Inhibition of mild steel corrosion in acid mine waters containing ferric ions

Abstract

Corrosion of underground mining equipment by acid mine waters in some north eastern Indian coal mines can be quite serious. It has been found that the presence of Fe³⁺ (concentration levels up to 500 mg l⁻¹) in acid mine waters is mainly responsible for rapid corrosion of mild steel through its reduction to Fe²⁺. Microbial aspects of Fe³⁺ generation in acid mine waters are also considered. Inhibition of the corrosion of mild steel in synthetic as well as original acid mine waters containing Fe³⁺ has been investigated from the aspect of the chemical species (complexes, compounds) formed with Fe²⁺ and Fe³⁺. The results of this investigation show that not all the inhibitors which gave acceptable protection to mild steel in sulphuric acid solutions were successful in acid mine waters containing Fe³⁺. Complete protection of mild steel was achieved with chromate, oxalate, and gluconate while quaternary ammonium compounds, thiourea, diaminoacetyl thiourea, and amines offered fair inhibition. Scanning electron microscopic examination of inhibited mild steel specimens revealed that chromate and oxalate cover the entire metal surface by forming stable complexes. Atomic absorption spectrophotometric evaluation revealed that oxalate and gluconate prevent the reduction of Fe³⁺ to Fe²⁺ by forming multilayers over the mild steel surface. The corrosion products present in these multilayers, as identified by X-ray analysis, are β-FeC₂O₄·2H₂O and β-Fe(C₆H₁₁O₇)₂ respectively.