troubleshooting, testing, & analysis
BY RAYMUND SINGLETON
SINGLETON CORP., CLEVELAND
Accelerated corrosion testing of metals and coatings was first developed in the late1890s and early 1900s for testing products to increase their useful performanceand service life. This method of testing the corrodibility of ferrous and nonferrousmetals and organic and inorganic coatings has since been improved and variationsadded to better test new materials and help operators understand how they mayperform in, and withstand, a wider range of real world environments.Accelerated corrosion testing has also evolved from providing informationto help determine durability of products, and their quality assurance duringmanufacturing, to use in product research and development. Professional trade,industry, military and standards organizations, such as: ASTM International(American Society for Testing and Materials), the Society of AutomotiveEngineers (SAE), the Federation of Societies for Coatings Technologies (FSCT),NACE International (National Association for Corrosion Engineers ), Society forProtective Coatings (SSPC) and others have developed a number of acceleratedcorrosion tests to meet the demands of changes, and advances, in technology,materials, and other performance requirements and environmental regulations.For instance, water-based coatings have been developed as replacements forsolvent-based liquid coating materials in response to environmental concerns.Some powder coatings and their application methods reduce the use of solventsand ease application. In addition, the automotive industry has improvedcoatings and increased the number of surfaces coated as part of the responseto market pressures for longer warranties. Some of the more widely used testsare the representative ASTM Standards detailed in this article and well knownindustry cyclic corrosion tests such as: General Motors GMW14872 (formerlyGM9540P), and SAE J2334. There are many other tests in use; however, thosementioned also exhibit wider applicability and versatility and are, therefore,some of the more popular procedures. Other than the ASTM B117 Salt Fog test,most widely used for on-line process and product quality control, no other singlecorrosion test is as dominant because of the wide variety of service conditions,product industries and specific issues such as various materials, applicationsmethods, properties, and varying surfaces. Other considerations to considerwhen choosing or specifying a particular accelerated corrosion test includethe multitude of environments and customer demands faced by a product andindustry. A variety of standard tests has beendeveloped to address these issues.One of the first—and still most widely utilized—accelerated corrosion testsdeveloped is ASTM B117 Operating Salt Spray (Fog) Apparatus. Early developmentof corrosion tests was initiated in order to evaluate the corrodibility ofmetals and protective properties of coatings in a marine or “near shore” environment.As corrosion testing was increasingly used also to evaluate productquality and materials used in product development, variations on the basic, ororiginal, corrosion test standards have been added to increase their usefulnessfor a wider range of environments and materials. This process has resulted inthe development of the various representative accelerated corrosion tests to bediscussed in this article.The usefulness of testing coatings for resistance to water, or high humidity,as a good indicator of their service life in service environments of that typeresulted in the ASTM D1735 Standard Practice for Testing Water Resistance ofCoatings Using Water Fog Apparatus and the ASTM D2247 Standard Practicefor Testing Water Resistance of Coatings in 100% Relative Humidity. Demandfor improved evaluation of decorative copper/nickel/chromium or nickel/chromiumcoatings on steel, zinc alloys, aluminum alloys, and plastics designed forsevere service resulted in the ASTM B368 Standard Test Method for CopperAccelerated Acetic Acid-Salt Spray (Fog) Testing, widely known by its acronymas the “CASS” test. Additional needs for testing of product resistance to harshindustrial environments led to the development of the ASTM G85 StandardPractice for Modified Salt Spray (Fog) Testing with its 5 included corrosion testvariations and the ASTM G87 Standard Practice for Conducting Moist SO2Tests. The ASTM G85 variations (tests): Annex 2, Annex 3, Annex 4, and Annex5 along with ASTM G87 were developed to subject test pieces to cycles of differingenvironments to more closely resemble the changes occurring in pertinentreal-world environments. See Table I for information on “static” versus “cyclic”environment tests. With the wide variety of materials and coatings, their uses, and the service environments in which they may beused, manufacturers andproduct development personnel now can select an accelerated corrosion testthat best suits their particular product and applicable environmental conditions.Table I is provided to give a guide to which ASTM test standard can beused for the representative situations listed. Along with the development of newaccelerated corrosion tests ASTM Standards have been developed to give directionand resources for the preparation of test pieces prior to testing and evaluationof post test results. A good example is ASTM D609 Standard Practice forPreparation of Cold-Rolled Steel Panels for Testing Paint Varnish, ConversionCoatings and Related CoatingProducts. This standard provides guidelines forthe preparation of test pieces before they are introduced into the acceleratedcorrosion test. ASTM Standards: D1654 Standard Test Method for Evaluationof Painted or Coated Specimen Subjected toCorrosive Environments; D610Standard Practice for Evaluating Degree of Rusting on Painted Steel Surfaces,and; D714 Standard Test Method for Evaluating Degree of Blistering of Paintswere developed to provide common standards for analyzing and reporting theresults obtained from the corrosion tests and observed on the tested specimens.These standards are summarized below.
