Potential utilization of sludge extract as corrosion inhibitor for reinforcing steel in alkaline environment

The proper disposal and utilization of sludge is a critical issue around the world. This study aims to evaluate the potential of sludge extracts as inhibitors against chloride-induced corrosion of low-carbon steel. Acidic extract (ACE), alkaline extract (ALE) and water extract (WE) were obtained from pretreated sludge using nitric acid, sodium hydroxide and deionized water as extractants, respectively. Steel specimens were pre-passivated in simulated concrete pore solutions (SCPS) with and without sludge extracts. After the addition of chloride, electrochemical tests and surface analyses were performed. Results show that ACE has the highest phosphate and organic nitrogen contents, followed by ALE and WE. Electrochemical tests revealed enhanced corrosion resistance in SCPS with sludge extracts. ACE and ALE demonstrated higher corrosion inhibition efficiencies at 89.2% and 87.4%, attributed to synergistic effect of phosphate and organic ammonia nitrogen. WE exhibited reduced efficiency (69.8%) due to its low phosphate content. After long-term storage, the efficiency of WE declined significantly compared to ACE and ALE, probably due to the degradation of organic corrosion inhibition components. Mott-Schottky analysis indicated improved oxide passive film formation with all extracts, but phosphates may diminish this effect, necessitating further investigation.