Microbiological influenced corrosion (MIC) has gotten much attention as one of the unique forms of corrosion. It occurs as a result of the life processes of several types of microorganisms or it can occur due to the simple formation and growth of colonies of these organisms on wetted metal surfaces. Different microorganisms can produce a variety of effects that lead to accelerated corrosion. These include formation of black iron sulfide deposits that create corrosion cells on carbon steel; production of acids and the resulting lowered pH; formation of hydrogen sulfide and corrosion in oil and natural gas applications; driving the electrode potential of stainless steels more anodic and out of their passive state into the pitting range or by generating hydrogen so that high strength steels are made more susceptible to hydrogen embrittlement or SCC. These organisms often form slime deposits on metallic surfaces called biofilms. The presence of the films alone – without added other effects – can cause crevice corrosion.    

Most alloys are subject to MIC although titanium alloys offer resistance. Carbon steels and cast irons are the materials where MIC is most often encountered. These materials typically suffer general corrosion while stainless steels and aluminum alloys experience pitting and crevice attack. If MIC is confirmed there is little or no value in switching from carbon steel to stainless steel to gain more resistance.

The microbes involved with MIC need three conditions to initiate and reproduce on a surface. These are water, some type of nutrient, e.g., a source of hydrocarbons, nitrogen or phosphorus, and some source of energy. The latter can be supplied by sunlight or by the redox (oxidation and reduction) chemical reactions that occur in all corrosion. Some microbes are anaerobic such as the very common sulfate reducing bacteria (SRB) and need an oxygen-free environment to live and grow while others require oxygen to live. MIC microbes can survive over a wide range of temperatures – from subzero up to maximum of about 200 degrees F. They cannot live at very high temperatures. Submerged colonies of microbes attach to metal surfaces and grow quickest when exposed to stagnant or low velocity liquids. They can survive over a wide range of pH values.

MIC may occur in several types of applications. Industrial water-handling systems are probably the most common. Others include the exterior of underground pipelines, in fire sprinkler systems and on marine piers and other structures exposed to seawater. In the past MIC has been a significant problem on aluminum in the lower sections of commercial aircraft below galleys and restrooms when adequate constraints on liquid spills have been overlooked. Stainless steels are particularly susceptible to MIC at welds.

Control methods for MIC include using biocides to kill the microbes and adding chemical corrosion inhibitors to retard corrosion. Coatings as well as design changes, e.g., assuring there are no internal areas in a piping system where there is incomplete drainage or other stagnant flow conditions exist, can be useful. Specific and diverse knowledge is needed to effectively analyze and combat MIC problems. The individuals that do the work need to be competent in the process chemistry of the given system, laboratory methods to identify and quantify specific microorganisms, effects of different corrosion inhibitors combined with biocides and methods to assess and assure the continuing success of different treatment schemes.  Once implemented it is essential that trained personnel continue to do sampling and monitor the effectiveness of the given control method. Adequate control of MIC is definitely not something that the user can “install and forget about”.

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SCC has been one of – if not the – most widely studied forms of corrosion. This is because it is can occur in so many different applications and it is an insidious type of attack that often leads to metal failure before the process of cracking is detected.

The mechanism of SCC combines corrosion and mechanical cracking such that each accelerates the other. Initiation generally occurs on a susceptible metal’s surface due to corrosion of that particular alloy by a particular corrosive medium. A small crack then starts. If aggressive conditions remain in effect, the crack grows and progresses through the metal until too little sound material remains to resist the stresses acting and final failure occurs. The rate of crack growth depends on several factors including the level of tensile stress and the values of other service conditions. The latter include service temperature, the level of concentration of aggressive ions in the corrosive or its pH and the time of exposure to the specific conditions. 

A unique feature of SCC is that it occurs only for particular combinations of metal or alloy, specific corrosive media and other service conditions. For example, copper alloys when exposed to ammonia compounds have long been recognized as likely to produce SCC if tensile stress is also present. Similarly, common austenitic grades of stainless steels, e.g., 304 and 316, in corrosive media with high concentrations of chloride ions and high temperatures will readily crack if tensile stress is present. However these same stainless steels will not crack in ammonia solutions at least up to about 212 F. Carbon steel is not subject to SCC in seawater but it will crack in caustic (NaOH) solutions at certain temperatures and NaOH concentrations. There are many susceptible combinations of alloys, corrosive media and service conditions that can cause cracking and more susceptible combinations are regularly being discovered. Corrosion handbooks provide data on the several combinations that are known to produce SCC.  