ISSUES
Primary issues that arise when discussing the usefulness of accelerated corrosiontests are comparison to real-world exposures and the consistency of test results.Accelerated corrosion tests are performed under controlled conditions with alimited number of variables, which generally do not duplicate the vast variety ofvariables that occur in any one location in a real- world environment. Comparisonstandards can be used to compensate for this. Comparison standards are properlyprepared, representative test pieces from a product with a known “track record”of exhibiting acceptable performance from exposure in applicable real-world environmentsand/or also acceptable corrosion resistance after being exposed to thespecified accelerated corrosion test(s). They then can be used as standards againstwhich other tested pieces are compared. This process helps especially productdevelopers and also quality professionals predict how a new product or productionrun of an existing product will perform when used. It is usually not possible, unlessadditional “long-term, real-time”, comparative outdoor or “in-service” exposuredata is available, to directly compare any given number of hours in an acceleratedcorrosion test with a given number of years exposure in real-world environments.The development of additional variations on basic accelerated corrosion tests haveallowed engineers, technicians, and quality professionals to better evaluate durabilityof products to be used in a wider variety of environments, under multiplecorrosive influences, and under constantly changing service conditions.The issues are: consistency in the accuracy of test results from subsequent runsof the same test in a single test chamber; and consistency for comparison of testresults obtained from the same test run in other chambers. These issues have beenaddressed with the development of ASTM methods for evaluating this necessaryconsistency. This verification is called repeatability of results from subsequentruns of the same test in a single test chamber and reproducibility of results fromsubsequent runs of the same test in other test chambers (regardless of how manyother corrosion test chambers are involved). These important verification proceduresare covered in Section X3 of the Appendix of the ASTM B117 test standardand incorporate “Mass Loss” Corrosion Coupons described in that section. Asimilar variation of this Mass Loss Corrosion Coupon procedure is described inParagraph 8.7 of ASTM B368, using nickel coupons for the “CASS” test.
CRITERIA FOR CYCLIC TESTS
The American Iron and Steel Institute, in cooperation with the AutomotiveCorrosion and Prevention Committee of the SAE, conducted a study of existingcyclic corrosion tests (completed in 2003, revised in 2007) in order to developtests that could be used to rank performance of materials used in automotive“cosmetic” applications. Approximately 20 existing corrosion tests were selectedfor study. These included “static” exposure condition tests (tests where theexposure conditions do not vary over the duration of the test), such as ASTMB117, and “cyclic” exposure condition tests such as ASTM G85 Annex 5 andGMW14872 (formerly GM9540P), (tests which subject test pieces to a varietyof different exposure atmospheres and conditions over the duration of the test).Ten different test materials were exposed in each test. The results of the testswere compared to real-world exposure results from 10 sites around the globeutilizing duplicates of the same materials. It was concluded from results thatan acceptable corrosion test must: (1) simulate the rank order of performanceof coatings in actual service; (2)produce the same type (character)of failure as found in real-worldexperience (example: blistering);(3) be reproducible; (4) be of relativelyshort duration; and (5) befeasible and cost effective.The test established as a resultof this effort is the SAE J2334Laboratory Cyclic CorrosionTest. The SAE J2334, GMW14872(formerly GM9540P), and ASTMG85, Annex 5 meet the above criteriaand are some of the morecommonly used cyclic corrosiontests in the U.S. and around theworld. The SAE J2334 and theGMW14872 were originally developedfor testing cosmetic finisheson automotive components. Fortuitously, they have been shown to producemore “realistic” corrosion results and, therefore, are used to test a much widervariety of materials, finishes, and applications. ASTM G85, Annex 5, is knownto be particularly useful for testing both: architectural coatings and industrial maintenance paints as well as other applications.
