Hydrogen Embrittlement – Part 3
High Strength Steels Achilles Heel

Susceptible Materials

While some stainless steel grades are susceptible, high strength steels with tensile strengths and hardness above 130,000 psi and Rockwell C35, respectively, are the most prone to hydrogen embrittlement. Steels below these tensile and hardness levels are generally immune. Why?

Increasing hardness, most commonly by heat treating, is accompanied by a corresponding decrease in ductility. In simple terms, ductility is the ability to deform under stress rather than crack or fracture. When hydrogen atoms combine into molecules in a steel that exceeds the tensile strength and hardness threshold, the steel cracks under the pressure increase. But if the tensile and hardness levels are below the critical threshold, the higher degree of ductility allows the steel to deform, absorbing and redistributing the pressure increase, rather than cracking.

Susceptibility to hydrogen embrittlement increases in alloy steels with heat treatment to higher strength. The strength/susceptibility relationship, in fact, approaches exponential levels. In other words, doubling the heat treated strength, quadruples the steel’s susceptibility to hydrogen embrittlement.

Identifying and sorting embrittled parts from good components before they fail is virtually impossible. The detection limit of chemical analysis for hydrogen is generally well above the 5 to 10 ppm level at which embrittlement has been shown to occur. Even if such detection capability was available, hydrogen tends to concentrate at isolated locations within the part. This leaves the majority of the part at “low” or undetectable hydrogen levels. Chemical analysis of parts after failure, to determine if hydrogen embrittlement is the cause, is also not viable since hydrogen diffuses from the part after fracture.

Since most hydrogen embrittlement results from hydrogen absorbed during the manufacturing process, parts which are “batch” processed are usually either all embrittled or all “good”. The failure of one part from a “batch”, therefore, is usually a good indication that others from the same batch will fail over a similar time period.

Prevention

The two keys to avoiding hydrogen embrittlement are at the design stage and during the manufacturing process. Designers with little metallurgical training may not realize the implications of the materials and manufacturing processes they call for in the drawing specifications. As with other failure modalities, hydrogen absorption can inadvertently be “designed into” a part.

On the manufacturing side, avoiding reducing acids where possible removes an abundant source of hydrogen from potential exposure to the part. Where electro- plating is required, minimizing plating time and maximizing current density reduce the volume of absorbed hydrogen. Consideration of electroless plating or vapor deposition as alternatives eliminates the possibility for hydrogen absorption from the coating process altogether.

Protecting furnace charges or components that will be welded from corrosion, or cleaning them prior to use will avoid hydrogen introduction by these routes. This applies equally to parts that will be heat treated. Any processing that will elevate the temperature of the part, and most do, will also raise hydrogen to a higher level of mobility, increasing the potential for absorption.

Hydrogen Management

Despite the most stringent precautions, processing requirements will sometimes introduce hydrogen to parts that exceed the hydrogen embrittlement tensile and hardness threshold. Fortunately, there is a procedure that will effectively remove absorbed hydrogen. This oven heating process, referred to as “baking”, is performed as follows:

1. Parts must be baked within 4 hours of hydrogen exposure. Less is better.
2. Parts must be baked at 400º F.
3. Parts must be held at 400º F for a minimum of 4 hours. Longer may be required depending on part size, processing, etc.

To be effective, the time and temperature parameters must be strictly followed. Short-cuts or delays on any step will dramatically reduce the effectiveness of the entire process. For example, twice as much hydrogen will be baked out at 400º F versus 350ºF, and doubling the bake time doubles the amount of hydrogen that is baked out.

The sooner baking begins after exposure to hydrogen, the better. The 4 hour “window” is a maximum. Note that baking must be performed after each hydrogen exposure if 4 hours will elapse between multiple exposures. No amount of baking will salvage embrittled parts if these time and temperature parameters have not been met.

A final word of caution. A 30 year analysis of hydrogen embrittlement failures in the aircraft industry found that over 70% resulted from improper baking procedures.

Closing

There are competing theories on the mechanism by which hydrogen embrittlement occurs. That presented here appears to be the most widely accepted by the scientific community.

