Corrosion Challenges from the Animal Kingdom

When the engineering community designs a bridge or other structure or a piece of industrial equipment, they take into consideration many factors, including the service loads, material properties, and increasingly, the potential for corrosion and other environmentally-driven effects.

Often, a corrosion-related design review addresses the chemical environment and the electrochemical aspects of corrosion on the material properties, and occasionally, the potential for environmentally-induced cracking. But despite the best efforts and foresight of the material scientist/designer to cover all the bases and avoid future failures, nature always seems to throw a curve ball. In this issue, we would like to share with our readers a few interesting cases of equipment failures caused by members of the animal kingdom.

Our first example represents the earliest known incidence of stress corrosion cracking (SCC) and dates back hundreds of years, long before the phrase was even coined. Interestingly enough, this first known incidence of SCC had a biological origin. In the sixteenth century, cavalry officers of the East India Trading Company (precursor to the British Raj, which lasted until India's independence in August 1947) were experiencing extensive failures of brass shell cartridges due to cracking. The cracking only occurred during the monsoon season, and, as a result, it became known as "season cracking."

The problem was eventually recognized as somehow being related to the practice of storing the brass cartridges in the cavalry horses' stalls. But why should this be a problem?

In hindsight, the answer is obvious to material scientists. Brass alloys are quite susceptible to SCC in the presence of ammonia. And what is one of the primary liquid by-products of animal metabolism? Ammoniacal compounds! So what was causing the cracking? Ammoniacal compounds carried over from the horses' urine, particularly during the very wet and humid monsoon season, were sufficient to cause SCC in the brass cartridges. The solution? Don't store the cartridges in the horse stalls!

Recently we saw an example of history repeating itself at one of the hydroelectric dams on the Columbia River. Spillway gate operation was being hampered by repeated failures of the thrust washers which support the gate swing arm swivel (see figure right). The thrust washers dated back to the dam's construction in the 1950s and were made of manganese bronze, with lead/graphite inserts for self-lubrication.

The failures were from numerous, multiple-branching, intergranular stress corrosion cracks. (see figures left). The cracking had initiated from the insert holes and propagated across the thickness of the washers.

Initially, we were quite surprised to see such a failure mode in the benign, outdoor Columbia River water environment. Elemental analysis of the lead/graphite inserts revealed the presence of ammoniacal compounds, which we quickly recognized as the likely cause for the SCC in the surrounding bronze material. But where were the ammoniacal compounds coming from out there in the "fresh air" environment of the spillway gate?

Further inquiry revealed the culprits: (No, it wasn't horses out there on the spillway gates!) Pigeons! Lots and lots of Pigeons! It turns out that Pigeon colonies had found the spillway gates to make comfortable gathering sites. However, unfortunately for the gate thrust washers, the resulting Pigeon excrement had provided the necessary aggressive ammoniacal environment for the occurrence of SCC.

In hindsight, it seems like a fairly obvious problem, but it was probably a long shot for the dam designers to have anticipated this back when they designed the dam! Potential corrective measures included the use of a replacement material for the thrust washers with a higher resistance to SCC. Another solution, of course, was to relocate the pigeon colonies.

While the "higher" forms of animals provided the aggressive environment for these first two corrosion failures, the "lower" life forms, in particular, microbes, also participate in causing corrosion havoc to metals. This so called microbial induced corrosion (MIC) is increasingly being recognized as the cause for previously unexplained corrosion in a number of cases, particularly within the chemical processing and oil industries.

Recently, we encountered an interesting example of this. One of our clients called with a problem: One of his storage tanks had sprung a leak, and when he had drained it and gone inside, he had found a peculiar localized area of severe corrosion on the floor; could we come over and take a look?

When we arrived on site, we went through the usual failure analysis procedures of establishing the age, service environment, and material of the tank; we then examined the tank to characterize the extent of the failure and its morphology.

The tank was about 15 years old, it was made of Type 317L stainless steel, and it was being used to store an acidic sulfite solution used in a pulping process. Over 50 percent of the bottom floor plate thickness had been lost to corrosion over about a quarter of the floor area. In a couple of spots, the corrosion had penetrated completely through the floor, which had resulted in the leak.

The corrosion was confined to an area of dark deposits that had accumulated at the low points of the tank bottom; outside of these areas, the stainless steel surfaces were completely intact. In fact, you could still see the bright, reflective polishing swirls from when the tank was built! Collecting samples of the corrosion products, we returned to our laboratory for a more detailed, microscopic examination.

The dark deposits were determined to be sulfates of the stainless steel base metal; even the weld contours were retained in the (converted) sulfate corrosion products (see figure left). The base metal sulfates were crystalline (see figure right) and had a pH of 2, somewhat more acidic than the bulk solution pH of 3.5.

The corrosion morphology was characteristic of the damage caused by anaerobic sulfate-reducing bacteria. Basically, the "food" for the bacteria consists of sulfite/sulfate, which, through a complex process, they convert to sulfide. The result of their "feeding" process is an increase in the acidity of the local environment, to the extent that the stainless steel is subject to aggressive attack.

Given the right conditions and nutrients, these microbes thrive in a wide range of environmental conditions. And even the high alloy stainless steels such as Type 317L, while offering superior corrosion resistance in many applications, are not immune to microbial corrosion.

We have seen microbial corrosion in both carbon steel and stainless steel alloys in a variety of other application too, including repulpers and paper machines. The "bugs" seem to survive in a wide range of media, from acidic to alkaline, and at temperatures where the higher life forms would immediately perish.

These types of challenges from the biosphere are just now beginning to be widely recognized among material scientists and engineers. Perhaps one day it will become common knowledge, and we'll take a call from a plant operator saying, "My equipment is suffering from a bacterial infection. Can you check it out and prescribe a remedy?"

