Failure Analysis

Biological Corrosion of Metals

Posted on 02. Jun, 2010 by Rob in Corrosion Analysis

Desulfovibrio vulgaris, a sulfur reducing bacteria, generate hydrogen sulfide as a metabolic bi-product. This species has been implicated in MIC – Microbiologically Induced Corrosion – in iron, steel, stainless steel, aluminum, zinc and copper alloys.

The cost of corrosion to the US economy is estimated at 4.2% of the Gross National Product according to a recent study. That amounts to over $350 billion annually which, until this year, exceeds the cost of all oil imports into the US.

Corrosion is the most common and costly failure mode impacting engineered and structural materials, yet it tends to be accepted as inevitable precisely because it is so pervasive. The same study, however, indicates that 40% of these costs, or $140 billion, could be saved through the application of existing practices and technologies. Although numbers of this magnitude tend to be overwhelming, they translate into real costs and lost savings by industry, right down to individual manufacturers and product end users.

The term “corrosion” describes a number of processes driven by a wide range of electro-chemical factors. At the root of these is the inherent instability, at the atomic level, of most industrial metals which predisposes them to return to their naturally occurring form, oxides.

One of the more unusual forms of corrosion results from the interaction of bacteria with a wide range of metals and alloys. Microbiologically Induced Corrosion (MIC) “technically” functions as an accelerant to more conventional corrosion processes. The rate of acceleration, however, may be from 10 to 1000 times conventional corrosion rates, requiring that MIC be addressed as a distinct corrosion process, from a practical standpoint.

MIC initiates and propagates primarily by two processes. The first is the formation of corrosion cells on a metal surface. Colonies of micro-organisms generate sticky biofilms which adhere them to their host surface and create a micro-environment that is significantly different from the surrounding metal. Variations in dissolved oxygen, pH, and organic and inorganic compounds in these micro-environments result in electrical potential differences with the surrounding metal, producing highly active corrosion cells.

The second is by direct chemical attack.  The metabolic by-products of many micro-organisms are highly corrosive. Two related organisms, sulfur reducing bacteria (Disulfovibrio) and sulfur oxidizing bacteria (Thiobacillus thiooxidans), produce hydrogen sulfide and sulfuric acid respectively. Localized sulfuric acid concentrations as high as 10% have been observed from these by-products. Other bacteria species produce a wide range of organic acids such as acetic acid, as well as ammonia.

Both aerobic bacteria, which thrive in an oxygenated environment, and anaerobic bacteria, which thrive in a minimal or non-oxygen environment have been documented in MIC. In some cases, these two bacterial types share a symbiotic relationship as aerobic bacteria deposit biofilms under which an oxygen depleted zone is formed at the metal interface. This oxygen depleted zone then becomes an ideal environment for the growth of anaerobic bacteria colonies.

The formation of tubercles is also often associated with MIC.  Tubercles resemble blisters of corrosion product and are initiated from biofilm deposits and iron oxidizing bacteria, particularly at low flow velocity areas in fluid piping systems. The growth and decomposition cycle of the tubercle releases sulfates and provides a site for anaerobic sulfate reducing bacteria on the interior of the blister. Tubercles also form an efficient oxygen concentration cell, dissolving iron under the blister.  Unchecked tubercle growth in fluid transport systems will severely limit or even completely block fluid flow.

Click image to enlarge.

Classic hallmark of a common MIC failure type. The cross-section above shows a Type 304 stainless steel heat exchanger tube that failed by MIC at the longitudinal seam weld, perforating the 0.065” thick tube wall.  MIC began at the tube ID due to an anaerobic bacteria species introduced through incomplete drying following hydrostatic testing.

Contact of the metal’s surface with water is a pre-condition to MIC. Since the bacteria species responsible for MIC pose no human health risk, “safe” drinking water systems are just as much at risk as non-potable water systems. Cooling systems and heat exchangers, wells, fire and agricultural automatic sprinkler systems and liquid storage tanks are among the more obvious potential sites for MIC to develop. However, fluid products not normally associated with water such as gasoline, oil and machining and cutting lubricants all contain at least trace levels of water which are sufficient to support bacteria that initiate MIC. Virtually all processed fluid products including food and beverage, petrochemical and other commercial and industrial products also contain varying amounts of water and are susceptible to MIC.

See Part 2 of this series – Microbiologically Induced Corrosion

See Part 3 of this series – Microbiologically Induced Corrosion Prevention and Analysis

Tags: biological corrosion, Corrosion Analysis, featured, metal corrosion