Tensile stresses are essential to the incidence and rate of SCC growth. They act to pry open the microscopic-scale tip of a crack and expose “new” metal to the corrosive medium. The corrosive then can attack this uncovered metal so that crack growth is promoted. Frequently corrosion pits on a metal surface will be the initiation points for SCC because they can act to concentrate and magnify tensile stresses. A crack will often begin and grow from the bottom of a corrosion pit. Other stress concentration areas on a susceptible metal surface can also promote crack initiation, e.g., at sharp corners or small radii on a stepped shaft or from undercuts or lack of penetration at a weld. 

Applied tensile stresses often are not the primary source of stress that causes SCC. Instead residual tensile stresses, i.e., those left in the metal after some manufacturing process such as welding or due to plastic deformation caused by cold working the metal, frequently are the more important sources. The designer may overlook residual stresses. These stresses can be essentially eliminated or reduced to minimal levels if a sufficient stress relief heat treatment is accomplished after the offending manufacturing process is completed.  

A metal that that has become sensitized and susceptible to another form of corrosion – intergranular attack (IGA) – is much more likely to experience SCC. Sensitization is a metallurgical degradation process in which areas nearby the grain boundaries of the metal become deficient in an alloying element that is essential to its corrosion resistance. Thus these deficient grain boundary areas become much more susceptible to the initiation and growth of a SCC crack that will follow this intergranular path through the metal.  

The basic control measure for SCC is to be aware of and avoid the susceptible combinations of alloy, specific corrosion medium and service conditions. Another approach is to minimize applied stresses and be particularly careful to avoid (or stress relieve) the often-overlooked residual stresses that may be present in the metal. Other measures include minimizing stress concentration features (both geometric and those caused by pitting) on the metal surface, preventing sensitization of the alloy, avoiding thermal insulation that can leach out and concentrate aggressive ions onto to metal it covers or inputting helpful compressive stresses to the metal surface via shot peening or cold rolling to counteract harmful tensile stresses. Finally, as in any corrosion process, minimize conditions that can accelerate attack, e.g., high temperatures, very low or very high pH (depending on the given metal) and where possible avoid high concentrations of aggressive ions, such as chlorides and fluorides, in the bulk corrosive medium. SCC cracking can initiate at crevices in metallic equipment because the partially closed geometric features that crevices create concentrate aggressive ions to much higher levels than in the bulk liquid. Thus eliminating all possible crevices is important.             

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There are several forms of  metallic wear. In each case the process can be defined generally as the unwanted removal of material from a surface due to mechanical action. This is in contrast to desirable manufacturing processes that occur by similar mechanisms such as machining, grinding or shot blasting.  The most common harmful forms of wear are abrasion, erosion, adhesion and fretting. 

Abrasive wear entails cutting or gouging of the surface of a solid metal when in contact with partially constrained but slowly moving, hard non-metallic or metallic particles from an adjacent, moving surface. The particles are confined between two surfaces being pressed together. Generally no lubricants are used. Heat due to friction is always created. Wear debris from the solid, softer metal is destroyed in the process. Abrasion often occurs in ore or earth moving equipment.

Erosion is similar to abrasive wear but the shearing force that degrades the metal surface is provided by free-moving solid particles that impact the wearing surface as they are conveyed by a flowing fluid – either liquid or gas. Larger, harder particles cause more damage than smaller, softer particles. If the fluid is corrosive to the wear surface then another mechanism – erosion-corrosion (E-C) – occurs that combines both wear and corrosion so that the result is worse than either acting alone. Erosion and/or E-C often occur on pump impellers, marine vessel propellers and in certain pressure let down valves. 

