Metallurgical Associates is in the business of solving problems, in both service failures and manufacturing process problems. We don't actively pursue the routine testing market in chemical analysis and tensile testing. However, many clients call on us to provide these services because of the "value added" analysis we provide by reviewing "routine" test results with an eye towards the processing and service they will be subjected to. As a result, we often recommend a change in material or tightening of the material specifications. This often provides a significant savings "up front" before processing or service failure costs are incurred.

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 billon annually which, until this year, exceeds the annual 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 revenue 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 to the host surface and create a microenvironment 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, 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 bacteria 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.

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.

MIC occurs as both general corrosion and pitting corrosion, though localized pitting is the more definitive form and more likely to result in dramatic system failures. Low flow areas in circulating systems such as heat exchangers and process piping are particularly susceptible since these "stalled flow" locations provide bacteria with the opportunity to attach to the tube or pipe surface. At both microscopic and macroscopic features, fluid flow "stalling" occurs at any 

crevice, joint, weld, or imperfection and these are typical locations for MIC. Interrupted flow in circulating fluid systems such as weekend, over night, or even brief maintenance shutdowns, also provides the opportunity for bacterial adhesions and the initiation of MIC. Once the bacteria are established, the corrosion process will proceed even after flow is restored. Hydro-static testing, in which a system is filled with fluid, pressurized, leak tested and drained - but often not completely dried - is a sequence repeatedly seen in the initiation of MIC failures. This testing usually immediately precedes placing the system in service, and failure may not occur for several months. When failure does eventually occur, the hydrostatic test and stagnant fluid residue are often overlooked and the cause of failure misdiagnosed as chloride induced corrosion.

Static fluid systems such as sumps and storage tanks are receptive environments for MIC. Corners, fittings, joints and welds are again vulnerable and in the case of fuels and non-water soluble fluids, the interface between the fluid and any water contaminant is particularly susceptible. MIC in underground storage tanks and pipelines, particularly in moist clay soils, has been widely observed despite protective tar, asphalt or polymeric coatings. While effective in preventing conventional corrosion, any delamination or bond failure of the coating provides an ideal bacterial growth environment.

Virtually all industrial metal alloys are subject to MIC, with the exception of titanium alloys. Testing suggests that the few stainless steel alloys containing molybdenum at levels of 6% or more are also highly resistant to MIC. These limitations severely restrict material substitution as a strategy to resolve MIC failures.

Carbon Steels - Generally more susceptible to conventional corrosion processes, carbon steels are also widely affected by a broad range of MIC implicated bacteria. Considerations of cost and ease of fabrication make carbon steel the material of choice in many water storage and transport applications, as well as the most widely reported material in MIC failures. Protective coatings generally have limited preventive value.

Stainless Steels - These alloys develop tough chromium oxide surface layers from which they derive their corrosion resistance. Once the oxide layer is breached, however, they are particularly vulnerable to both conventional and MIC corrosion. Welds are highly susceptible due to potential alloy inhomogenaity. Highly stressed components are potential initiation sites for MIC induced stress corrosion cracking.

Aluminum Alloys - One of the earliest recognized high profile cases of MIC was of aluminum jet aircraft fuel tanks in the 1950's. Water contaminant in the kerosene based fuel and condensation in the tanks provided the media in which 

the bacteria multiplied. Research indicates some bacteria species may utilize kerosene and other fossil fuels as a nutrient source. Since this landmark case, MIC has been widely recognized as a significant problem in both tank and structural aircraft components.

Copper Alloys - Typically, higher alloy content lowers the corrosion resistance of copper alloys, although relatively pure copper is also susceptible to MIC. Copper and copper alloys are affected by a wide range of bacterial bi-products including carbon dioxide, hydrogen sulfide, and organic and inorganic acids. Cold worked or stressed copper alloy components are especially susceptible to stress corrosion cracking from ammonia and the bacteria that generate it. Selective corrosion, such as de-zincification in brass alloys, has also been observed in MIC failures.

Nickel Alloys - These alloys are often used in high pressure, high flow rate applications such as pumps, turbine blades, valves and evaporators. Nickel alloy components in these systems are vulnerable to MIC during shut down intervals and stagnant water conditions. Nickel-chromium alloys exhibit a degree of resistance to MIC.

PREVENTION AND ANALYSIS

The first line of defense against MIC is cleanliness. General corrosion prevention techniques are a good starting point since once corrosion begins, the introduction of MIC producing bacteria will greatly accelerate the process. Once bacteria are established, both anaerobic bacteria which "tunnel" into metal, and other forms which adhere under biofilms, are extremely difficult to completely remove from the affected system. Water and other fluids should be monitored for solids and debris content. These contaminants provide nutrients to bacteria, accelerating their proliferation. Filtering of fluids is useful in this respect. Water content in fuel, lubricants and similar products should be monitored and removed when excessive levels are reached. Material substitution is of limited value since, as noted, MIC effects almost all industrial metals. There are, however, several materials which are impervious or resistant to MIC where cost and compatibility justify their use. These materials are generally extremely expensive and in some cases, such as titanium, require specialized fabrication methods. In the case of underground pipelines and other fluid transport and storage systems, alternate non-metallic materials such as PVC have significantly limited MIC where these materials can be substituted. Local building codes, however, often exclude this option in structural applications. Design to minimize low-flow areas, crevices, welds, etc. can reduce the likelihood of MIC but there are severe limitations to how far this approach can be taken in the design and manufacture of practical systems. Biocides are widely used to treat incoming water. These, however, are highly toxic and expensive and require regular monitoring of concentration. Their toxicity and potential contaminative effect precludes their use in any food products and many process fluids.

