f ature oeus

BY RAYMUND SINGLETON, SINGLETON CORP., CLEVELAND, OHIO

Accelerated Corrosion Testing

A ccelerated corrosion testing of metals and coatings was first

developed in the late 1890s and early 1900s for testing products to

increase t heir useful perfo rmance and service life. This method of test­ing the corrodibility of ferrous and nonferrous metals and organic and inorganic coatings has since been improved and variations added to

bener rest new materials and help operators understand how they may perform in, and wirhsmnd, a wider range of real world environments.

Accelerated corrosion testing has also evolved from providing informa­tion to help determine durability of products, and their quality assurance during manufacturing, 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 Automotive Engineers (SA E), the Fedetation of Societies for Coatings Technologies (FSCT), NACE International (National Association for Corrosion Engineers ») Society for Pro tective Coatings (SSPC) and others have developed a number of accelerated corrosion 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 for solvent-based liquid coating materials in response to environmen­tal concerns. Some powder coatings and their application methods reduce the use of solven ts and ease applica­tion . In addition) the automotive industry has improved coatings and increased the number of surfaces coated as part of the response to mar­ket pressures for longer warranties.

121 metalfinishing I November/ December 2012

Some of the more widely used tests are the representative ASTM Standards detailed in this article and well known industry cyclic corrosion testS such as: General Motors GMW14872 (fotmerly GM9540P), and SAE J2334. There are many other tests in use; however, those men­tioned also exhibit wider applicability and versati li ty and are, therefore, some of the more popular proce­dures. Other than the ASTM B1l7 Salt Fog tes t, most widely used for on-line process and product quality control, no other single corrosion test is as dominant because of the wide variety of service conditions, product industries and specific issues such as various materials, applications meth­ods, properties) and varying surfaces.

O ther considerations to consider when choosing or specifYing a partic­ular accelerated corroSIOn tes t include th e multitude of environ­ments and customer demands faced by a product and industry. A variety of standard tests has been developed to address these issues.

One of the first- and still most widely utilized- accelerated corro­sion tests developed is ASTM B 117 Operating Salt Spray (Fog) Apparatus. Early development of corrosion tests was initiated in order to evaluate the corrodibility of met­als and protective properties of coat­ings in a marin e or "near shore)) envi­ronment. As corrosion testing was increasingly used also to evaluate product quality and materials llsed in product development) variations on the basic) o r original, corrosion test standard s have been added to

increase their usefulness for a wider range of environmen ts and materi­als. This process has resulted in the development of the various repre­sentative accelerated corrosion tests

to be di scussed in this update. The usefuln ess of testing coatings

for resistance to water, o r high humidity) as a good indicator of their service life in service environments of that type resulted in the ASTM 01735 Standard Practice for Testing Water Resistance of Coatings Using Water Fog Apparatus and the ASTM 02247 Standard Practice for Testing Water Resistance of Coatings in 100% Relative Humidity. Demand for improved evaluation of decorative copper/nickel/chromium or nickel/chromium coatings on steel, zinc alloys, aluminum alloys, and plastics designed for severe service resulted in the ASTM B368 Standard Test Method for Copper Accelerated Acetic Acid-Salt Spray (Fog) Testing, widely known by its acronym as the "CASS" test.

Additional needs for testing of product resistance to harsh industri­al environments led to the develop­ment of the ASTM G85 Standard Practice for Modified Salt Spray (Fog) Testing with its 5 included corrosion test vatiations and the ASTM G87 Standard Practice for Conducting Moist S02 Tests. The ASTM G85 variations (tests): Annex 2, Annex 3, Annex 4, and Annex 5 along with ASTM G87 were developed to subject test pieces to cycles of differing envi­ronments to more closely resemble the changes occurring in pertinent real-world environments. See Table I for in formation on "static" versus "cyclic') environment tests.

With the wide variety of materials and coatings, their uses, and the serv­ice environments in which they may be lLsed, manufacturers and produce development personnel now can select an accelerated corrosion test [hat best suits their particular prod­uct and applicable environmental conditions. Table I is provided to give a guide to which ASTM test standard can be used for the representative sit­uations listed. Along with the devel­opment of new accelerated corrosion tests ASTM Standards have been developed to give direction and resources for the preparation of test pieces prior to testing and evaluation of post test resu lts. A good example is

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featurefocus Table I. Static Versus Cyclic Tests

Static Corrosion Tests In static corrosion tests the exposure zone conditions of the test chamber stay the same (within

defined tolerance limits) throughout the duration of the test. The test duration (length of exposure) is the only variable.

Examples of Static Condition Corrosion Tests ASTM B 117 Salt (Fog) Spray ASTM B 368 Copper Accelerated Salt Spray ASTM D 1735 Water Fog Humidity ASTM D 2247 Condensing 100% humidity ASTM G 85, Annex 1 Acetic Add Salt Fog

Cyclic Tests In cyclic corrosion !ests the conditions in the exposure zone change for each step in the cycle in an

ordered, predefined sequence. The changeable conditions can include temperarure, various types of moisture application, different levels of humidity, different types of pollutant or electrolyte such as various salts, gases, etc" and various lengths of exposure for each,

Cycle Type A tests Temperature is constant Various types of moisture application, diff~rent levels of humidity, different types of pollutant or

electrolyte such as various salts, gases, etc., and various lengths of exposure for each step. Total test duration (length of exposure) is also a variable

Examples of Cyclic Type A Condition Corrosion Tests ASTM G 85 Annex 2 Acetic Acid Annex 3 Acidified SF Annex 4 SaltiSOz Spray

Cyclic Type B Tests Temperature is different for at least some steps in the cycle Various types of moisture application, different levels of humidity, different types of pollutant or

electrolyte, such as various salts, gases, etc., and differing lengths of exposure for various steps. Total test duration (length of exposure) is also a variable.