Abrasion, erosion and E-C are typically controlled by selection of a sufficiently resistant material for the given application. This material’ s key parameter is its high overall hardness or its surface hardness. Options include high hardness steels, steels with tungsten carbide particles added, high manganese steels, certain austenitic stainless steels and cobalt-based alloys. A case hardened surface layer can also be effective. In the case of E-C, the corrosion resistance of the selected material for the given service conditions is usually more important than its abrasion resistance although both affect results. 

Adhesive wear is the result of micro-welding of the microscopic high points, called asperities, on two metal surfaces sliding on one another without a sufficient lubricant.  As this continues asperities break off and are transferred to the softer metal while frictional heat causes interfacial temperature to increase. Failure is relatively common due to the many dynamic machine applications where a proper lubrication system is essential.  Thus the primary control measures are selecting a proper lube, assuring it is cooled to control interfacial temperature and providing a filter for most wear debris generated. 

Fretting is a form of adhesion that occurs due to micro-motion between two contacting metal surfaces that are not expected to move during a given period. Generally, no lubricant is used. Powered wear debris is generated that stays at the interface. This process often occurs while mechanical devices are being stored or transported and subject to external vibrations that create very small relative movements. Surface damage creates stress risers that act when the equipment is put into service. Premature failure by fatigue can result from this assumed, “static” exposure. Control can be difficult but some actions include temporarily inserting non-metallic materials between contacting surfaces or increasing the hardness or shot peening both surfaces to minimize the chance of in-service fatigue crack initiation. It is important not to overlook this potential threat.

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The most current (2002) study of the cost of corrosion in the United States found the associated loss was $276 billion in 1998 – 3.1% of the country’s total Gross Domestic Product that year. The National Association of Corrosion Engineers (NACE International) and the Federal Highway Administration (FHWA) jointly coordinated that study.

In 2008 a study by the U.S. Department of Defense indicated the return on investment for corrosion mitigation projects over a three-year period in military applications averaged 50 to 1, i.e., savings-to-mitigation expenditures. Given these data why does corrosion continue to be a major economic sinkhole for private industry and our government?

A 2011 report, entitled “Research Opportunities in Corrosion Science and Engineering”, published by National Academic Press in Washington, D.C. (www.nap.edu) provides important facts and opinions relevant to this question. Some of these results follow: 

-Presently corrosion engineering is not a required course in most U.S. undergraduate engineering curriculums. Often such a course is not available at all. This is true even for students in material science and engineering that one might expect would have this as a fundamental area of study. 

-Industry and government employers recognize they need employees with competence in corrosion control but the engineering graduates they hire generally don’t. The employers’ main concern is that those making design decisions involving materials and corrosion often lack minimum knowledge necessary to provide resistance to attack. The report notes some employers surveyed said many of their engineers, “don’t know what they don’t know”. 

-Employers attempt to solve this problem in two ways. First with on-the-job training (OJT) and short courses on corrosion for engineers and technicians that make critical materials decisions and/or monitor existing equipment for potential failure by corrosion. Outside consulting corrosion experts are also used but, unfortunately, these people are most frequently used as failure analysts after a loss. Their input would be more useful during the early design stage of a project when the opportunity to avoid later costs of  litigation or shutdown of in-service equipment would be most effective. 

-OJT, short courses and the use of consultants provide stopgap solutions. However, fewer people with corrosion engineering backgrounds are available as in-house mentors, as outside consultants or as short-course trainers. Many of those with this experience are senior engineers that have or soon will retire and, compared to most other engineering disciplines, they were always a small group. Often their replacements are from other disciplines in which little or no training in corrosion was provided. 

-The report concludes that stopgap tactical measures must continue by industry and government but strategic progress in corrosion control requires additional actions. These include clearly defining for universities what engineering graduates need in terms of awareness and knowledge of corrosion and providing financial support for corrosion research. The latter is essential to gaining better corrosion control and prediction methods but it has another major benefit. Quality faculty members and students are attracted to well-funded areas in universities. This will eventually result in a larger population of qualified engineering graduates to effectively address corrosion.

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Typically an engineering root-cause failure analysis (RCFA) is performed for one of two purposes. These are for use in litigation or for internal use in various industrial and related organizations. 

