Acid-Proof Liners: ZG25Cr7 Corrodes 15× Slower than Hadfield Steel
Wet grinding in acidic pulp destroys liners through a mechanism that no hardness chart will predict: corrosion and abrasion working together. The measured weight loss in corrosive-abrasive duty can run several times, to more than ten times, the simple sum of pure corrosion plus pure wear. The two damage modes feed each other — abrasion strips protective layers, fresh metal corrodes, corroded surface abrades away more easily.
If your mill runs acid sulfide ore or acidic process water, this is the physics your liners are living with. And it’s why simply buying a harder steel doesn’t fix the problem.
The steel built for it
ZG25Cr7 is a low-carbon, medium-chromium alloy steel designed for weak-acid wet grinding duty:
| Element / Property | Value |
|---|---|
| C | 0.20–0.28 % |
| Cr | 6–8 % |
| Heat treatment | 960 °C oil quench + 240 °C temper |
| Impact toughness | ak 173.6 J/cm² |
That toughness figure deserves a second look: 173.6 J/cm² is extraordinary for a corrosion-resistant wear steel. Most acid-capable materials trade away impact margin — this one keeps it, which matters because wet mill liners still take constant ball strike on top of the chemistry.
The corrosion data
Static corrosion testing in pH 4.5 weak-acid media, three liner materials side by side:
| Liner material | Relative corrosion rate |
|---|---|
| Hadfield steel ZGMn13 | 1 (baseline) |
| Si-Mn alloy steel | ~1/2.24 of baseline |
| ZG25Cr7 medium-Cr alloy steel | 1/15.61 of baseline |
A 15.6-fold reduction in corrosion rate against the most common liner steel in the world. And roughly 7× better than the silicon-manganese steel that many buyers upgrade to as their “first improvement” — a step that only partially addresses the corrosion side while doing nothing about the interaction effect.
Matching the material to the pH
A rough selection guide from our grade table:
| Pulp condition | Recommended direction |
|---|---|
| pH ~7, neutral | manganese or standard alloy steels — corrosion is secondary |
| pH 5–6, mildly acidic | Cr-bearing multi-alloy steels (see our CHMe case) |
| pH 4–5, acidic, sulfide ore | medium-chromium grades like ZG25Cr7 |
| Strongly acidic / extreme duty | higher-chromium systems — ask us, this is case-by-case |
The economics follow the same logic as our other alloy-steel cases: ZG25Cr7 costs more per tonne than manganese steel, but in the right pulp it removes a wear mode that manganese steel simply cannot survive. When corrosion is 15× slower, the reline calendar stretches accordingly.
QM Mill Liners casts ZG25Cr7 and related medium-chromium liners for acidic wet grinding. Send your pulp pH, ore sulfur level and mill drawing, and we’ll tell you whether this grade or the CHMe-class steels fit your duty better — the honest answer depends on your impact level too. Start here.
FAQ
Why do liners wear out so fast in acidic slurry? Corrosion and abrasion interact: each mechanism amplifies the other. Total loss can exceed the sum of pure corrosion plus pure wear by several to more than ten times. Hardness alone doesn’t address it — the steel needs corrosion resistance too.
How much better is ZG25Cr7 than Hadfield steel in acid? In pH 4.5 static testing, its corrosion rate was 1/15.61 that of Hadfield steel — a 15.6-fold reduction.
Does a corrosion-resistant liner still need toughness? Yes. Wet mill liners take continuous ball impact. ZG25Cr7’s ak of 173.6 J/cm² means the corrosion resistance doesn’t come at the price of cracking in service.