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Fig. 1. Schematic drawing of a typical
salt fog test chamber.
of coatings in actual service; (2)produce the same type (character)of failure as found in real-worldexperience (example: blistering);(3) be reproducible; (4) be of relativelyshort duration; and (5) befeasible and cost effective.The test established as a resultof this effort is the SAE J2334Laboratory Cyclic CorrosionTest. The SAE J2334, GMW14872(formerly GM9540P), and ASTMG85, Annex 5 meet the above criteriaand are some of the morecommonly used cyclic corrosiontests in the U.S. and around theworld. The SAE J2334 and theGMW14872 were originally developedfor testing cosmetic finisheson automotive components. Fortuitously, they have been shown to producemore “realistic” corrosion results and, therefore, are used to test a much widervariety of materials, finishes, and applications. ASTM G85, Annex 5, is knownto be particularly useful for testing both: architectural coatings and industrialmaintenance paints as well as other applications.
TYPES OF TESTS
For the purposes of this discussion ASTM B117 is described first. The otherASTM Corrosion Test Standards discussed are described in terms of how theyvary from the basic B117 Standard. Only those aspects of these standards, which are different from the B117 standard, will be discussed.
ASTM B117 — StandardPractice for Operating SaltSpray (Fog) Apparatus
The ASTM B117 Standard isprimarily used to test the corrosionresistance of organic andinorganic coatings on metals.Examples are paint or plated finisheson metal items and exposureof products to “marine” or“near-shore” environments. Thisstandard will also be the baseagainst which this article willdescribe other corrosion tests.The B117 Standard is a “static”(constant) condition, continuoustest, preferably run in multiplesof 24-hour periods. Thetest pieces are exposed to a saltfog atmosphere from atomizedsolution made up of 5% sodiumchloride and 95% ASTM D1193 Type IV water by mass and held to purity standardsspecifying that it contain: not more than 0.3% by mass total impurities.Halides (Bromide, Fluoride, and Iodide) other than Chloride, shall constituteless than 0.1% by mass of the salt content. Copper content shall be less than0.3ppm by mass. Sodium Chloride that has had anti-caking agents added shallnot be used because such agents may act as corrosion inhibitors. This solutionis then atomized into the chamber exposure zone, which is maintained at atemperature of 35 +/- 2°C (95 +/ - 3°F) and a relative humidity of at least 95%,creating a fog that has a condensate collection rate of 1.0 to2.0 ml/hr per 80 m2of horizontal collection area. To maintain these conditions, the chamber
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Fig. 2. Typical water-jacketed-type corrosion test chamber
mostly used for static condition tests.
chloride and 95% ASTM D1193 Type IV water by mass and held to purity standardsspecifying that it contain: not more than 0.3% by mass total impurities.Halides (Bromide, Fluoride, and Iodide) other than Chloride, shall constituteless than 0.1% by mass of the salt content. Copper content shall be less than0.3ppm by mass. Sodium Chloride that has had anti-caking agents added shallnot be used because such agents may act as corrosion inhibitors. This solutionis then atomized into the chamber exposure zone, which is maintained at atemperature of 35 +/- 2°C (95 +/ - 3°F) and a relative humidity of at least 95%,creating a fog that has a condensate collection rate of 1.0 to2.0 ml/hr per 80m2of horizontal collection area. To maintain these conditions, the chamberis heated and best maintainedwith a wet bottom condition inthe exposure zone. The equipmentneeded to maintain thesetest conditions will meet therequirements described in TableII and be made such that allcomponent parts that come incontact with the exposure zone,water, or solutions are made of,or coated with, inert materials.Typical test chamber componentsand operational connections are illustrated in Fig.1.
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Fig. 3. Typical large-size drive-in-type
corrosion test chamber.
ASTM D1735 — Standard Practice for Testing Water Resistance of Coatings
Using Water Fog Apparatus
The ASTM D1735 Standard uses a standard fog-generating chamber as used forthe ASTM B117 Standard. The primary variations are the chamber temperature,which is held at 38°C (100°F) and the elimination of the corrosive salt (electrolyte).The fog is created by atomizing ASTM D1193 Type IV water without anysalt whatsoever. This test method is used to gauge the life expectancy of coatingsystems by measuring their resistance to water penetration.
