Stainless Steel in Municipal Sewage-How to Recognize Favorable Corrosion Conditions

Area of Science:

• Materials Science
• Environmental Science
• Microbiology

Background:

• Chromium-nickel steel, specifically 304 stainless steel with delta ferrite, is generally corrosion-resistant.
• Wastewater environments can present unique challenges leading to unexpected material degradation.
• Understanding factors influencing corrosion is crucial for infrastructure longevity.

Purpose of the Study:

• To investigate the corrosion behavior of 304 stainless steel exposed to wastewater under different submersion conditions.
• To identify the microbial factors contributing to observed corrosion, particularly pitting.
• To evaluate the potential of microbiological analysis for assessing wastewater corrosivity.

Main Methods:

• Exposure of 304 stainless steel samples to real wastewater for 18 months in three configurations: above surface, half-submerged, and fully submerged.
• Visual and surface analysis of corroded samples.
• Fluorescence in situ hybridization (FISH) to identify bacterial classes in wastewater sludge and on sample surfaces.

Main Results:

• Samples above wastewater (A-series) showed intensive pitting corrosion.
• Half-submerged (B-series) and fully submerged (C-series) samples exhibited minor mechanical erosion and no significant surface changes, respectively.
• FISH analysis revealed increased populations of delta-proteobacteria, known for sulfur/sulfate reduction, in sludge and on submerged samples.

Conclusions:

• The wet hydrogen sulfide environment, influenced by specific bacterial activity, is a key factor in the pitting corrosion of 304 stainless steel above wastewater.
• Delta-proteobacteria play a significant role in the corrosive conditions observed.
• Microbiological evaluation, focusing on bacterial classes, offers a promising method for assessing wastewater aggressiveness and predicting corrosive environments.