The subject of this article has been limited to hydrogen embrittlement in low alloy steels. Other alloys are subject to hydrogen embrittlement, thought the mechanisms discussed are the same. However, other hydrogen driven failure mechanisms have been observed, though they appear to be less common. These include hydrogen induced blistering, internal hydrogen precipitation and hydride formation in nonferrous metals.

Part 4 will briefly discuss the analysis of hydrogen embrittlement failures and illustrate several “infamous” and unusual failures.

The Metallurgical Phenomenon

Hydrogen atoms are the smallest of any element. So small, that they easily travel between iron atoms. The boundaries between crystals, or grains, which are the structure of metals, are gapping canyons in relative size to hydrogen atoms. Once absorbed, hydrogen atoms are attracted to microscopic crystal defects, or misalignments, where there is slightly more space between grains. They are also attracted to areas under tensile stress that cause a very slight increase in the space between grains from the opposing “pull” of the stress.

As more hydrogen atoms accumulate at these areas, they combine to form relatively very large hydrogen molecules (H₂) which raises internal pressure, expands the size of the defect or grain boundary interface and attracts still more hydrogen atoms, accelerating the process. This cycle produces a raising tensile stress inside the component which eventually results in a micro-crack. These micro-cracks grow rapidly and simultaneously at numerous locations within the part, reducing the actual intact load bearing cross section by as much as 10-20%.

In order for hydrogen embrittlement to occur, three conditions must coincide:

1. The part must have a tensile strength in excess off approximately 130,000 psi. This generally corresponds to a hardness of Rockwell C 35.
2. The part must be in contact with a source of hydrogen. This may occur during manufacture, in service, or both.
3. The part must be subjected to a tensile stress.

This last condition can be deceptive because parts do not need to be assembled or in service to be under tensile stress. Residual internal stresses from casting, forging, welding and other manufacturing processes are significant and, in fact, are probably the root cause of most hydrogen embrittlement failures. Heat treating to raise strength levels above 130,000 psi induces substantial levels of residual stress. The disturbing phenomenon of “shelf popping”, unassembled parts cracking in storage or inventory with an audible “pop”, results from hydrogen embrittlement associated with residual stress.

Since the majority of the hydrogen is absorbed through and accumulates at the grain boundaries, hydrogen embrittlement cracking is primarily intergrannular (fracture at the grain boundary) rather than transgrannular (fracture through the grains) as in some other forms of brittle cracking.

Hydrogen Sources

One of the challenges in predicting and preventing hydrogen embrittlement is the wide range of available sources of hydrogen, both in the manufacturing and service environment.

Sources from manufacturing include the original steel making process, subsequent casting or forging, grinding operations, soldering and brazing fluxes, blasting and tumbling media, welding electrodes, acid cleaning or pickling and electro-plating, etc.

Service related sources of hydrogen may include incidental contact with acids or hydrogen containing cleaning solutions, or absorption from hydrogen containing product by equipment used in its processing. The most common source in service by far, however, is corrosion. Corrosion can also act as a source of hydrogen in the manufacturing process. Rusted ingots and scrap used in casting melts, welding on parts that have corroded, and heat treating corroded parts, are potential sources of absorbed hydrogen, particularly when exposed to elevated temperatures which increase the mobility of hydrogen atoms.

In Part 3, we will discuss the conditions that render steels susceptible to hydrogen embrittlement, and how to prevent its occurrence in both the manufacturing and service environments.

Part 4 will briefly discuss the analysis of hydrogen embrittlement failures and illustrate several “infamous” and unusual failures.

The Phenomenon

Sudden brittle fracture in high strength steels resulting from hydrogen embrittlement represents an extremely dangerous phenomenon to industry, particularly since it is usually the result of factors that occur during the manufacturing process.

Hydrogen embrittlement reduces ductility, often to the point where metals behave like ceramics. Consequently, fatigue strength and fracture toughness are also dramatically reduced. Brittle fracture occurs without warning and can be immediate, within hours of manufacture, or after years in service. Hydrogen embrittlement failures have even been observed in unassembled parts in inventory, a phenomenon known as “shelf popping”.