ASTM D2247 — Standard Practice for Testing Water Resistance of Coatings
in 100% Relative Humidity
The ASTM D2247 Standard is used to investigate how a coating resists water,which can help predict its useful life. This is generally a pass/fail test; however,the time to failure and degree of failure can be measured. A coating system typicallypasses if there is either no sign of water-related failure after the specifiedtest period or less than an established specified amount of failure.For the ASTM B2247 Standard the ASTM D1193 Type IV water is placed inthe bottom of the exposure zone to a depth of 6 to 8 in. and then heated. Thearea of the exposure zone abovethe water is not heated directlyin order to cause condensationon the test pieces. The wateris introduced into the chamberbefore the test pieces andmaintained at a temperatureapproximately 5 to 10°C (10–20°F) higher than the temperatureof the vapor above it. Themajor variation in the chamberis the elimination of the foggingtower. The 100% humidityresults from the vaporization ofthe heated water in the bottomof the chamber and/or havingheated humidified air pumped through the water. The humidifying tower isused when the option of having air pumped through the heated water is chosen.The exposure zone is maintained at 38° ± 2°C (100° ±4°F) and 100% relativehumidity. This test method is performed in water-jacketed chambers (see Fig.2)with the 6 to 8 in. of water mentioned above in the bottom of the exposurezone and the water jacket filled to 12 in. above the bottom to ensure that theimmersion heaters are submerged. It can also be run in larger chambers, suchas walk-in or drive-in size units (see Fig. 3), equipped with heated water tanks inthe exposure zone to generate the required humidity. A primary concern whenconducting the ASTM D2247 test is that the chamber be sufficiently insulated.Figure 4 is a schematic of a typical ASTM D2247 humidity test chamber configuration.To ensure that the ASTM D2247 test is being performed consistently,it is recommended that a control test piece with known durability be includedin each test run.
ASTM B368 — Standard Test Method for Copper Accelerated Acetic Acid-
Salt Spray (Fog) Test (or CASS Test)
The ASTM B368 Standard, originally developed by the National Associationfor Surface Finishing (NASF), is used primarily in the development of coated metal products, including decorative coatings, which will be exposed to severeservice and more corrosive environments and also to evaluate the performanceand protective properties of more durable coatings which would not be affectedby the typical ASTM B117 type salt fog test. This test method is particularlyapplicable to the evaluation of the performance of decorative copper/nickel/chromium or nickel/chromium coatings on steel, zinc alloys, aluminum alloys,anodized aluminum and plastics designed for severe service. This test meets thechallenges of providing a simulated service evaluation to test product specificationsin research and development and provide manufacturing controls forenvironmental changes caused by human intervention. The B368 Standard is avariation on the basic ASTM B117 Salt Spray (Fog) Test. For this variation, adjustthe pH of the 5% salt solution into the range 6.0 to 7.0, and then add 0.25 gramsof reagent-grade copper chloride per liter of salt solution. The test is not consideredto start until the temperature within the exposure zone has reached 49°C+/- 1°C (120° +/- 2-°F) and is then run continuously for 6 to 170 hours as agreedupon between purchaser and seller prior to the start of the test. It is requiredthat the temperature within theexposure zone be checked twicea day only on regular businessdays when the chamber may beperiodically opened for “short”durations (preferably less than5 min) to work with test piecesand/or replenish solution.Another variation from thestandard salt spray (fog) test isin the mandatory verificationfor consistency of test exposureconditions. In ASTM B using“Nickel Mass Loss Coupons”rather than steel (as used inthe B117 Practice, Section X3[non-mandatory]). The B368Standard uses corrosion testchamber apparatus that meetsthe requirements for the ASTMB117 Standard and will withstandthe increased temperature and stronger electrolyte solution used for this test.
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Fig. 5. Typical cyclic-type corrosion test chamber
with PLC control for automatic operation of
changeable atmospheres, temperatures, and
durations of various steps in cycle.
