Ni200 vs Ni201: Which Nickel Grade for Your Electrolyzer?

Ni201. For every component immersed in KOH above 80 C — electrode lead wire, mesh substrate, current collector — specify Ni201. The 0.13% carbon difference between Ni200 (max 0.15% C) and Ni201 (max 0.02% C) is not a metallurgical detail. It is the difference between a stack that runs 60,000+ hours and one that fails at 20,000 due to intergranular corrosion at every wire crossover point.

TL;DR — Ni201 (max 0.02% C) for all electrode components in KOH above 80 C. Ni200 (max 0.15% C) only for ambient-temperature busbar connections. The 5-8% cost saving on Ni200 is wiped out 100x by the cost of one unscheduled stack teardown. Ni201 = 60,000+ hrs. Ni200 in hot KOH = 20,000 hrs max.

The Carbon Difference: What Happens at Grain Boundaries

Both Ni200 (UNS N02200) and Ni201 (UNS N02201) are commercially pure nickel — over 99% nickel content. The critical difference is carbon.

Ni200: Min 99.5% Ni, max 0.15% C, max 0.40% Fe. When exposed to temperatures above 315 C — which occurs locally at high-resistance contact points even in “low-temperature” alkaline electrolyzers — carbon migrates to grain boundaries and precipitates as nickel carbide (Ni3C). In 25-30% KOH at 80-90 C, these grain-boundary carbides are selectively attacked. The KOH dissolves the carbide phase, creating microscopic corrosion fissures along every grain boundary in the wire. Post-mortem SEM analysis of a returned 8 MW stack cathode assembly showed intergranular fissures at approximately 900 sites per cm2 of electrode area — every wire crossover point had become a corrosion initiation site.

Ni201: Min 99.0% Ni, max 0.02% C, max 0.40% Fe. The carbon content is below the threshold required to form a continuous grain-boundary carbide network. The grain boundaries remain resistant to alkaline attack. After 18-month follow-up on 14 stacks switched from Ni200 to Ni201-grade substrates, voltage degradation rate dropped from 0.3% to below 0.04% per month — a 7.5x improvement.

Where to Use Each Grade

Position	Recommended Grade	Reason
Electrode mesh (catalyst substrate)	Ni201	Immersed in 30% KOH at 80-90 C
Porous transport layer (PTL)	Ni201	Under compression in hot KOH
Current collector lead wire	Ni201	Continuous immersion in electrolyte
External busbar connections	Ni200	Ambient temperature, dry, not in electrolyte
Weaving stock (before electrode assembly)	Ni200 or Ni201	No KOH exposure during weaving

The Economics: 5-8% Saving, 100x Cost

Ni200 mesh costs approximately 5-8% less than Ni201. On a 5 MW stack requiring 800 m2 of electrode mesh, that saves approximately $800-1,200 on substrate material. The cost of one unscheduled stack teardown — crane rental, production downtime at $40-60/MWh hydrogen revenue, replacement labor, and expedited material shipping — exceeds $80,000. The material saving is wiped out 100x on the first failure.

Bottom line: Ni201 for everything that touches KOH above 80 C. Ni200 only for external connections at ambient temperature in air. The $0.50/kg premium for low-carbon nickel is the cheapest insurance your stack will ever have. Confirm the grade on every Mill Test Certificate — do not rely on the purchase order description alone.