Generally, the higher the strength of the steel, the more at risk it is to hydrogen embrittlement and the more vulnerable it is to lower levels of hydrogen. Embrittlement at levels of 10 parts per million and less are not uncommon. Some research suggests this relationship is exponential. In other words, doubling the strength, quadruples the susceptibility to hydrogen embrittlement.

Although hydrogen embrittlement occurs in many different metal alloys, high strength steel appears to be the most sensitive, is the most widely used and accounts for the largest number of hydrogen embrittlement failures. This article offers an overview on hydrogen embrittlement as it relates to high strength steels only, though details of the phenomenon generally apply to other susceptible metals.

First Appearance

In the late 1940’s a revolution was underway in aviation. Jet propulsion was rapidly replacing the old piston engine driven propeller technology and aircraft performance began to exceed levels that had been considered physically impossible just ten years earlier. Weight reduction and more power propelling airframes that could withstand higher loading were critical to these improvements. This resulted in demands for higher strength alloys from which smaller, lighter and stronger components could be made.

Low alloy steels such as 4130 had been used in aviation in the past. However, these materials were typically used in the normalized heat treated condition, at tensile strengths in the 90,000 to 120,000 psi range – well below levels susceptible to hydrogen embrittlement. In response to demands for more strength, “radical” heat treatments to tensile strengths approaching 200,000 psi were applied to 4130 and other “anemic” low alloy steels. Some of the first hydrogen embrittlement failures appeared, though they weren’t initially recognized as such.

Enhanced low alloy steels, such as 4140 and 4340 were used in response to these failures, and the cycle was repeated, with the demand for more performance from smaller components resulting in processing to ever higher strength levels.

One of the unfortunate consequences of increasing the strength of low alloy steels is a corresponding reduction in corrosion resistance. To combat increased corrosion in service, a variety of electroplated coatings, such as chromium, nickel and cadmium, were applied. With a new potent source of hydrogen now available from the plating baths used in these processes, a dramatic increase in hydrogen embrittlement failures occurred in both the aerospace industry and in other industries to which the new materials technology had filtered down.

Part 2 in this series will discuss this phenomenon from a metallurgical perspective – what actually occurs, on a microscopic scale, that causes hydrogen embrittlement?

In Part 3, we will discuss the conditions that render steels susceptible to hydrogen embrittlement, and how to prevent its occurrence in both the manufacturing and service environments.

Part 4 will briefly discuss the analysis of hydrogen embrittlement failures and illustrate several “infamous” and unusual failures.

A gas transmission compressor shaft in a facility in China began showing excessive wear at the bearing journals during initial mechanical testing (spinning) of the compressor. The plant operator suggested the tin based babbitt bearings were the source of the wear. Our analysis of the bearings showed that the babbitt and bond fully met industry specifications. Our analysis also revealed hard abrasive particles, composed of aluminum oxide (alumina) and silicon dioxide (quartz) embedded in the surface of the babbitt (Figures 1 and 2). These materials are consistent with grinding and sanding media, and caused the bearings to act as a cutting tool on the shaft journal surface. Metallographic examination of cross sections taken from the bearing confirmed that these particles were present at the surface only, and not throughout the babbitt layer verifying that they were introduced during the commissioning or initial operation of the compressor rather than during manufacture of the bearings. These results indicated that contamination of the lubrication system feeding oil to the bearings occurred during installation or maintenance by the plant operator’s personnel.

Figure 1 – Embedded silicon dioxide particle in the bearing surface. 450X

Figure 2 – EDS analyses of the embedded particles indicated a mixture of aluminum oxide (Al and O)
and silicon dioxide (Si and O), or quartz (shown above).

Fingerprints

Fingerprints are usually not a source of what we typically think of as contaminants. However, perspiration and the oils from our skin contain chlorine, potassium, sodium and other elements in sufficient concentrations to contaminate and initiate corrosion in sensitive components such as electronic contacts and data storage media. Highly polished parts are also susceptible to cosmetic degradation from fingerprints if no protective clear coat is present. Plating defects such as blisters and poor plating adhesion can result when these “bio-contaminants” are present on parts prior to immersion in the plating bath. This was highlighted in MAI’s recent analysis of blister defects (Figure 3) on a nickel plated exhaust manifold.

