Understanding Mil-Spec: MIL-A-18001

The Benchmark for Zinc Anode Quality

In the early 1950s, the U.S. Naval Command partnered with the International Zinc Institute to investigate the inconsistent performance of zinc anodes in marine environments. Their research uncovered a key insight: even small amounts of iron contamination in zinc (greater than 0.0014%) could cause passivation—a condition where the anode becomes inactive and stops protecting the structure it’s attached to.

To address this, they developed the first U.S. military specification for zinc anodes: MIL-A-18001A.

Why Anodes Used to Fail

Before this specification, the effectiveness of sacrificial anodes was unpredictable. Two seemingly identical vessels using zinc anodes could have completely different corrosion outcomes. The issue?

Many zinc anodes weren’t staying electrochemically active over time. A dense film—mainly iron oxide—would form on the surface, caused by excess iron in the zinc itself, rendering the anode inert.

The Birth of MIL-A-18001

The new military spec required:

• Maximum 0.0014% iron content
• Strict limits on copper, silicon, and other impurities
• Use of Special High Grade (SHG) zinc—the purest commercially available zinc
• Stringent manufacturing controls to prevent contamination during casting

This created a reliable standard that dramatically improved the consistency and protection of zinc anodes in service.

Refinement Through Alloying

Over time, metallurgists discovered that by adding controlled amounts of aluminum and cadmium, anodes could tolerate slightly higher iron content—up to 0.005%—without passivating. These alloyed zinc anodes form a loosely adherent zinc oxide layer that naturally sheds in water, exposing fresh metal and keeping the anode continuously active.

Why MIL-Spec Still Matters Today

Only through continuous electrochemical activity can a zinc anode provide lasting protection. Anodes that don’t meet MIL-A-18001 risk being contaminated, underperforming, or worse—failing to protect your structure altogether.

That’s why every Reliance Zinc Anode is certified to meet or exceed MIL-A-18001 standards—because when it comes to corrosion protection, there’s no room for compromise.