Chemical Treatment and Corrosion Inhibitors
What you must know
Chemical treatment covers water chemistry control, inhibitor selection, and the interaction between treatment and corrosion mechanism. Understand the three broad inhibitor classes: anodic (passivating, e.g., nitrite, molybdate, chromate historically), cathodic (e.g., zinc salts, polyphosphates that form films), and mixed/organic film-formers (amines, azoles for copper). Anodic inhibitors can be dangerous at underdosing: too little inhibitor can shift the potential into a pitting regime rather than passivating fully.
Know scaling and its relationship to corrosion: calcium carbonate scale can be protective but reduces heat transfer; over-treatment with scale inhibitors can leave metal bare and vulnerable. Understand Langelier Saturation Index and how pH, alkalinity, calcium hardness, and temperature affect scaling tendency. Know oxygen scavengers (sulfite, hydrazine substitutes) and their role in boiler and closed-loop systems, and the pH/alkalinity targets that keep carbon steel in its passive range.
Be able to reason about biocide programs and microbiologically influenced corrosion (MIC): biofilm control is a treatment problem, not just a materials problem. Understand that chlorination can be ineffective in biofilms and that alternating biocides help.
Traps
Questions often ask which inhibitor is "best" without specifying the mechanism; the correct answer depends on whether the system is anodic, cathodic, or mixed control. Another trap: assuming more inhibitor is always safer. Underdosed anodic inhibitors can worsen localized attack. Also watch for confusing scale control with corrosion control.
Rule of thumb
If pH and alkalinity put the water near calcium carbonate saturation, carbon steel tends to form a protective film; if the water is aggressive (low LSI, low pH), expect general or localized attack and treat accordingly.