Handbook of Corrosion Engineering Episode 1 Part 14 pps

Many variations are possible, but thetechnique most applicable to corrosion detection is shadow moirésometimes called projection moiré for surface height determination.The structured lig

ing area Different borescopes are designed to provide direct, forwardoblique, right angle, and retrospective viewing of the area in question.

Fiberscopes. Fiberscopes are bundles of fiber optic cables that transmitlight from end to end They are similar to borescopes, but they are flex-ible They can be inserted into openings and curled into otherwiseinaccessible areas They also incorporate light sources for illumination

of the subject area and devices for bending the tip in the desired tion Like borescope images, fiberscope images are formed at an ocular

direc-or eyepiece

Video imaging systems. Video imaging systems (or “videoscopes”) consist oftiny charge-coupled device (CCD) cameras at the end of a flexible probe.Borescopes, fiberscopes, and even microscopes can be attached to videoimaging systems These systems consist of a camera to receive theimage, processors, and a monitor to view the image The image on the monitor can be enlarged or overlaid with measurement scales.Images can also be printed on paper or stored digitally to obtain a per-manent record Video images can be processed for enhancing and ana-lyzing video images for flaw detection Specialized processingalgorithms may be applied which can identify, measure, and classifydefects or objects of interest

Advanced methods. Moiré interferometry is a family of techniques thatvisualize surface irregularities Many variations are possible, but thetechnique most applicable to corrosion detection is shadow moiré(sometimes called projection moiré) for surface height determination.The structured light technique is geometrically similar to projected orshadow moiré methods, and can be thought of as an optical straight-edge Instead of fringe contours, the resultant observation is thedeparture from straightness of a projected line The surface profile can

be calculated using image processing techniques

D-Sight has the potential to map areas of surface waviness as well

as to identify cracks, depressions, evidence of corrosion, and othersurface anomalies D-Sight is a method by which slope departuresfrom an otherwise smooth surface are visualized as shadows It can beused in direct visual inspection or combined with photographic orvideo cameras and computer-aided image processing The concept ofD-Sight is related to the schlieren method for visualizing index ofrefraction gradients or slopes in an optical system One possible prob-lem with D-Sight is that the technique shows virtually every devia-tion on the surface, regardless of whether it is a defect or a normalresult of manufacture

Liquid penetrant inspection. The liquid penetrant NDE method isapplied to detection of faults that have a capillary opening to the testobject surface The nature of this NDE method demands that attention

be given to material type, surface condition, and rigor of cleaning.Liquid penetrant inspection can be performed with little capital expen-diture, and the materials used are low in cost per use This technique

is applicable to complex shapes and is widely used for general productassurance

This technique is easy, completely portable, and highly accurate ifperformed properly It detects open-to-the-surface crack indications.Rigorous surface cleaning is required This technique is applicableonly to cleaned surfaces; unclean ones will give unsatisfactory results

It is readily used on external and accessible surfaces that have beensubjected to minimal corrosion deterioration and can be cleaned Itreadily detects any open-to-the-surface cracks, surface defects, andpitting

Magnetic particle inspection. Magnetic particle inspection is applied tothe detection of surface-connected or near-surface anomalies in testobjects that are made from materials that sustain a magnetic field.Special equipment is required in order to induce the required magnet-

ic field Procedure development and process control are required inorder to use the proper voltage, amperage, and mode of induction Testobject materials must be capable of sustaining an induced magneticfield during the period of inspection The concentration and mode ofapplication of the magnetic particles must be controlled Materialcharacteristics or surface treatments which result in variable magnet-

ic properties will decrease detection capabilities Magnetic particleinspection can be performed with little capital expenditure and, aswith the liquid penetrant technique, the materials used are low in costper use, the technique is applicable to complex shapes, and it is wide-

ly used for general product assurance

Magnetic inspection can be portable It requires only a tion power source, such as that provided by an electrical outlet It ismost frequently used in evaluating the quality of weld deposits andsubsurface weld indications such as cracks This is the preferredmethod for detecting cracks in deaerators, for example

magnetiza-Radiographic inspection. Radiographic inspection is a nondestructivemethod of inspecting materials for surface and subsurface disconti-nuities This method utilizes radiation in the form of either x-rays orgamma rays, both of which are electromagnetic waves of very shortwavelength The waves penetrate the material and are absorbed,depending on the thickness or the density of the material being

examined By recording the differences in absorption of the mitted waves, variations in the material can be detected The varia-tions in transmitted waves may be recorded by either film orelectronic devices, providing a two-dimensional image that requiresinterpretation The method is sensitive to any discontinuities thataffect the absorption characteristics of the material.