The parameters in which MIC can occur are extremely varied and include multiple bacteria species, a broad range of affected materials and almost endless environmental diversity. As a result, MIC prevention and mitigation is equally varied. Accurate analysis of the cause and effects of each individual MIC failure is an essential first step in selecting from this range of solutions.

On May 11^th, 1842 the first major railroad disaster in history set off a chain of events which led to the discovery of the phenomenon that we now know as fatigue failure.  The Paris - Versailles Express, hurtling down the tracks at the then astounding speed of 50 miles per hour, exploded in flames when the drive axle on the lead locomotive broke, digging its front end into the railbed.  The second locomotive in the tandem drive set smashed into the firebox of the lead engine along with the first three cars, killing 57 passengers outright and injuring over a hundred more.  It was the 1800's equivalent of a jumbo jet crash, and the great scientific minds of the day focused their collective wisdom on perhaps the first major failure analysis in history.  The result of their decade long investigation produced the beginnings of our understanding of fatigue.

Fatigue is the most common type of fracture in engineered components.  Fatigue fractures are also particularly dangerous because they can occur under normal service conditions, with no warning that a progressively growing crack is developing until the final catastrophic failure.  The component, whether it's the outer aluminum skin of a commercial jet or a simple tubular chair leg, often appears to be perfectly sound with no visible distortion to warn of impending failure. 

A technical understanding of fatigue requires a comprehensive knowledge of metallurgy, physics, and phenomena like plastic deformation, slip planes and dislocation theory.  In fact, there are several competing theories on exactly what happens at a microscopic level when a fatigue crack initiates.  But a practical understanding of the process is extremely beneficial and has direct application to its prevention, and the manufacturing environment, as discussed below.

To the non-technically inclined, the term "fatigue" suggests this type of failure is related to the age of a component, that the material is "tired".  In fact, fatigue fracture can occur within hours of a component going into service.  Conversely, even large, highly stressed components can operate for decades with no fatigue cracking or failure.

Fatigue fractures result from repeated, or cyclic, stresses.  These stresses can take a variety of forms, such as bending (in one direction), reverse bending (back and forth in two directions), torsion (twisting in one or more axis) and rotation.  Regardless of the variation in direction, the stress on the component at the point of fatigue fracture is always tensile stress, in which the fracture initiation site is being "stretched", or pulled in opposite directions.  To illustrate this, visualize a tube which is being repeatedly bent in one direction.  The side of the tube that is concave when it is bent is being compressed.  The side of the tube which is convex is being "stretched", or subjected to a tensile stress.  This is the side on which a fatigue crack will initiate.

Fatigue cracks initiate at stresses below the tensile strength of the material.  Tensile strength is the stress, or load, at which a material breaks when pulled in two opposing directions.  This load is a specific value for each metal alloy, varying somewhat depending on heat treating and other processing operations.  These values are widely available in engineering reference manuals, typically expressed as pounds per square inch in American references.  The fact that fatigue cracks can occur at stress levels below the tensile strength of a material is difficult to explain.  Theories on this focus on physical and structural changes at the microscopic (0.0001" or less) area of crack initiation.

Fatigue is a progressive fracture mechanism.  Once a fatigue crack initiates, it is driven further into the component with each stress cycle.  This crack growth process continues as long as the component is subjected to cyclic stress.  Depending on the magnitude and frequency of these stresses, the crack may grow over time ranging from hours to years.  Eventually, the crack advances to a point where the remaining intact cross section of the component can not sustain the next cyclic stress load - "the straw that breaks the camels back" - and complete fracture of the component occurs.

In the "real world" fatigue usually - that's usually, not always - initiates at a location that acts as a stress concentration, or focal point, to the stresses imposed on a component.  Stress concentrations take a wide variety of forms.  They include geometric features (such as holes, slots, corners and radii), rough areas of surface finish, welds, corrosion pits, and microstructural defects such as inclusions.

The exception to "usually", the cases where fatigue fractures initiate from component surfaces that are free of stress concentrations, typically result from one of two causes; under-design of the component, or abusive service conditions.  Just as all materials have an ultimate tensile strength, they also have a fatigue strength, sometimes called the fatigue limit or endurance limit.  Once a component is subjected to cyclic stresses that exceed this limit, fatigue fracture occurs.  

Fatigue failures of this type are less common than fatigue failures initiating from stress concentrations.  Usually components are intentionally over-designed to deal with stresses several times greater than what they would be subjected to in service as a safety margin.