Figure 3 – SEM image of blistered nickel plating. 4000X

Analysis of opened blisters by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) revealed the presence of chlorine, potassium, calcium and sulfur in small, but significant, concentrations. Both the plating and pre-plating rinse bath chemistries were well within optimal parameters. Visual examination of rinsed parts which were staged for plating showed no indications of contamination. Examination of these parts by SEM and EDS, however, revealed extensive fingerprint residue which contained the same elementals as that observed in the opened blisters (Figures 4-6).

Figure 4 – SEM image of fingerprint on rinsed manifold.
Somewhat optically transparent, fingerprints are readily visible in SEM images. 8.0X

Figure 5 – SEM image of dark area on the fingerprint shown in Figure 4 at high magnification
showing potassium chloride salt crystals from exuded perspiration and oils. 7000X

Figure 6 – EDS analysis of crystals shown in Figure 5 composed of sulfur (S), chlorine (Cl at arrow),
potassium (K) and calcium (Ca) in addition to the steel substrate.

Because the plating vendor’s customer set particularly demanding cosmetic standards, manual inspection of the parts had been instituted between each step in the process. Manual inspection of the parts following rinsing, but prior to plating however, nullified an otherwise effective rinsing procedure by introducing residue from the inspectors fingertips. In an attempt to find the source of the problem, the plating vendor initiated a more rigorous manual inspection which only resulted in more defects from greater amounts of handling. Interestingly, the original plating operation with manual inspection presented no problems when started in early January. Blister defects only began to appear as ambient temperatures rose in early summer, reaching a peak in late July when perspiration and exuded oils from the inspectors fingertips increased.

“Micro Soccer Balls”

The root cause in our failure analysis of a gearbox transfer case was straight forward. The unit had been in service for only a short time before failure and was one of several cases which had failed in nearly identical circumstances. Gross abrasive wear of the bearings and gear teeth was obvious. What was less obvious was the source of the contaminant. SEM examination of debris from the gear case contained the expected wear debris from the gears and bearings, considering the degree of damage they exhibited. Also present, however, were numerous spherical particles exhibiting a morphology that resembled microscopic soccer balls (Figure 7).

Figure 7 – One of the many “micro soccer balls” found in the in the gearbox transfer case. 825X

Analysis of the spherical particle by EDS indicated a composition of iron with approximately 2% manganese. Metal will take on a spherical form only when it solidifies from a molten state free of any outside force, including that of gravity when on a horizontal surface. That fact limited our potential sources. Temperatures generated from friction during the failure sequence would not begin to approach the melting point of iron, so that eliminated an internal source. No melting or foundry facilities were present at the manufacturing site where the gearboxes were assembled, furthermore, the particles size and morphology was inconsistent with a foundry source. That left arc welding spatter which, based on our experience with previous analyses, was consistent with the particle size and morphology. The problem here was that no welding was performed in the fabrication of the gearbox and the manufacturing site performed only mechanical assembly operations. The contaminant particles, however, said otherwise. A review of the affected gearboxes production dates, and comparison to plant maintenance records eventually solved the mystery, revealing that a facility upgrade involving welding had been performed near the assembly line on the affected dates.

Unknown Residue

A manufacturer of aluminum high performance automotive pistons noticed a white residue on finished parts during the final inspection process. A review indicated that no changes in the manufacturing process or materials had been implemented, but despite an enhanced cleaning procedure the residue persisted. Piston samples exhibiting the white residue (Figure were sent to MAI to determine its composition and source.

Figure 8 – White residue deposit on the piston at the ring lands. 4.0X

Examination of the residue by SEM revealed a crystalline morphology as shown in Figure 9. Chemical analysis by EDS (Figure 10) indicated phosphorus, potassium and oxygen as the primary components of the residue with trace levels of carbon, sulfur and other elements.