trans-The techniques and technologies of x-ray radiography have most to

do with the design of the x-ray tube itself There are many differenttypes of tubes used for special applications The most common is thedirectional tube, which emits radiation perpendicular to the long axis

of the tube in a cone of approximately 40° Another type is thepanoramic tube, which emits x-rays in a complete 360° circle Thistype of tube would be used, for example, to examine the girth welds in

a jet engine with a single exposure

■ Real-time radiography. This is the new form; it presents an instantimage, much like a video camera It is mostly used for examining thesurfaces of piping beneath insulation with the insulation in place It

is completely portable, and its operators are required to be licensed.This technique allows the instant viewing of a radiographic image

on a cathode-ray tube The image may be captured on any

electron-ic medium in use today This electronelectron-ic/digital imaging technique isthe only data retention system available

■ Classical radiography. This is similar to a medical radiograph thatgenerates a film record It is a completely portable inspection proce-dure, and extensive training and licensing of personnel are required.This technique is used to examine piping for interior corrosion anddeposits, weld quality, and conditions of internal valving or compo-nents A limitation is that it cannot be used on piping systems filledwith water or other liquids, since the radiation cannot penetratewater Extensive calibration and destructive verification of actualconditions allow achievement of a high level of confidence in theradiographic technique

Advances in the use of radiography are being made that involveusing computers and high-powered algorithms to manipulate the data.This is termed computed tomography, or CT scanning By scanning apart from many directions in the same plane, a cross-sectional view ofthe part can be generated, and a two-dimensional view of the internalstructure may be displayed The tremendous advantage of this method

is that internal dimensions can be measured very accurately to mine such conditions as wall thinning in tubes, size of internal dis-continuities, relative shapes, and contours More advanced systemscan generate three-dimensional scans when more than one plane isscanned CT scanning is costly and time-consuming Radiography in

deter-general and CT scanning in particular are extremely useful in dating and calibrating other, less complex and less costly methods.Radioisotope sources can be used in place of x-ray tubes.Radioisotope equipment has inherent hazards, and great care must betaken with its use Only fully trained and licensed personnel shouldwork with this equipment As with x-rays, the most common method

vali-of measuring gamma ray transmission is with film

Compton backscatter imaging (CBI) is emerging as a near-surfaceNDE measurement and imaging technique CBI can detect criticalembedded flaws such as cracks, corrosion, and delaminations in metaland composite aircraft structures In CBI, a tomographic image of theinspection layer is obtained by raster scanning the collimated source-detector assembly over the object and storing the measured signal as

a function of position Rather than measuring the x-rays that passthrough the object, CBI measures the backscattered beam to generatethe image This enables single-sided measurement

Eddy-current inspection. When an electrically conductive material isexposed to an alternating magnetic field that is generated by a coil ofwire carrying an alternating current, eddy currents are induced onand below the surface of the material These eddy currents, in turn,generate their own magnetic field, which opposes the magnetic field ofthe test coil This magnetic field interaction causes a resistance to cur-rent flow, or impedance, in the test coil By measuring this change inimpedance, the test coil or a separate sensing coil can be used to detectany condition that would affect the current-carrying properties of thetest material Eddy currents are sensitive to changes in electrical con-ductivity, changes in magnetic permeability (the ability of a material

to be magnetized), the geometry or shape of the part being analyzed,and defects Among these defects are cracks, inclusions, porosity, andcorrosion

Eddy-current methods are used to measure a variety of materialcharacteristics and conditions They are applied in the flaw detectionmode for the detection of surface-connected or near-surface anomalies.The test objects must be electrically conductive and be capable of uni-form contact by an eddy-current probe Special equipment and spe-cialized probes are required to perform the inspection Proceduredevelopment, calibration artifacts, and process control are required toassure reproducibility of response in the selected test object

Initially, eddy-current devices utilized a meter to display changes ofvoltage in the test coil Currently, phase analysis instruments provideboth impedance and phase information This information is displayed on

an oscilloscope or an integrated LCD display on the instrument Results

of eddy-current inspections are obtained immediately The other type of

eddy-current instrument displays its results on planar form on a screen.This format allows both coil impedance components to be viewed Onecomponent consists of the electrical resistance due to the metal path ofthe coil wire and the conductive test part The other component consists

of the resistance developed by the inducted magnetic field on the coil’smagnetic field The combination of these two components on a singledisplay is known as an impedance plane

