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Corrosion is an electrochemical reaction: metal gives up electrons and dissolves. Where water and oxygen meet steel, tiny anodes and cathodes form on the surface, iron goes into solution, and the metal is eaten from the inside out. Treatment does not abolish the reaction — it interferes with it, by forming a protective film on the metal, controlling dissolved oxygen, holding pH in a sensible band, and denying corrosion the deposits and bacteria that accelerate it.
Put steel in aerated water and it sets up a corrosion cell on its own surface. At the anode, iron dissolves:
Fe → Fe²⁺ + 2e⁻
Those electrons travel through the metal to a cathodic area, where dissolved oxygen consumes them. The products react on to form the rust you eventually see. Three things follow from this:
Not all corrosion looks the same, and the causes differ:
| Form | What it is | Where you see it |
|---|---|---|
| General | Fairly even loss across a surface | Predictable, and the least dangerous |
| Pitting | Deep, localised penetration | Perforates a wall while most of the pipe looks fine |
| Galvanic | Two dissimilar metals coupled in water | Copper and steel joined in one circuit |
| Under-deposit | Attack sheltered beneath scale or sludge | Invisible until failure. Ties corrosion to scale control. |
| Microbiological (MIC) | Bacteria create corrosive local conditions | Ties corrosion to biofilm — and to Legionella control |
It is tempting to treat these as three separate issues. They are not.
Scale gives corrosion somewhere to hide — under-deposit attack proceeds happily beneath a mineral layer while a surface inspection looks clean. Biofilm does the same thing and adds bacteria that generate their own corrosive micro-environments. Meanwhile corrosion products themselves become deposits, which shelter more biofilm.
This is why a programme has to balance all three at once. Push pH down hard to stop scale and you accelerate corrosion. Push it up to protect metal and you precipitate scale — and, as it happens, you also cripple your chlorine (see effects of pH on various biocides). Every lever moves the others.
A closed system should be nearly corrosion-free: seal it, remove the oxygen, and the reaction starves. In practice they corrode anyway, and the reason is usually make-up water. Every leak that is topped up brings in fresh oxygen and fresh minerals, and quietly dilutes the inhibitor.
A closed loop whose inhibitor keeps disappearing is telling you it is not closed. Find the leak. See chemical flushing and boiler water treatment.
Corrosion is slow and invisible, so it must be measured rather than assumed. Corrosion coupons — pre-weighed metal strips placed in the flow and removed after a set period — give a measured corrosion rate rather than an opinion. Continuous data on pH, conductivity and inhibitor residual shows whether protection is actually being maintained between visits, which is what real-time monitoring is for.
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An electrochemical reaction between metal, water and dissolved oxygen. Iron dissolves at anodic sites while oxygen is consumed at cathodic sites. It is accelerated by dissolved oxygen, higher conductivity, dissimilar metals in contact, deposits that shelter localised attack, and bacteria.
Corrosion that proceeds underneath scale, sludge or biofilm. It is dangerous because the sheltered environment differs chemically from the bulk water and the attack is hidden — a surface inspection can look clean while the metal beneath is being penetrated.
Almost always because it is not truly closed. Leaks force make-up water in, which dilutes the inhibitor and introduces fresh oxygen and minerals. Persistent inhibitor loss is a symptom to investigate, not a dosing problem to top up.
With corrosion coupons — pre-weighed metal strips placed in the flow and removed after a set period, which give a measured corrosion rate rather than an opinion — alongside continuous data on pH, conductivity and inhibitor residual, which shows whether protection is actually being maintained between visits.
We measure corrosion rates with coupons and build a programme that protects the plant — not just the paperwork.