INTRODUCTION

ABSTRACT

The rapid and unexpected failure of AISI type 304 stainless steel in a wastewater treatment system was investigated in the laboratory by simulation studies for a period of 4 months. Slime and water samples from the failure site were screened for corrosion causing bacteria. Pseudomonas sp. and Bacillus sp. were the most common aerobic bacteria isolated whilst sulfate reducing bacteria (SRB) were the major anaerobic bacteria. Compared to sterile controls, metal coupons exposed to Pseudomonas sp and Bacillus sp. showed initiation of severe pitting corrosion suggesting the possibility of MIC. As the appearance of pits was different in both the tested strains, it was thought that the mechanisms of corrosion are different.

Microbiologicaly influenced corrosion (MIC) is one of the most deleterious effects of metal microbe interactions. Unexpected and rapid failure cases occur in otherwise resistant materials even in mild environment (Walsh, 1999). MIC occurs often at or near welds (Borenstein 1991). Materials affected by MIC include stainless steels among many other structural materials (Borenstein 1991). This phenomenon is reported in different industries such as power, chemical, shipping, oil and gas and potable water supply lines. Localized corrosion in the form of pitting in stainless steels is a common failure cause due to the influence of microbes present in the surroundings. When a fresh metal surface is exposed to a non-sterile medium, a conditioning film is formed followed by biofilm. The first ones to attach over the surface is bacteria, followed by unicellular algae, fungi etc. (Borenstein, 1994). Biofilms are never uniform and they form patchy non uniform areas with or without biofilm. A change in the chemistry of the surface follows and consequently aggressive micro niches results. Depending on the nature of metabolism and the community interaction, the characteristic reactions makes the surface corrode. Though the focus at the electrochemical aspects of MIC is in limelight in the recent past (Dexter et al, 1989; Little et al, 19990; Mansfeld and Little 1990; Videla 2001), emphasis on the aftermath of biofilm formation on it is lacking attention. We had attempted to investigate on a corrosion failure case from a point of view of presence of bacteria and film formation.

Background

An unexpected and rapid failure case occurred in a wastewater treatment plant. The post combusted ash is collected in a reservoir; rainwater sweeps through it and the seepage water is collected and purified before it is let to the river water system. Leaking in the disc used to stir the wastewater as it is being purified made the shaft not rotating and consequently stopped running. The failure was observed in less than six months period which was unexplainable from the material side as the structural material was AISI type 304 stainless steel. On examination, severe pitting was seen at and near the welds completely covered with slime. The appearance of corrosion sites on the failed samples and the surroundings represented typical MIC features. Pits were typical in having tiny openings outside which on internal observation led to wide and severe damage (Fig.1). Anticipating the possibility of microbiologically influenced corrosion, laboratory simulation studies were carried out using bacterial isolates from the water and slime samples brought from the corroded site. Results were compared with that obtained in the case of sterile medium to elucidate the effect of microorganisms present in the environment.

MATERIALS & METHODS

Enumeration, isolation and identification of bacteria

Slime and water samples from the corrosion

This content is only available via PDF.
You can access this article if you purchase or spend a download.