Automated scanning is performed using an instrumented scannerthat keeps track of probe position and automated signal detection sothat a response map of the test object surface can be generated.Resolution of the inspection system is somewhat dependent on thefidelity of the scan index and on the filtering and signal processingthat are applied in signal detection A scan map can be generated byautomated eddy-current scanning and instrumentation systems.The results of eddy-current inspection are extremely accurate if theinstrument is properly calibrated Most modern eddy-current instru-ments are relatively small and battery-powered In general, surfacedetection is accomplished with probes containing small coils (3 mmdiameter) operating at a high frequency, generally 100 kHz and above.Low-frequency eddy current (LFEC) is used to penetrate deeper into apart to detect subsurface defects or cracks in the underlying structure.The lower the frequency, the deeper the penetration LFEC is general-

ly considered to be between 100 Hz and 50 kHz

A major advantage of eddy-current NDE is that it requires only imal part preparation Reliable inspections can be performed throughnormal paint or nonconductive materials up to a thickness of approxi-mately 0.4 mm Eddy-current technology can be used to detect surfaceand subsurface flaws on single- and multiple-layered materials

min-Advanced methods

Scanned pulsed eddy current. This technique for application of current technology uses analysis of the peak amplitude and zerocrossover of the response to an input pulse to characterize the loss ofmaterial This technology has been shown to measure material loss onthe bottom of a top layer, the top of a bottom layer, and the bottom of abottom layer in two-layer samples Material loss is displayed according

eddy-to a color scheme eddy-to an accuracy of about 5 percent A mechanical bond

is not necessary, as it is with ultrasonic testing The instrument andscanner are rugged and portable, using conventional coils and commer-cial probes The technique is sensitive to hidden corrosion and provides

a quantitative determination of metal loss

Magneto-optic eddy-current imaging. Magneto-optic eddy-current(MOI) images result from the response of the Faraday magneto-opticsensor to the weak magnetic fields that are generated when eddy cur-rents induced by the MOI interact with defects in the inspected mate-

rial Images appear directly at the sensor and can be viewed directly

or imaged by a small CCD camera located inside the imaging unit Theoperator views the image on the video monitor while moving the imag-ing head continuously along the area to be inspected In contrast toconventional eddy-current methods, the MOI images resemble thedefects that produce them, making the interpretation of the resultsmore intuitive than the interpretation of traces on a screen Rivetholes, cracks, and subsurface corrosion are readily visible The image

is in video format and therefore is easily recorded for documentation

Ultrasonic inspection. Ultrasonic inspection, one of the most widelyused NDE techniques, is applied to measure a variety of materialcharacteristics and conditions Ultrasonic examination is performedusing a device which generates a sound wave through a piezoelectriccrystal at a frequency between 0.1 and 25 MHz into the piece beingexamined and analyzes the return signal The device measures thetime it takes for the signal to return and the amount and shape ofthat signal It is a completely portable device that requires only thatthe probe be in direct contact with a clean surface in order to obtainaccurate information

Test objects must support propagation of acoustic energy and have ageometric configuration that allows the introduction and detection ofacoustic energy in the reflection, transmission, or scattered energyconfigurations The frequencies of the transducer and the probe diam-eter have a direct effect on what is detected Lowering the testing fre-quency increases depth of penetration, while increasing the probediameter reduces the beam spread Increasing the frequency alsoincreases the beam spread for a given diameter

Manual scanning is performed using instruments that have an loscope-type readout Operator interpretation uses pattern recogni-tion, signal magnitude, timing, and respective hand-scan position.Variations in instrument readout and variations in scanning can besignificant Automated scanning is performed using an instrumentedscanner that keeps track of probe position and automated signal detec-tion (time, phase, and amplitude), so that a response map of the inter-nal structure of the test object can be generated The resolution of thesystem is somewhat dependent on the fidelity of the scan index and onthe filtering and signal processing that are applied in signal detection

oscil-A scan map may be generated by automated ultrasonic scanning andinstrumentation systems