In the legal arena RCFA’s are part of the input that an engineering expert witness contributes to define the physical circumstances that caused the accident, injury or other loss in question. Litigation may also develop out of subrogation that an insurance company makes following a property and casualty claim. After the RCFA, the retaining attorney gains definitive information on liability for his or her case. The expert obtains a solid basis for offering fully objective opinions during sworn testimony. 

The second use of a RCFA is in several types of organizations where a physical failure often involves a financial loss but no legal action is involved. Here the objective is to gain a fundamental understanding of the failure so as to make changes that will prevent a reoccurrence. An organization that employs the engineer, either in-house or as an outside consultant, is most interested in practical recommendations to prevent a future failure of the given equipment. By contrast, the attorney in a legal case likely will concentrate primarily on the cause(s) of the incident as a basis for assigning liability and not what the best “fixes” might be.

When starting any failure investigation the engineer should consider just what is a “failure” in the given situation. Essentially it is whatever the owner or user of the device, system or process defines as a failure. For materials and mechanical design applications this might include such conditions as fracture, permanent deformation or dimensional change, inadequate service life, unacceptable appearance, contamination of the process stream or excessive vibrations during operation. 

Completing a RCFA typically follows the scientific method. This entails defining the specific problem or failure to be evaluated; proposing alternative hypotheses, i.e., causes in the case of failures; testing or evaluating each alternative; and, finally, forming a conclusion based on which alternative or cause is best supported by the total evidence assessed.  

For failures of engineering materials or mechanical designs the sequential steps in a thorough RCFA will generally flow as follows: 

• Gather and protect for analysis all possible physical evidence and information. If possible this should include samples of failed parts, similar but unaffected parts, i.e., exemplars, any relevant documents (drawings, manuals, etc.) and verbal descriptions of the circumstances from the best-available sources. Take digital photos of “everything” using macro and close-up views for comparison. Always take more photos than you think you will need,
• Complete detailed, non-destructive visual examinations of all physical evidence. Assess these macro-scale results in terms of all written, photographed and verbal information gathered. Often it is advisable to seek more facts about the general type of equipment or process in question for comparison to the verbal and written data provided by those persons directly involved in the failure, 
• Define the most probable cause (and/or contributory causes) of the failure and develop a plan for doing specific analyses. The goal of the planned work is to support or refute the possible cause(s) defined. The actions in the analysis plan should always proceed from the least to the most destructive level of destruction of the physical evidence,
• Carry out the analysis plan. Various laboratory tests often are used,
• Integrate all findings and information obtained and come to a conclusion about the most probable and contributing causes of the failure,
• For industrial failures, complete sufficient research to establish practical recommendations to minimize the chances of future failures. Generally it is advisable for the engineer to offer different corrective actions in a written report – each alternative with its own probability of success and cost – along with his professional opinion as to which best meets the needs of the given client.                       

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Many negative consequences can result from failures of a range of different products that often lead to property & casualty insurance disputes or product liability legal actions. The products may include consumer or industrial goods. Typically the private user or worker is injured or a business incurs significant financial loss. The injured individual, insurance carrier, attorney(s) or industrial manufacturer and user are all interested to confirm what went wrong – and who is liable. Resolution often depends on engineering analysis.

Engineers that specialize in performing root-cause failure analyses frequently assist insurance representatives or attorneys in these situations. They are known as forensic engineers in a legal action. Generally there are four broad causes for failures:
· Defective design or inadequate instructions
· Defective materials
· Defective manufacturing
· Misuse or abuse by the user

Defective design may include a variety of factors for which the designer is responsible. For example, incorrect specification of a part for both normal and unintended but foreseeable forces and stresses, specifying the wrong material or choosing a physical configuration that is inherently unsafe. Lack of clear instructions and warnings for safe use are sometimes an issue. Too often technically correct decisions by the design engineer are overridden for cost-cutting reasons and compromises are made that produce accidents.

Defective materials can cause premature failures. Normally this is because of inadequate quality control by the material vendor or unclear specifications for quality by the purchaser. Use of poor quality foreign-supplied materials and parts is a reoccurring issue. Forensic engineers that specialize in materials usage can apply an array of laboratory analysis techniques to define the properties of the failed parts and compare them to specified properties. Obtaining an exemplar of the same, specified material and part to illustrate any differences is very useful.

