Chemical Treatment
Chemical treatment controls corrosion, scaling, and biological activity in water and process systems. Candidates should know corrosion inhibitors (anodic, cathodic, and mixed), scale inhibitors, biocides, oxygen scavengers, and pH adjusters, and how each works and where it applies.
Anodic inhibitors suppress the anodic reaction and can be dangerous if underdosed, because they may cause severe localized attack by leaving an incomplete protective film. Cathodic inhibitors reduce the cathodic reaction. Mixed inhibitors act on both. Scale inhibitors (e.g., phosphonates, polymers) interfere with crystal growth rather than removing deposits. Biocides control MIC and biofouling; oxidizing and non-oxidizing types have different application regimes. Oxygen scavengers (e.g., sulfite, hydrazine substitutes) remove dissolved oxygen.
Traps: confusing scale control with corrosion control, ignoring the critical concentration for anodic inhibitors, and forgetting that biocide efficacy depends on contact time and resistance. Also watch the interaction between chemicals and materials (e.g., chlorination affecting stainless steels) and the importance of monitoring residuals.
Worked example: A cooling system needs a residual of 5 mg/L of an inhibitor in 1,000 m³ of water. Mass required = 5 mg/L × 1,000,000 L = 5,000,000 mg = 5 kg of active inhibitor, ignoring losses. If product is 25% active, dose 20 kg of product. Rule of thumb: always convert residual target to active mass, then account for product concentration and system losses.
Understand dosing control, residual monitoring, and the role of treatment programs in an overall corrosion management plan.