The most fundamental technique used is that of thickness testing Inthis case, the ultrasonic pulse is a compression or longitudinal wavethat is sent in a perpendicular direction into the metal being measured.The signal reflects off the back wall of the product being analyzed, and

the time of flight is used to establish the thickness There are ments that allow the testing to be conducted through paint coatings.This is done by looking at the waveform and selecting the area that rep-resents the actual material, not the signal developed by the coatings.Techniques have been developed that employ different types ofwaves, depending on the type of inspection desired Compressionwaves are the type most widely used They occur when the beamenters the surface at an angle near 90° These waves travel throughmaterials as a series of alternating compressions and dilations inwhich the vibrations of the particles are parallel to the direction of thewave travel This wave is easily generated and easily detected, andhas a high velocity of travel in most materials Longitudinal waves areused for the detection and location of defects that present a reasonablylarge frontal area parallel to the surface from which the test is beingmade, such as corrosion loss and delaminations They are not veryeffective, however, for the detection of cracks which are perpendicular

instru-to the surface

Shear or transverse waves are also used extensively in ultrasonicinspection; these are generated when the beam enters the surface at amoderate angle Shear-wave motion is similar to the vibrations of arope that is being shaken rhythmically: Particle vibration is perpen-dicular to the direction of propagation Unlike longitudinal waves,shear waves do not travel far in liquids Shear waves have a velocitythat is about 50 percent of that of longitudinal waves in the samematerial They also have a shorter wavelength than longitudinalwaves, which makes them more sensitive to small inclusions This alsomakes them more easily scattered and reduces penetration

Surface waves (Rayleigh waves) occur when the beam enters the rial at a shallow angle They travel with little attenuation in the direc-tion of the propagation, but their energy decreases rapidly as the wavepenetrates below the surface They are affected by variations in hard-ness, plated coatings, shot peening, and surface cracks, and are easilydampened by dirt or grease on the specimen

mate-Lamb waves, also known as plate waves and guided waves, occurwhen ultrasonic vibrations are introduced at an angle into a relative-

ly thin sheet A lamb wave consists of a complex vibration that occursthroughout the thickness of the material, somewhat like the motion ofsurface waves The propagation characteristics of lamb waves depend

on the density, elastic properties, and structure of the material as well

as the thickness of the test piece and the frequency of the vibrations.There are two basic forms of lamb waves: symmetrical (dilational) andasymmetrical (bending) Each form is further subdivided into severalmodes, which have different velocities that can be controlled by theangle at which the waves enter the test piece Lamb waves can be used

for detecting voids in laminated structures, such as sandwich panelsand other thin, bonded laminated structures.

Advanced methods

Dripless bubbler. One of the most promising improvements inultrasonic testing technology is the dripless bubbler This is a devel-opment not in the ultrasonic probe itself but in the mechanism foremploying it consistently on curved, irregular, vertical, and invertedsurfaces The dripless bubbler itself is a pneumatically powered devicethat holds a water column between the ultrasonic probe and theinspected surface With software control of the movement of the probe,

a fast and accurate map of the inspected surface can be obtained

Laser ultrasound. There is also emerging interest in the area oflaser ultrasonics, or laser-based ultrasound (LUS) The innovation isthe use of laser energy to generate sound waves in a solid This obvi-ates the need for a couplant between the transducer and the surface ofthe inspected material The initial application of this new technologyseems to be directed toward process control However, the technologycan also be applied for thickness measurement, inspection of weldsand joints, surface and bulk flaw detection on a variety of materials,and characterization of corrosion and porosity on metals

Thermographic inspection. Thermographic inspection methods areapplied to measure a variety of material characteristics and condi-tions They are generally applied in the flaw detection mode for thedetection of interfaces and variation of the properties at interfaceswithin layered test objects Test objects must be thermally conductive,and the test object surface must be reasonably uniform in color andtexture This technique uses the infrared energy associated with thepart or system being examined It is noninvasive and gives a photo-graphic image of the thermal conditions present on the surface beingexamined It can be used to accurately measure metal temperatures toestablish whether brittle or overheated conditions exist The method is

a volume inspection process and therefore loses resolution near edgesand at locations of nonuniform geometry change

Manual inspection is performed using manual control of the thermalpulse process and human observation and interpretation of the thermalimages produced as a function of time A false-color thermal map pre-sentation may be used to aid in discrimination of fine image featuresand pattern recognition The thermal map may be recorded on video-tape as a function of time Automated scanning is performed using aninstrumented scanner which reproducibly introduces a pulse of ther-mal energy into the test object and synchronizes pulse introductionwith the “start time” for use in automated image readout Automatedreadout is effected via preprogrammed digital image processing and is

test object– and inspection procedure–specific Several techniques havebeen developed that use this temperature information to characterizethe thermal properties of the sample being tested.

