# Ni200 vs Ni201: Which Nickel Grade for Your Electrolyzer?

> The 0.13% carbon difference between Ni200 and Ni201 determines whether your electrode wire survives 60,000 hours or fails at 20,000. Selection guide with chemical composition, grain-boundary corrosion data, and application recommendations.


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.