |
7 |
4 |
0.3 |
2906 |
S32906 |
< 0.03 |
29 |
7 |
2.3 |
0.35 |
Table 3 . Typical chemical composition for four duplex grades used in caustic service conditions
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