Many defects affect the thermal properties of materials Examplesare corrosion, debonds, cracks, impact damage, and panel thinning.With judicious application of external heat sources, these defects can bedetected by an appropriate infrared survey Uses of thermography tech-niques currently range from laboratory investigations to field equip-ment Thermography, in its basic form, has the limitation that itmeasures only the surface temperature of the inspected structure orassembly Therefore, it does not provide detailed insight into defects

or material loss located more deeply in the structure Because it is anarea-type technique, it is most useful for identifying areas that should

be inspected more carefully using more precise techniques, such aseddy-current and ultrasonic methods

Thermal wave imaging overcomes some of these limitations by suring the time response of a thermal pulse rather than the tempera-ture response The thermal pulse penetrates multiple layers whenthere is a good mechanical bond between the layers The benefits ofthermal wave imaging technology include the ability to scan a widearea quickly and to provide fast, quantitatively defined feedback withminimal operator interpretation required

mea-Advanced methods. The raw image displayed by an IR camera conveysonly information about the temperature and emissivity of the surface

of the target it views To gain information about the internal structure

of the target, it is necessary to observe the target either as it is beingheated or as it cools Since it takes heat from the surface longer toreach a deeper obstruction than to reach a shallow one, the effect of ashallow obstruction appears at the surface earlier than that of a deepone The thermal response to a pulse over time, color-coded by time ofarrival, is displayed as a two-dimensional, C-scan image for interpre-tation by the operator

Dual-band infrared computed tomography uses flash lamps to excitethe material with thermal pulses and detectors in both the 3–5- andthe 8–12-m ranges to obtain the results This technique gives three-dimensional, pulsed-IR thermal images in which the thermal excita-tion provides depth information, while the use of tomographicmapping techniques eliminates deep clutter

6.6.3 Data analysis

When an NDE process is applied to a test object, the output response

to an anomaly within the test object will depend on the form of

detec-tion, the magnitude of the feature that is used in detecdetec-tion, and therelative response magnitude of the material surrounding the anom-aly In an ultrasonic inspection procedure, for example, the ampli-tude of the response from an anomaly within a structure may beused to differentiate the response from the grain structure (noise)surrounding the anomaly If the ultrasonic procedure (measure-ment) is applied repetitively to the same anomaly, a distribution ofresponses to both the anomaly and the surrounding material will beobtained.

The measured response distribution reflects the variance in theNDE measurement process and is typical of that obtained for any mea-surement process The response from the surrounding material con-stitutes the baseline level for use in discrimination of responses from

internal anomalies The baseline response may be termed noise, and

both the discrimination capability and anomaly sizing capability of theNDE procedure are dependent on the relative amplitudes and the rate

of change of the anomaly response with increasing anomaly size(slope) The considerable flaw-to-flaw variance and the variance in sig-nal response to flaws of equal size cause increased spread in the prob-ability density distribution of the signal response If a thresholddecision (amplitude) level is applied to the responses, clear flaw dis-crimination (detection) can be achieved, as shown in Fig 6.42 If thesame threshold decision level (acceptance criterion) is applied to a set

of flaws of a smaller size (as shown in Fig 6.43), clear discriminationcannot be accomplished

In this example, the threshold decision level could be adjusted to

a lower signal magnitude to produce detection As the signal tude is adjusted downward to achieve detection, a slight increase inthe noise level will result in a “false call.” As the flaw size decreas-

magni-es, the noise and signal plus noise responses will overlap In suchcases, a downward adjustment in the threshold decision level (todetect all flaws) will result in an increase in false calls Figure 6.44shows an example in which the threshold decision level (acceptancecriterion) has been adjusted to a level where a significant number offalse calls will occur In this example, a slight change in flaw signaldistribution will also result in failure to detect a flaw The NDE pro-cedure is not robust and is not subject to qualification or certifica-tion for purposes of primary discrimination The procedure may,however, be useful as a prescreening tool, if it is followed by anoth-

er procedure that provides discrimination of the residuals Forexample, a neural network detection process structured to providediscrimination at a high false call rate may be a useful in-line tool ifother features are used for purposes of discrimination after theanomaly or variance is identified

Probability density distribution

Signal + Noise Noise

Signal amplitude

Threshold Decision Level

Figure 6.42 Flaw detection at a threshold signal level.