Figure 9 – White residue on the piston exhibited an acicular crystalline morphology. 1000X

Figure 10 – EDS analysis of the residue showed oxygen (O), phosphorus (P) and potassium (K)
as the main constituents. The aluminum (Al) is from the piston substrate.

The oxygen, phosphorus and potassium are characteristic of tribasic potassium phosphate, a common component of soaps and detergents. Analysis of the detergent used to clean the pistons following machining confirmed that this was the source of the residue. Persistent questions to vendor eventually produced an admission that a change had been made to the formulation of this detergent without notification of their customers. This change resulted in a less soluble reside which the original rinse procedure did not completely remove. An extension of the rinse time resolved the problem in the short term and got pistons shipped. An eventual change in detergent brought the rinse time, and production rate, back to the original schedule.

Read part 1 of Analyzing Contaminants, Stains and Debris

The objective in analyzing contaminants is the identification and elimination or isolation of their source. Techniques used make this identification include Optical Stereomicroscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive  X-Ray Spectroscopy (EDS). The accompanying case studies describe a selection of contaminant investigation and analyses performed here at Metallurgical Associates. Part 2 of this discussion will describe examples of the analysis of contaminants for a variety of sources.

Read part 2 of Analyzing Contaminants, Stains and Debris.

Material substitution is of limited value since, as noted, MIC affects 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 system and with 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.

Microbiologically Induced Corrosion (MIC) Failures

MIC Failure Example 1

This sequence shows several steps in the analysis of pitting corrosion in stainless steel tubing from a water bottling plant. The plant processes purified water, normally a media relatively immune to MIC. However, hydrostatic testing, performed during installation of the process piping, introduced anaerobic bacteria which adhered to several tube ID welds and adjacent areas, resulting in MIC and perforation of the tubes (above).

The perforations were examined using a Scanning Electron Microscope (SEM) which revealed biological adhesions in and around the pits. Several entries leading to apparent sub-surface voids were also observed (shown at arrows).

Micro-chemical analysis of the biological adhesions, by Energy Dispersive Spectroscopy (EDS), identified high levels of carbon (C), oxygen (O) and sulfur (S). These elements are consistent with sulfur reducing and oxidizing anaerobic bacteria species implicated in MIC.

Polished cross sections through the pits revealed internal cavities in the 0.060” thick tube wall, again, a hallmark of anaerobic bacteria which adhered to the tube ID surface and migrated to these oxygen depleted cavities formed by corrosive attack from their acidic bi-products. Because MIC usually initiates at the ID of tubing, extensive corrosion and eventual perforation occur before any visible evidence of attack is apparent externally.

MIC Failure Example 2

“Weeping” of fluid from systems is a precursor to full blown perforation by MIC. The source of this “weeping” is often a subtle discoloration of the tube or vessel surface as shown at the center of the circled area.

Examination of these features by Scanning Electron Microscopy reveals fine micro-pitting and a “sponge” like morphology as the interior MIC attack nears the outer surface.

Probing of this “sponge” like surface collapsed the thin crust of remaining metal, exposing the sub-surface cavity created by anaerobic bacteria and their sulfuric acid bi-products.

A cross section of the discolored feature reveals the extent of corrosive MIC damage which has penetrated completely through the tube wall thickness.

MIC Failure Example 3

Pitting and general corrosion are both associated with MIC, sometimes in the same corrosion failure. The interior of this carbon steel storage tank exhibits extensive general corrosion.

Examination by Scanning Electron Microscopy revealed numerous tubercles on the corroded tank ID surface. Tubercles are found in association with MIC producing iron oxidizing anaerobic bacteria.

The interface of the tubercle with the metal substrate beneath it offers an oxygen depleted environment that is ideal for anaerobic MIC bacteria. Ultrasonic cleaning of a section of the corroded tank to remove the tubercles revealed small deep pits suggesting connected sub-surface cavities consistent with MIC by sulfur reducing bacteria.

Cross sections of the tank confirm anaerobic MIC bacterial activity by the presence of characteristic sub-surface voids. This failure demonstrates the symbiotic relationship often found between two or more MIC implicated bacterial species, producing two corrosion modes (general and pitting) in a single corrosion failure.

