24 INSPECTION TRENDS

Many factors influence which ultrasonic couplant will per-form best for each specific application. Couplants canbe selected by utilizing the following ten criteria tomatch the testing requirements:

1. Corrosion Inhibition. The couplant selected must notcause corrosion. Why is corrosion important? Corrosion dam-age from ultrasonic couplants can take many forms, from reduc-ing the useful life of the component, such as interfering with anapplied coating or plating, to catastrophic failure of the part dueto hydrogen embrittlement, stress corrosion cracking, or deeppitting. (Detailed information on corrosion follows this list.) Corrosion damage is often not visible (crevice corrosion,

hydrogen embrittlement; titanium stress corrosion crackingat elevated temperature).

Corrosion may take place over a long period of time. Hydrogen embrittlement is a concern with low-alloy, high-

strength steel such as bolts and landing gear, while stress cor-rosion cracking is a concern with titanium components thatare subjected to elevated temperatures.

Corrosion specifications can be used as a guideline for cou-plant suitability; for example, ASTM F519, HydrogenEmbrittlement of Steel, and ASTM F-945, Titanium StressCorrosion Cracking.2. Acoustic Impedance. Greater couplant acoustic imped-

ance reduces surface noise and improves coupling on roughand/or curved metal surfaces. Very high acoustic impedance(Z) couplants provide improved results for concrete testing andrough, pitted, or corroded metal surfaces.

3. Viscosity. Higher viscosity couplants provide enhancedcoupling and reduced surface noise on curved surfaces.Viscosity selection involves exposure (overhead surface drips),adhesion to the part (vertical surface), and ease of application(spreading couplant over the area to be inspected). Viscositymeasurements are a relative indicator between couplants usingthe same polymers (i.e., comparing couplants that use acrylicpolymers). Couplants using different polymers have very differ-ent flow (rheology) characteristics and do not necessarily relateby viscosity measurement numbers. In considering viscosityrequirements, the presence of surface salts from corrosion orthe environment influence couplant selection. Cellulose poly-mer couplants (such as Echogel, UT-X Powder, and SonoGlide)will provide better viscosity stability and will not thin when incontact with salts.

4. Temperature Range. Temperature of the part to be test-ed, length of time the inspection will take, and whether the cou-plant will be left in place (flow meters or acoustic emission) allinfluence the temperature range required of a couplant.

Remember, some couplants have a minimum as well as maxi-mum operating temperature.

5. Drying Time or Evaporation Rate. Couplant liquidretention reduces the need to reapply and increases efficiency.A higher upper limit temperature range in a couplant usuallyequates to a longer drying time. Drying time is a crucial con-sideration when performing a lengthy inspection.

6. Couplant Removal. Consider whether the couplant is tobe removed and how: Wiping (incomplete removal). Water wash. For water-soluble products, remove before the

couplant dries. Dried couplant films can be removed withpressure washing.

Solvents. Selection based on the specific couplant and mater-ial compatibility.

Complete removal required. For mandatory removal of alltraces of couplant, select a couplant with a fluorescent tracerand inspect the part with ultraviolet light after cleaning.

Leave couplant on the part. In many applications, couplantsare selected for long-term compatibility and are not removedafter the inspection. 7. Toxicity. When there is the potential for animal contact or

ingestion, select an appropriate low-toxicity product. Never useethylene glycol antifreeze to lower freezing points or to extenddrying time. For pharmaceutical and food-processing equip-ment, use a couplant with claims specific for the application. Forhigh-temperature couplants, beware of products that contain flu-orocarbons as they can cause polymer fume fever in the ultra-sound inspector. Look for the fluoro in the chemical name onthe material safety data sheets (MSDS).

8. Environmental Sensitivity. When couplants will beintroduced directly into the environment without sewage treat-ment, use products designed for reduced environmental impact.Typically these products will have low (or no) nitrates or tri-ethanolamine and no heavy metals or chromium components inthe corrosion-inhibiting system.

9. Surface Wetting. If an oil film is present, the ideal cou-plant quickly cuts through the film to provide an unobstructedacoustic interface between the transducer and test surface.Couplants need low surface tension to wet the surface, espe-cially on plastics.

10. Reliability. Choose a couplant supplier that providescouplants that are homogeneous throughout the entire batch,thus ensuring product reliability. This typically requires valida-tion of the production process as follows: Product consistency between and throughout batches is criti-

cal. Drop by drop, gallon by gallon, batch by batch product

MARIAN LARSON (mlarson@sonotech-inc.com) is marketing and sales manager, Sonotech, Inc., Bellingham, Wash.

Ten Criteria for Accurate UltrasonicCouplant Selection

Selecting the correct couplant specific to an application is critical to both ensure you dono harm to the environment and to obtain optimal results

BY MARIAN LARSON

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chemical uniformity is critical to corrosion inhibition properties.

Choose a couplant manufacturer that is certified to ISO9001:2000.

Consider a vendor audit of your couplant supplier.

