Theory 051/Engineering & Materials /Corrosion
Pitting Follows a Ratio
Chloride limits are written as absolute numbers. The damage may depend on what it is competing with.
Pitting corrosion initiates at a threshold in the ratio of chloride to inhibitor concentration rather than at a threshold chloride concentration, so the same chloride level can be safe or dangerous depending on what else is in the water.
01The evidence
Inhibitors raise the chloride concentration needed for pitting, and the effect is not linear in inhibitor dose.
02The tension
Standards specify chloride limits for alloys, which predicts pitting from chloride alone and fails when inhibitor levels vary between systems.
03The mechanism
Chloride attacks the passive film by competing with oxygen and hydroxide for adsorption sites. Inhibitors occupy the same sites, so the outcome depends on the competition rather than on the absolute chloride concentration.
That makes the ratio the threshold variable, and predicts that adding inhibitor should shift the critical chloride upward in a predictable way.
04What this is not
Absolute limits ignore the competing species. This makes the ratio the threshold, which explains failures within limits.
05Predictions
- P1Pitting should track the chloride-to-inhibitor ratio.
- P2Inhibitor addition should raise the critical chloride.
- P3Failures within chloride limits should occur at low inhibitor.
06Refutation
Pitting tracking absolute chloride at controlled inhibitor would restore the limit.
07What it changes
It would change water chemistry specifications.