Defective manufacturing can take many forms. Included could be surface defects left by machining or damage during handling that cause early fatigue failure of parts, improper welding practices that degrade material properties, defects due to improper casting procedures, improper case hardening that causes early wear failure or improper heat treatment of the alloy that produces inferior material properties. Most of these issues are due to inadequate quality control or final testing prior to release of the product. The forensic engineer can generally detect them by careful questioning of the responsible parties coupled with personal inspections and/or laboratory analyses of failed components.

Potential misuse or abuse by the user requires a sensitive approach by the investigating engineer. The injured person may have been seriously harmed by the product failure and thus he or she is deserving of compassion. However, that person also may be defensive if he or she had a role in the accident. The forensic engineer must analyze all available information to include the victim’s explanation but also other findings. The attorney that initially retained the engineer may or may not like his final conclusions. The engineer’s integrity and professionalism demand that he provides truthful, objective input based on all the evidence. The attorney then can designate the engineer as his expert witness (or not), pursue the case further or seek a settlement.

Depending on whom he has as his insured, the insurance representative may also have options when the engineer has concluded his work and found which of the four root-causes applies. One option may be to pursue subrogation against the responsible party.

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A variety of low-voltage electronic devices may fail to operate properly because of exposure to certain corrosive, gaseous environments. Susceptible items include consumer products such as microwave ovens, audio equipment, TV’s and personal computers as well as many types of industrial sensors and automatic control devices. Failure generally occurs because of specific application factors. These include the severity of the environment, the presence or absence of atmospheric sealing of components within the device and the metals used at critical internal locations. Frequently complete failure doesn’t occur but normal operation is erratic.

Typically these problems occur due to raised electrical resistance at contact junction points within the device that would normally allow essentially unrestricted passage of electrical current. Particular chemical constituents in the air attack common metals used at contact points and form very thin films, i.e., corrosion products, which increase electrical resistance. Either failure of the device or irregular function then results. Low electrical resistance at contact points is essential in many electronic devices because they operate at relatively low voltages. Voltage is the driving force for electric current flow across an electrical contact point just as pressure is the driving force for delivering water through the resistance offered by a given length or diameter of piping. 

Corrosion product film thickness as low as several hundred angstrom units can create irregular operation of low-voltage electronic devices in even relatively low humidity gaseous environments. One angstrom equals 3.9 X 10^-9 inch (or 1 X 10^-7 millimeter). Obviously such thicknesses are not what most persons generally consider as evidence of corrosion. Copper and silver are normally the preferred metals used in electronic contacts. Both of these metals (and some others) are subject to corrosive attack and thus formation of these very thin corrosion product films in atmospheric air containing parts per billion (ppb) levels of reduced sulfur compounds. Copper is also subject to attack when ppb levels of chlorine gas or chlorides, nitrogen oxides or ozone are present in air. High humidity, i.e., above about 50%, in air that also contains one or more of these contaminants greatly accelerates the rate of corrosion. Often in industrial applications synergistic interactions between the several gaseous contaminants cited – with or without high humidity – intensify the attack. 

The Instrumentation, Systems and Automation Society (ISA) has developed a widely used engineering standard (ISA-71.04-1985) that provides more details on these effects. It also provides practical guidance on rating the relative corrosivity of gaseous atmospheres in terms of ppb quantities of contaminants present and the thickness of corrosion films formed on copper test coupons exposed to such conditions.

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Most technical persons in various industries are aware of the importance of the several forms of corrosion and their effects on the reliability of products or processes and thus the bottom line. Generally most people think about some of the well-known forms such as general corrosion, pitting or stress-corrosion cracking (SCC). However, crevice corrosion is often overlooked. This form of attack occurs when an area on a susceptible metal is partially, but not completely, closed off from full access to the liquid corrosive medium. Some typical crevice sites occur at the exposed, wetted edge of two plates of material (both don’t necessarily have to be metals) that are bolted together; under sand or debris on a wetted metal surface; at the gaps in a joint between two wetted metals that are tack welded together (rather than being joined with a continuous weld) or in the wetted interface spaces between the external threads on a bolt and the mating internal threads of a nut.