ASTM G85— Standard Practice for Modified Salt Spray (Fog) Testing
Five variations of this standard have been developed to accommodate its useto different purposes. The variation that is used by interested parties in anysituation must be agreed upon prior to the start of testing. Following are briefexplanations of each of the five individual test variations:
Annex 1—Acetic-Salt Spray (Fog) Testing
The Acetic Acid-Salt Spray (Fog) Test is used to test ferrous and nonferrousmetals and inorganic and organic coatings for resistance to additional and morecorrosive environments than the ASTM B117 Standard. This test is designed asa continuous exposure test without interruptions. It uses a 5 ±1 part by masssolution of sodium chloride in 95 parts of ASTM D1193 Type IV water. The pHof the solution is adjusted with the addition of glacial acetic acid to be withinthe range of 3.1 to 3.3. This solution is then atomized into the exposure zoneto create a fog that has a condensate collection rate of 1.0 to2.0 ml/hr per 80m2of horizontal collection area while the temperature of the exposure zone isbeing held at a constant 35° +/- 2°C (95° +/- 3°F). This variation is particularlyuseful in research when evaluating the effect of altering the parameters of anelectroplating process or evaluating the quality of the process on decorativechromium plating. It is used primarily for steel and zinc die-casting substrates.It should be noted by users that even though this test is utilized as a more severealternative to the ASTM B117 Salt Spray (Fog) test, the type of actual corrosionproduced is not necessarily of a “like-kind” to that resulting from the B117 teston the same test specimen material.
Annex 2—Cyclic Acidified Salt Fog Test
For this variation the pH of the test solution is adjusted to a range of 2.8 to 3.0,the exposure zone temperature is raised to 49°C (120°F), and the humidifyingtower temperature is raised to 57°C (135°F); however, the most significantchange is the wet and dry cyclic nature of the test. This change in the testrequires that the test chamber be equipped with apparatus to produce differingatmospheres for the various steps in the test cycle and timing apparatus thatwill cause the atmosphere within the chamber to vary as follows. Every 6 hours,the test pieces will be exposed to 3/4 of an hour spray of atomized salt solution,then a 2-hr dry period, during which the chamber is purged of humidity. Thefinal cycle is 1 ¾ hours of high humidity as described by the temperature andcollection rates specified. Adding the cycles to this test increases its effectivenessif evaluating how products perform in a continuously changing environment.
Annex 3—Acidified Synthetic Sea Water (Fog) Test
The addition of 42 g of synthetic sea salt and 10 ml of glacial acetic acid per literof solution, in this variation, is to increase its usefulness for production controlof exfoliation-resistant heat treatments used in producing 2000, 5000, and 7000series aluminum alloys. The pH is adjusted into the range 2.8 to 3.0 and the testis performed at a temperature of 49°C (120°F). When using this variation to testorganic coatings on metallic substrates the test is performed at a pre-selectedtemperature in the range 24 to 35°C (75–95°F). The collection rate specificationfor fog cycles of this test is unchanged from the B117 Standard at 1 to 2 ml/hrof operation; however, 2-hr cycles are used throughout the test period. Becauseof the cyclic nature of this test, prior to starting the test a separate 16-hour saltfog test is necessary to establish and verify proper condensate collection rates.The test chamber must be equipped with apparatus and controls that will cyclethe exposure zone repeatedly through a ½ -hr spray step then 1 ½ hours of soaktime at 98% relative humidity (see Fig. 5).
Annex 4—Salt/SO2Spray (Fog) Test
In this test either a sodium chloride or synthetic sea salt solution can be used.The determination of which will be used is dependent on the product beingtested and the requirements of the interested parties. A primary difference inthis test procedure is the addition of sulfur dioxide (SO2) to the exposure zoneusing a predetermined cycle. The requirement of the ASTM G85, Annex 4 stan-dard is that all the cycles during the test period be equal in length. IntroducingSO2into the exposure zone on a periodic basis requires additional apparatus.The chamber can be equipped with a device that will disburse the SO2evenlythroughout the chamber without directly impinging on any test pieces. The SO2supply source attached to the chamber must have a regulator, flow meter, andtimer to allow for accurate introduction of SO2in accordance with any set cycle.It is imperative to address all safety issues: equipment, personnel, apparatus, andprocedures prior to testing with SO2, since it is a very hazardous highly toxicgas. Note that the primary changes to the chamber are to allow for the correctintroduction of the SO2. All parts of the chamber that come into contact withthe SO2must be made of materials that are inert to SO2gas.