Dr. John Simmons Joins MEI-C Staff

We are pleased to announce that Dr. John W. Simmons has joined our staff as a senior metallurgical engineer. Dr. Simmons has more than 15 years of industrial, manufacturing, and research experience in metallurgy and materials science. His special area of expertise involves failure analysis, materials characterization, structure-property relationships, alloy development, and powder metallurgy.

Most recently, Dr. Simmons was Technology Manager for a medical implant device manufacturer specializing in porous metal coatings. Prior to that, he was a Senior Research Scientist with the U.S. Department of Energy's Albany Research Center in Albany, Oregon.

He is a graduate of the Oregon Graduate Institute of Science & Technology, with Masters and Doctorate degrees in Materials Science and Engineering. He also holds a Bachelors degree in Metallurgical Engineering from California Polytechnic State University in San Luis Obispo.

Dr. Simmons has been actively involved in the International Metallographic Society (IMS), an affiliate society of ASM International, for a number of years. He is currently serving a four-year term as a member of the IMS Board of directors and served as the General Chairman for the highly successful 30th Annual International Metallographic Society Convention held in Seattle (1997). He received the 1998 "Presidents Award" from the International Metallographic Society for his contributions to the Society.

Dr. Simmons has been recognized on numerous occasions for his work in metallography/materials characterization; he received the prestigious Jacquet-Lucas Award for Excellence in Metallography from ASM International/The International Metallographic Society (1994) and the Grand Prize at the 1997 International Powder Metallurgy Metallographic Competition.

He has more than 30 technical publications in the areas of metallography, sensitization and corrosion of stainless steels, thermal stability, microstructural development, structure-property relationships, strain hardening, and novel powder processing techniques, much of the work involving nitrogen-alloying of stainless steels. He developed mechanical fluidized bed technology for the production of high-strength nitrogen-alloyed austenitic stainless steels for powder metallurgy, synthesis of Ni-Al powders for thermal spray applications, and synthesis of other compounds. He has also been involved with the development of system documents for ISO 9000 as well as safety and health program.

John and his wife Qinghua, a senior engineer with a local semiconductor firm, enjoy spending time with their four year old son Daniel, and anxiously await the arrival of his sibling at the end of July.

Biodegradation of Materials

As our feature article discussed, the scientific community is becoming increasingly aware of the biosphere's attack on metallic materials. More generally well known are the deteriorating effects of the biosphere on nonmetallic materials such as wood, adhesives, sealants, paint and coatings, rubber, plastic, coolants, and fuels.

All of these materials can be degraded by bacteria, fungi, or algae, depending on the material and the exposure history. We have conducted many investigations in which materials have failed to perform as expected, and many of these cases have involved sorting out whether the degradation was caused by biological or other agents. About 90 percent of the time, the cause of the deterioration falls into the "other" category, but sometimes biodegradation contributes partially or wholly to the problem.

The most commonly encountered material suffering biodegradation is your house. Any paint or roof after a few years of exposure builds up a surface film of dust and dirt, which provides a nutrient medium for various bacteria, fungi, and algae.

Of course, manufacturers have had many years experience in formulating paint and roofing materials to minimize degradation. And in our wet Pacific Northwest climate, paints and roofs need more biocides or bio-resistant materials than in dry climates.

Inside your house, wood and wallboard exposed to moisture will biodegrade. Bathrooms, kitchens, windows, and sliding doors are primary targets for fungal attack. Not only do structural materials degrade, but also people exposed to high levels of fungal spores and bacteria can develop medical problems such as chronic respiratory ailments or pneumonia.

Many of our biodegradation jobs have involved indoor air problems or stains. Water incursion or condensation is almost universally present, and removing the water or lowering the humidity is usually the solution.

We have had several cases where occupants of office buildings have complained of odors and recurring headaches; these problems are sometimes traced to faulty construction where an inadequate (or nonexistent) moisture barrier between the ground and the building has resulted in excessive interior moisture levels and consequent growth of bacteria, fungi, or other biological agents.

We have encountered several cases of bacteria clogging airplane fuel filters at critical times. With fuels, the key is usually not to store them for long periods of time.

In one project, a barge explosion during welding was traced back to anaerobic bacteria working on water and oil to generate explosive amounts of methane. A similar project involved an explosion of a house in Portland many years ago due to seepage of methane out of the ground under the house. The house had been built in a swampy area where bacteria beneath the house produced methane, or "swamp gas".

Years ago, we had a project involving a dark stain on the surface of a donut. We set out to analyze the elemental composition of the stain in the electron microscope to see what type of noxious chemical was responsible, but found instead that the so called stain was a spectacular "forest" of mold—a great example of nature in her full glory!

Sometimes, the biological agent is several steps up the evolutionary ladder from bacteria and molds. A recent complaint about a "dark speck" on a food container resulted in our analyzing the speck and finding it to be a particular gnat native to eastern Oregon, Idaho, and Utah trapped under the polyethylene coating.

MEI-C People

Chris Gerdes has joined MEI-C as an engineering technician. Chris has an Associate degree in Nondestructive Testing Technology from Southeast Community College in Milford, Nebraska and studied metallurgical engineering at Iowa State University for two years. In his spare time, Chris enjoys nature and building custom cars.

Mark Habel, engineering technician, received his certificate as a Certified Welding Inspector (CWI) from the American Welding Society (AWS).

Dr. D. G. Chakrapani, president of MEI-C, attended the 31st National Symposium on Fatigue and Fracture Mechanics sponored by ASTM in Cleveland, OH, June 21-24.