Crevice attack occurs rapidly in such localized confined spaces that are open sufficiently to let the corrosive liquid enter and start the corrosion process but are not open enough to let that corrosive liquid freely exit the space. After it starts the crevice corrosion process grows or propagates very much like pitting. For example crevice corrosion is self-accelerating – called autocatalytic – just like pitting. In addition the appearance of crevice corrosion inside the confined space is similar to a pitted surface. Crevice attack usually is more dangerous than pitting. Assuming otherwise equivalent conditions, it will initiate sooner and then continue penetrating the metal inside a crevice before attack occurs on a freely exposed surface where pits might form. This confined-area form of attack is also a major practical threat because it is often difficult to fabricate something without creating one of the many types of crevices in wetted areas. Of course being aware of the danger of crevices and then seeking to avoid them during design is a major way to minimize the problem.

If some potential crevices cannot be avoided there are actions that can be practical in certain cases. One is to close off the opening or “mouth” of the crevice with a suitable sealant or a continuous weld. Another approach is to provide for regular removal of sand, dirt or debris that may form crevices over time, e.g., by designing for complete drainage. If a crevice is unavoidable – assure that its opening is not small but is open enough to allow free interchange – in and out – by the corrosive liquid. Finally, selecting a more corrosion resistant metal can be an option. For example, crevice attack often occurs on commonly used austenitic stainless steels in a corrosive liquid containing chloride ions. In these applications picking another alloy with higher percentages of chromium, molybdenum and nitrogen in its composition provides the most resistance. Molybdenum is, by far, the most effective of these elements but also the most expensive

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As engineers we are obviously conscious of the vital need to define and then design for the largest applied stresses that a product is liable to encounter in service. The product’s long-term reliability and safety for the user demands this acknowledgement. However, many of us may overlook residual stresses that can be created inside metallic materials.

If these residual stresses are of sufficient magnitude and if they are tensile in nature they will accelerate certain failure mechanisms. These forms of failure include “pure” mechanical fatigue (that is without a significant corrosion effect), corrosion fatigue, hydrogen embrittlement (HE) and stress-corrosion cracking (SCC). Tensile stresses can be created by a variety of manufacturing processes such as severe cold working, forging, casting, machining, grinding and electroplating. They may also be generated in service due to thermal growth and contraction.

By contrast, compressive residual surface stresses as created by shot peening, cold rolling or by case hardening processes such as carburizing or nitriding are beneficial. They protect the metal against failures that typically initiate at the metal surface, e.g., any variety of fatigue or SCC. HE originates further below the metal surface and its control generally does not benefit from surface compressive residual stresses.

Correct stress relief annealing heat treatments can be used to eliminate or greatly decrease both types of residual stresses. The temperature and duration of an effective stress relieve heat treatment depend on the specific alloy and dimensions of the part being treated. These treatments may then be followed with beneficial shot peening, cold rolling or a case hardening process if desired. Residual stresses, tensile and compressive, are algebraically additive to each other and to any applied stresses that may be acting. The key is – don’t overlook residual stresses – whether undesirable or intended.

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Various industrial firms employ different types of engineers, technicians and equipment maintenance personnel that directly or indirectly deal with the reliable operation of the physical products or assets of the organization. However, many of these folks typically have little background in the successful use of different metallic materials that are at the heart of either the success or failure of most products and assets. Many organizations face a related problem. They have one or more well experienced persons that are knowledgeable in materials engineering areas but they, like many technical specialists, have already or soon will be reaching retirement age and will no longer be available to mentor the younger employees that remain. How will the less experienced personnel perform with their limited materials knowledge and experience and what will be the effects on the business?

Continuing education via short courses in materials engineering areas such as basic metallurgy, corrosion and the characteristics of different materials’ failure mechanisms -and how to avoid them- can serve a fundamental need that directly affects the P&L of the business. Such short courses will not make overnight experts out of the students but they will provide some well-known tactics that specialists know but that non-specialists may not or that they overlook. If these tactics are applied many costly mistakes can be avoided. There are numerous sources for such training and if done well the return on the investment is high. This is especially true if the training is conducted on-site at the organization so as to minimize time away from the job and eliminate the direct costs of travel for the attendees.

Is such training something your organization ought to consider?

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