See Part 1 of this series – Biological Corrosion of Metals

See Part 2 of this series – Microbiologically 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 de-lamination 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 high profile cases of MIC was of aluminum jet aircraft fuel tanks in the 1950’s. Water contamination 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 effected by a wide range of microbial 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.

See Part 1 of this series – Biological Corrosion of Metals

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

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

There are three stages in the life of a fatigue failure; 1. Initiation, 2. Crack Growth (propagation), and 3. Final Fracture. These stages are illustrated in the SEM image of a fractured rectangular wire above. The Initiation is indicated by the large red arrow at the lower left. The area of progressive Crack Growth extends from this arrow to the line indicated by the three smaller red arrows. Final Fracture, the point at which the remaining intact cross section of the wire could not sustain the next cyclic stress load – “the straw that breaks the camels back” – and complete fracture occurred, is the light area above the three arrows. This fracture is an example of bending fatigue (in one direction) initiating from a single point of origin.

The fractured crane lifting hook above is an example of reverse bending fatigue (back and forth in two directions). In this case, the major bending stress was applied from the bottom of the fracture as oriented in this photo, and a minor stress from the top. The darker gray area indicates final fracture in a single stress cycle. The thin horizontal band at mid fracture indicates a significant “jump” in the fracture progression that occurred in the cycle proceeding final fracture which almost, but not quite, resulted in complete fracture. This fatigue fracture initiated from multiple origins. Multiple origins are indicated by the steps, or “ratchet marks”, at the outer diameter of the fracture indicated by the arrows. Ratchet marks occur when multiple fatigue cracks initiate at slightly different planes on a component’s surface. As these multiple cracks progress into the component, they eventually join into a single fracture plane as show above.

Rachet marks resulting from multiple fatigue origin locations are shown at high magnification in these images taken on our Scanning Electron Microscope (SEM). Fatigue cracking penetrated only a short distance into this NASCAR Racing suspension component before it failed completely in a single load cycle. As a result, the multiple origin fatigue cracks never progressed far enough to coalesce into a single fracture plane. Several of the individual origin sites are indicated by arrows.

The diagonal bands exhibited by the fatigue fracture of this compressor connecting rod are commonly called “arrest lines”. These indicate a change in the frequency of cyclic stresses, such as “stop-start” sequences, changes in RPM, or variations in load. The initiation site at the crankshaft journal bore (arrow) is heavily damaged. This is not uncommon in fatigue failures. As the first location to crack, the initiation site is exposed to potential relative movement of the two sides of the crack during propagation. This presents a significant challenge to the analyst in determining the root cause of fatigue cracking.

Fatigue fractures exhibit distinct features, called striations, when viewed at high magnification using Scanning Electron Microscopy. Striations appear as relatively evenly spaced parallel lines. Each striation is actually a shallow crack that results from a single load, or stress, cycle. Repetition of these cycles produces an advancing repetition of shallow cracks as shown above in this fatigue fracture in a hydraulic valve body. This process is characterized by the term, “fatigue crack propagation”.

The appearance, or morphology, of fatigue fracture striations varies depending on the magnitude and frequency of the applied load and the physical characteristics of the affected component such as hardness, microstructure and chemical composition of the alloy. These SEM images illustrate fatigue striations in an aluminum valve body (left) and an alloy steel high pressure hydraulic cylinder (right).

In some cases, the root cause of a fatigue failure can only be discovered by an analysis of internal characteristics of a component at the crack location. In this example, a metallographic cross section revealed decarburization (dark phase at arrow) of the surface of a steering arm due to faulty heat treating. This carbon depleted layer has significantly reduced hardness and strength, as well as residual tensile stress, conditions highly conducive to fatigue crack initiation.

Other types of internal defects which act as initiation sites for fatigue are apparent on the fracture surface. Examination of this brake return spring by SEM revealed fracture features which radiate from a single initiation point. Viewed at higher magnification, this initiation point exhibits a void containing a non-metallic inclusion which acted as a stress concentration.

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.

Read part 4 – Analyzing Material Fatigue