Annex 5—Dilute Electrolyte Cyclic Fog/Dry Test
For this test the electrolyte solution contains 0.05% sodium chloride and 0.35%ammonium sulfate in 0.60% bymass of ASTM D1193 Type IVwater. This solution is muchmore dilute than the standardsalt spray (fog) test and is runusing 2-hr cycle times comprisedof 1-hr fog at ambient 24°C ±3°Cand relative humidity below75%, followed by 1-hr dry off at35°C. That is, the test pieces areexposed to 1 hour of spray fog atcontrolled “room” temperature,then 1 hour dry-off time at 35°C(95°F). The collected condensateshall have a pH within the rangeof 5.0 to 5.4. Because of the cyclic nature of this test, a separate 16-hr salt fog testis necessary to establish and verify proper condensate collection rates prior tostarting the cyclic test. Because of the differences in humidity in this test methodand the cyclic nature of the test, the chamber will need to have a separate air lineand valve to allow the atomizing air to bypass the humidifying tower and timingapparatus to control the cycle times, temperature changes, spray, and airflow.This test is a modification of the British Rail “Prohesion” test developed in the1960s for the industrial maintenance coatings industry. This test is best suitedfor the testing of paints on steel.
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Fig. 6. Custom-sized, computer-controlled corrosion test
chamber for cyclic testing.
ASTM G87—Conducting Moist SO2Tests
The ASTM G87 standard is an adaptation of the sulfur dioxide tests used inEurope and is particularly effective in producing easily visible corrosion onmetals as would be observed in an industrial or marine environment. Thistest requires 2 ±0.2 L (122 in3 ±12 in3) Type IV water in the bottom of the testchamber and does not use a fogging apparatus. The apparatus used to generatethe necessary humidity is the same used in the ASTM D2247 test. The gasintroduction apparatus is the same as used in the ASTM G85 Annex 4 test. Oncethe test pieces are placed in the chamber 0.2, 1, or 2 L (12, 61, or 122 in3 ±12in3) of sulfur dioxide (SO2) with a purity of 99.9% (liquid phase) is introduced.hours and remains at this level for the duration of the test cycle. One test cycleis 24 hours. A test can be run for as many cycles as are agreed upon betweenthe purchaser and seller; however, the SO2and water must be changed for each24-hr cycle. The 24-hr testcycles are executed in one of the following ways: 1.Continuous — change of water and SO2with as little disturbance as possible2. Alternating Exposure — 8-hr exposure; drying in ambient for 16 hours with20–30°C (68–86°F) and 75% relative humidity The variations differing from theB117 practice are that for this test method all test pieces be placed on the samehorizontal plane and all test pieces in a given test run be of similar reactivity.
NOTE:While adding corrosive gases may expedite degradation of the protectiveproperties of the coating on the steel substrate, the purpose of the test must bekept in mind. This gas introduction may be acceptable when testing materialsused in an environment that may include similar acids, such as roofing materials,that could be exposed to acid rain.
EQUIPMENT
With continued developments in accelerated corrosion testing, basic changesand improvements have been made to the equipment. The basic requirementsfor accelerated corrosion testing equipment are listed on Table II under theASTM B117 Standard. The adjustments required for each variation on theaccelerated corrosion-testing model are then listed next to the appropriateASTM standard. Typical applications are listed in Table III. Along with thebasic operating requirements, improvements have been made in the materialsused in the construction of the chambers, the hardware and software controls,and data recording capabilities. Programmable logic controller (PLC) systemsprovide for increasingly sophisticated data acquisition, manipulation, and display(see Fig. 6). As a result, both test chambers with manually entered settingsfor automatic operation, and test chambers with increasingly sophisticatedprogrammable controls for automatic temperature changes; varying levels ofcontrolled humidity; automatic changes for different environmental atmospherecycles; and automatic data logging/accumulation, record keeping, and outputwith graphic data display, are available through a variety of suppliers. Thesecomprehensive capabilities allow each laboratory to find accelerated corrosiontesting equipment that meets their specific needs.