Signal + Noise Noise

Signal amplitude

Threshold Decision Level

Flaws not detected (misses)

Figure 6.43 Failure to detect smaller flaws at the same threshold signal level.

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Center (NTIAC), 1997.

Acceleration and Amplification of Corrosion Damage

7.2.1 Corrosion tests and standards 491 7.2.2 Examples of corrosion acceleration 500 The anodic breakthrough method for testing anodized

Intergranular anodic test for heat-treatable aluminum alloys 505 The corrosion resistance of aluminum and aluminum-lithium alloys in marine environments 507

High-temperature/high-pressure (HT/HP) testing 517 Electrochemical test methods 522

7.3.1 General sensitivity problems 566 7.3.2 Auger electron spectroscopy 566 7.3.3 Photoelectron spectroscopy 567 7.3.4 Rutherford backscattering 568 7.3.5 Scanning probe microscopy (STM/AFM) 569 7.3.6 Secondary electron microscopy and scanning Auger

sometimes receives a damaging heat treatment, and the heating andcooling causes residual stresses in the structure Weld spatter andweld oxides tend to drastically reduce the corrosion resistance ofstainless steels, for example.

■ Heat treatment. A large group of iron-based alloys has been found to

be susceptible to rapid intergranular attack in a wide range of plantenvironments when the compositions at the grain boundaries havebeen changed by equilibrium segregation of alloying elements, espe-cially the precipitation of carbides, nitrides, and other intermetallics.These changes are a result of exposure of the alloys during produc-tion of mill forms (rods, sheet, plates, and tubes) to temperatures atwhich solid-state reactions occur preferentially at grain boundaries.Because welding operations are used in the production of tubes fromsheet material and during shop fabrication and field erection, thereare further opportunities for the exposure of alloys to the range oftemperatures that may result in the depletion of essential chromium.Figure 7.1 illustrates the weld decay zone as a function of the weld-ing temperature of a stainless steel containing what was a commoncarbon content only a decade ago The extent of sensitization for a giv-

en temperature and time was found to depend very much on the bon content An 18-8 stainless steel containing more than 0.1% C may

car-be severely sensitized after heating for 5 min at 600°C, whereas a ilar alloy containing 0.06% C is affected less The physical properties

sim-of stainless steels do not change greatly after sensitization Becauseprecipitation of chromium carbide accompanies sensitization, the alloybecomes slightly stronger and slightly less ductile Damage occursonly upon exposure to a corrosive environment, with the alloy corrod-ing along grain boundaries at a rate depending on the severity of theenvironment and the extent of sensitization

Heat-affected zone (HAZ)

Corrosion tests are an important tool for a variety of industrialtasks that can vary greatly over the life of a system A decision thatmakes economic sense at design time may not make any sense by thetime the same system is in its 20th year of operation In some processapplications, the materials selected may have been the optimumchoice for the initial operating conditions However, unintendedminor changes in the operating conditions can easily increase the cor-rosivity of a process For tests to yield meaningful results, knowledge

of the environment that exists under actual service conditions is essary Quite often the water quality within a plant, under normaloperating conditions, differs significantly from that at the intake tothe plant In order to conduct realistic corrosion tests, these varia-tions must be taken into account The bulk environmental conditionscan be clean seawater, e.g., around offshore structures and some pow-

nec-er stations In othnec-er instances the watnec-er is polluted or brackish, while

in still other cases, e.g., ships, a variety of water qualities will beencountered during service.1

Some of the factors leading to corrosion damage can be reproducedrelatively easily by creating a situation favorable to their occurrence.However, other factors depend entirely on the development of localdefects that often become visible only after long and highly variableperiods of exposure, such as the effects caused by the neutral saltspray test commonly known as ASTM B 117, Method for Salt Spray(Fog) Testing When an experiment or test is planned, many factorshave to be considered The following list enumerates some of the moststandard considerations for the design of a test program:2

■ What are the objectives of the test?

■ How should the results be interpreted?

■ How can the information be integrated with earlier or other tests?

■ How many specimens are available, and what is their productionschedule (batch, sequential)?

■ How many factors control the specimen’s behavior?

■ How many factors are to be included in the tests?

■ Which of these factors interact and which have negligible interaction?

■ What type of data are to be measured?

■ Is the sample homogeneous?

■ How representative is the sample?

■ Are the tests destructive?

■ How expensive are the tests and/or specimens?