Ultrasonic Couplants and Ferrous Corrosion

Do No Harm. Hippocrates most famous dictum is primumnon nocere, meaning first do no harm. The same messageapplies to nondestructive examination. As implied by the name,NDE is intended to have no detrimental effect on the part beinginspected. With ultrasonic flaw inspection and thickness gaug-ing, the greatest possibility of causing harm is through corro-sion. Ultrasonic couplants are often not completely removedafter inspection. Thus, couplants must be selected that will notcause corrosion. Corrosion damage often requires a long timeframe to fully develop, may not be obvious, and can cause cata-strophic failure.

Critical Parts Inspections. Many metal parts in a broadrange of applications are ultrasonically inspected using cou-plants. Examples include jet engine turbine blades during boththe manufacturing process and routine maintenance, nuclearpower plant components, helicopter rotors, aircraft landing gear,automotive components, high-strength bolts, and welds on skilifts, ship hulls, amusement park rides, rocket engine casings,high-pressure steam piping and oil pipelines, etc. The list of met-als and alloys inspected is also broad and includes titanium, alu-minum, composite materials, and the ferrous metals cast ironand steel. Cast iron is commonly used for engine blocks andwind turbine blades; steel is used in high-strength bolts, boilertubes, aircraft landing gear, and aircraft engine fuse pins; titani-um is used for jet engines, turbine blades, and aircraft; stainlesssteels are used in nuclear power plants and nuclear submarines;aluminum is used in aircraft; and composite materials are usedin fixed wing aircraft, helicopters, etc.

Ferrous Corrosion Potential from Ultrasonic Couplants

Ferrous metals contain iron. Ferrous metals may be pureiron (like wrought iron), or may be alloys of iron such as graycast iron and steel, which are alloys of iron and carbon. Ferrousmetals are usually, but not always, magnetic. Oddities exist,such as austenitic stainless steel, which is ferrous but nonmag-netic while cobalt is magnetic but nonferrous.

Throughout the 1950s and 1960s, the most common ultra-sonic couplants used for ferrous metals were petroleum-basedoils and greases. Petroleum-based couplants provided superiorcorrosion inhibition, protecting machined surfaces and unpro-tected metal for very long periods of time. If the petroleum cou-plant was not abraded away and the part was stored out of theelements, corrosion inhibition would last for years. Inspectorscould be confident that ferrous materials, such as machinedparts, would not corrode between manufacturing operations asa consequence of ultrasonic inspection or thickness gauging.

Today, petroleum-based oils and greases are seldom useddue to environmental concerns, fire hazard, and operator der-matitis. Ultrasonic couplants must have a sophisticated balanceof corrosion inhibition agents, and uniformity of the couplant iscritical to ensure that each drop of couplant contains the sameamount of corrosion inhibitors.

Testing Ferrous Corrosion Properties of UltrasonicCouplants

Some inspectors conduct their own internal testing of NDEultrasonic couplants for compatibility with the parts inspected.Most inspectors using ultrasonic NDE, however, rely on the

couplant manufacturer to accurately state the known corrosioncharacteristics of the couplant.

One method of ferrous corrosion testing (as exhibited in Fig.1) is to use 1018 low-alloy steel and gray cast iron surface-groundcoupons. Couplant is applied onto the surface, in two beads. Aglass plate is then placed over one bead to simulate crevice cor-rosion or concentration cell conditions. The corrosion mecha-nisms of action differ significantly for crevice vs. surface corro-sion. Thus, a combination of both tests is a good predictor of thecouplants balance between cathode and anode corrosion inhibi-tion, and the negative effects of any chemicals or precipitates thatcontribute to accelerated corrosion in a concentration cell.Concentration cells are formed by pitting corrosion (a self-serv-ing crevice), and by crevices formed from washers, overlappingmetal parts, lifting paint chips, or any type of mating surface. In ametal-ion concentration cell, the accelerated crevice corrosionoccurs at the edge or slightly outside the crevice, while in an oxy-gen-concentration cell the accelerated corrosion usually occurswithin the crevice, between the mating surfaces.

Ultrasonic Couplant Quality and Reliability

It is not possible to test each drop of ultrasonic couplant forchemical and corrosion inhibition properties, just as it is notpossible to test drugs drop by drop, or pill by pill in the phar-maceutical industry. To accomplish and ensure a homogeneous,reproducible ultrasonic couplant, the couplant manufacturingprocess must establish a high degree of assurance that, even ina gelled or high-viscosity product, the entire batch will be homogeneous.

Ensuring product uniformity requires a quality managementsystem to establish and maintain a controlled process in whichthe key parameters include documented raw material specifica-tions, established purchasing and receiving inspection proce-dures, documented work instructions and procedures, docu-mented final product test procedures, use of ISO-compliantthird-party testing laboratories as required, and compliancewith international standards applicable to quality managementsystem ISO 9001:2000.

Selecting the correct couplant specific for an application is crit-ical to both ensure you do no harm and to obtain optimal results.Applying the ten criteria presented previously will help you selectthe most appropriate ultrasonic couplant for your job.

Fig. 1 Sonotechs Ultragel II (left) vs. a couplant withinadequate corrosion inhibition (right). Crevice corrosionsimulated under a glass microscope slide over the top bead ofcouplant on each coupon. (Photo courtesy of Sonotech, Inc.)

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