TEST PIECE PREPARATION/CLEANING
In order for any of the above accelerated corrosion tests to obtain comparableresults, all test pieces must start the test in a similar condition without contaminantspresent on the surface of interest. The ASTM test standards discussedabove indicate reference standards to be used for the preparation andcleaning of test pieces. Particular attention must be paid to the wide variety ofmaterials being cleaned and recognition of differing specialized material andcoating operations. ASTM test standards state that the cleaning method isto be dependent on theproduct being cleaned and then refer to ASTM D609Standard Practice for Preparation of Cold-Rolled Steel Panels for Testing Paint,Varnish, Conversion Coatings and Related Coating Products where required.This standard describes five cleaning and preparation methods. The preparationmethods described are: Procedure A—Conversion Coating; Procedure B—VaporDegreasing; Procedure C—Solvent Brushing; Procedure D—Solvent Wiping; andProcedure E—Alkaline Cleaning. Procedure A (Conversion Coating) addressesthe issue of ensuring that the test piece be protected from rusting between themanufacturing process and the corrosion test. Procedure B (Vapor Degreasing)exposes the test pieces to solvent vapors within controlled conditions to removecontaminants. This procedure is currently discouraged for environmental reasons.Procedures C (Solvent Brushing) and D (Solvent Wiping) both use MineralSpirits as the cleaning agent. Procedure E (Alkaline Cleaning) describes methodsfor cleaning with an alkaline cleaning agent that is appropriate for the product.The cleaning method and cleaning agent chosen for any test run should be asagreed upon between the purchaser and seller. ASTM D609 refers to the waterbreak test as a method of checking test pieces for contaminants. The procedureis simply to run a small amount of water over the surface of the test piece. If it isfree of oils and contaminants the water will run over it in a sheet and off in onefall with no breaks. Finally, ASTM D609 discusses the handling of cleaned testpieces and storage in paper impregnated with volatile corrosion inhibitor (VCI)if the test pieces are not to be run immediately after cleaning.
EVALUATION OF RESULTS
Some primary ASTM Standards that have been developed to assist in the evaluationof corrosion test results are: ASTM D1654—Standard Test Method forEvaluation of Painted or Coated Specimen Subjected to Corrosive Environments;ASTM D610—Standard Test Method for Evaluating Degree of Rusting onPainted Steel Surfaces; ASTM D714—Standard Test Method for EvaluatingDegree of Blistering of Paints;and ASTM B537—Standard Practice for Ratingof Electroplated Panels Subjected to Atmospheric Exposure. Irrespective of thetitle wording, the text of ASTM B537 states that it is also used to evaluate panelssubjected to laboratory accelerated corrosion tests. Other standards are alsoavailable for evaluation of specific materials and finishes.ASTM D1654 is used for the evaluation of substrates, pretreatments, coatingsystems, or a combination thereof. This standard is also used when the coatinghas been scribed prior to exposure to the corrosive environment. ASTM D1654recommends a straight-shank tungsten carbide tip or equivalent scribing tool toproduce a cut through the coating to the substrate of the test piece. The scribeserves as a deliberate failure of the coating system prior to corrosion testing.Once the test piece has been through the selected accelerated corrosion test itis then cleaned by rinsing with a gentle stream of water at 45°C (110°F). Loosecoating is then removed using compressed air to blow it off or a scraper to scrapeit off. The “creepback” (perpendicular distance of failure from the scribe line) ofthe coating is measured at uniform intervals and the rating system described inthe standard applied. ASTM D610 provides a series of photographic referencestandards that demonstrate degrees of rust on the surface of a test piece. Thetest pieces are first gently cleaned then the amount of rust observed is reportedfrom comparison to the photographic standards and the additional use ofestablished standardized terminology from the test standard. It is important tocompare only rust and not surfaces stained by rust. ASTM D714 is used to evaluatecoating adhesion by observing the amount of blistering that has occurred.The test pieces are first gently cleaned then compared to the series of includedphotographic references and the additional use of the appropriate “standardized”terminology indicated in the test standard. They are graded first by thesize of the blisters that appear on the surface and then the number of blistersthat appear per unit area.
CONCLUSION
The corrosion tests discussed in this article are some of the most utilized testsof this type. They also demonstrate the diversity of the tests and some of theadvancements in accelerated corrosion testing and complexity of the subject.They also provide a solid basis for evaluating the performance of a wide varietyof products and materials. These standards have been developed, evolved andrefined through cooperative efforts of experts and interested parties from: ASTM International, industry, government, and other trade organizations worldwide.