If the initiation stage can be prevented, fatigue fracture will not occur.  It sounds so obvious and simple.  It's not.  As noted above, initiation is the most complex stage of fatigue fracture.  A low magnitude load, which would have no effect whatsoever on a component in a single application, can be devastating when repeatedly applied as thousands or millions of cycles.  The cumulative effect of these cyclic loads are microscopic "shifts" in the material's structure which ultimately produce a "dislocation" - at this scale it is too small to be called a crack - and the focal point of stress concentration is born.  Corners, holes, rough surface finish, welds and other features only accelerate the process.  To further complicate the issue, vibration harmonics, dampening of the system and the environment in which the component functions add a large unknown factor.  Collectively, these affects become difficult to predict in the design stage.

From a practical standpoint, fatigue failures present a danger to you, the manufacturer, at three points in a components life.  These are the design stage, the manufacturing process, and the service environment.

Design

The design engineer is the first line of defense against fatigue fracture.  He or she can't prevent failures originating in the manufacturing process or service environment, but the designer lays the foundation of prevention.

In an ideal world, each design would be subjected to extensive stress calculations and fatigue testing.  In the real world this is rarely cost effective for non-critical components. Instead, accepted and "proven" parameters are applied.  These typically include safety margins which are more than adequate.  Typically, but not always.

Computer Aided Design (CAD), Finite Element Analysis (FEA) and a variety of other computer driven design and predictive technologies can greatly enhance the fatigue resistance of a component at the design stage.  But they can not prevent fatigue failures.  That's because the next two threats of fatigue failure are beyond the designer's control.

Manufacturing processes are a rich, though unintended, source of stress concentrations from which fatigue cracks can initiate.  The list is almost endless, and includes rough machined surfaces from dull tooling or excessive feeds and speeds, burrs from cutting or drilling operations, and insufficient chamfers or corner radiuses.  Welds, even when technically faultless, provide geometric stress concentrations.  Defective welds and welding procedures may result in porosity and high hardness heat affected zones from which fatigue can initiate.  Mechanical fasteners - bolts, screws, studs, and rivets- are highly prone to fatigue failure, either due to defects in the fastener itself, or to insufficient tightening torque during the assembly stage of the manufacturing process.

Care in manufacturing and a good quality control program will avert many of these potential sources of fatigue initiation.  However, despite the best quality control program, the manufacturer is often at the mercy of their raw material supplier.  These suppliers may open the door to fatigue failure through castings which contain excessive porosity or  microstructural defects, mill products which are work hardened,  forgings with undetected laps or seams, or gross non-metallic inclusions in any of these products.  Appropriate specifications on outsourced stock and components are vital in guaranteeing their quality, but as with so many aspects of production, they are a compromise.  Loose specs solicit low cost bids, but a potentially high percentage of defective products, while tight specs limit the number of vendors capable of meeting them and drive costs higher, cutting into profits.

Once a product leaves the factory you, the manufacturer, have lost control and all bets are off.  Abuse and inadequate maintenance are leading sources of failure by fatigue, as well as other failure modes.  Failure of components or assemblies "up stream" from your product may introduce higher loads than the product or component was designed to sustain.  Service environments, such as road salts or ocean front installations may instigate corrosive attack, with corrosion pits providing a fatigue initiation sight.  Analysis and identification of the root cause of fatigue failures in these cases is critical in educating  your customer in the appropriate use and maintenance of your product and getting them back on track as a satisfied customer.

Identifying the root cause of service environment initiated fatigue failures can be challenging.  Some years ago, we provided analytical support on a lawsuit which was filed after an individual sustained a back injury when the metal leg of a "stacking chair" fractured.  Stacking chairs are the type of institutional chairs you often see in school auditoriums, government office waiting rooms, etc. and are designed to be stacked, one upon the other, for more compact storage when not in use.  This particular chair came from a college in Ohio.  Our analysis proved low stress, high cycle fatigue as the failure mode.  In other words, low magnitude stresses applied at high frequencies, in this case over a million cycles.

The chair had been in use for a relatively brief time, and even if it had seen longer service, it seemed unlikely that it had been sat in on a million occasions.  This presented something of a mystery, as the failure mode was indisputable.  Investigation of the service environment revealed that the chairs were used sporadically and when not in use, were stacked in a storeroom.  The college staff was methodical in setting up the chairs in orderly rows in an adjacent auditorium, then stacking them from the same row end when they were no longer required, with the same chair ending up on the bottom of the stack before going back into storage.  The stack was higher than the maximum specified by the manufacturer, providing a load in excess of the design limit.  A survey of the area revealed that the storeroom was immediately above the main HVAC installation, the key and final piece of the puzzle.  Vibration from the HVAC system, transmitted through the storeroom floor, and loads from the weight of chairs stacked in excess of the design limit provided the stresses required to initiate the fatigue crack.  Once the crack grew to the point at which the remaining intact tubular leg could no longer sustain the load of a sitting person, final fracture occurred.

As with all failure analyses, the analyst must provide specific answers to three critical questions when evaluating a fatigue failure.  They are: 1. how did it fail?  2. Why did it fail?  and  3. What will prevent future failures?  If you have commissioned a failure analysis, and all three of these questions are not answered, all you have paid for is some interesting pictures and a possible lawsuit when your product fails again.