Examples of Cyclic Type B Condition ColTQ8ion Tests ASlM G 85 Annex 5 Modified SF (DEC) Dilute Electrolyte Cyclic FoglDry Test AS1M G 87 Moist SOz GM 9540P SAE J-2334

ASTM 0609 Standard Practice for Preparation of Cold-Rolled Steel Panels for Testing Paint Varnish, Conversion Coatings and Related Coating Products. This standard pro­vides guidelines for the preparation of test pieces before they are intro­duced into the accelerated corrosion test. ASTM Standards: 01654 Standard Test Method for Evaluation of Painted or Coated Specimen Subjected to Corrosive Environments; D610 Standard Pracrice for Evaluating Degree of Rusting on Painted Steel Surfaces, and; 0714 Standard Test Method for Evaluating Degree of Blistering of Paints were developed to provide common standards for analyzing and reporting the results obtained from the corrosion tests and observed on the tested speci mens. These stan­dards are summarized below.

ISSUES Primary issues that arise when dis­cussing the usefulness of accelerat­ed corrosion tests are comparison to real-world exposures and the consis-

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tency of test results. Accelerated cor­rosion tests are performed under controlled conditions with a limited number of variables, which general­ly do not duplicate the vast variety of variables that occur in anyone location in a real-world environ­ment. Comparison standards can be used to compensate for this. Comparison standards are properly prepared, representative test pieces from a prod uct wi th a known "track record" of exhibiting acceptable per­fOl'mance from exposure in applica­ble real-world environments and/or also acceptable corrosion resistance after being exposed to the specified accelerated corrosion test(s}. They then can be used as standards against which other tested pieces are compared. This process helps espe­cially product developers and also quality professionals predict how a new product or production run of an existing product will perform when used. It is usually not possible, unless additional "long-term, real­time)), comparative outdoor or "in­service" exposure data is available,

to directly compare any given num­ber o f hours in an accelerated corro­sion test with a given number of years exposure in real-world envi­ronments. The development of additional variations on basic accel­erated corrosion tests have allowed engineers, technicians, and quality professionals to better evaluate durability of products to be used in a wider variety of environments, under multiple corrosive influences, and under constantly changing service conditions.

The issues are: consistency in the accuracy of test results from subse­quent runs of the same test in a sin­gle test chamber; and consistency for comparison of test results obtained from the same test run in other chambers. These issues have been addressed with the development of ASTM methods for evaluating this necessary consistency. This verifica­tion is called repeatability of results from subsequent runs of the same test in a single test chamber and reproducibility of results from sub­sequent runs of the same test in other test chambers (regardless of how many other corrosion test chambers are involved). These important verification procedures are covered in Section X3 of the Append", of the ASTM B117 test standard and incorporate "Mass Loss" Corrosion Coupons described in that sect ion. A similar variation of this Mass Loss Corrosion Coupon procedure is described in Paragraph 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 Automotive Corrosion and Prevention Committee of the SAE, conducted a study of existing cyclic corrosion tests (completed in 2003, revised in 2007) in order to develop tests that could be used to rank pe·r­formance of materials used in auto­motive "cosmetic" applications. Approximately 20 existing corrosion tests were selected for study. These included "static" exposure condition tests (tests where the exposure con-

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featu refocus

Fig. 1. Schematic drawing of a typical salt fog test chamber.

di rions do not vary over the dura­tion of the test), such as ASTM B117, and "cyclic" exposure condi­tion rests such as ASTM G8S Annex 5 and GMW14872 (form erly GM9540P), (tests which subject test pieces to a variety of different expo­sure atmospheres and condit ions over the du ration of the test). Ten di ffe rent test materials were exposed in each test. The results of the tes tS were compared [0 real­world exposure results from 10 sites around the globe utilizing dupli­cates of the same materials. It was concluded from results tha t an acceptable co rrosion test must: (1) simulate the rank order of perform­ance of coatings in actual service; (2) produce the same type (charac ter) of failure as found in real-world experience (example: b listering); (3) be reproducible; (4) be of rela tively short duration; and (5) be feasib le and cost effective.

The test established as a result of this effort is t he SAE J2334 Laboratory Cyclic Corrosion Test. The SAE J2334, GMW14872 (fo r­merly GM9540P), and ASTM G85 , Annex 5 meet the above criteria and are some of the more com­monly llsed cyclic corrosion tests in the u.s. and around t he world. The SAE J2334 and the GMW14872 were ori ginally developed for test­ing cosmetic finish es on automo­tive components. Fo rtu itou sly, they have been shown to produce more "realis tic" corrosion resul ts and, therefore) are llsed to test a

14 I metalfinishing I November/December 2012

Table II. Equlpment RequlremeDts for Various Test Standards

Standard

B 117

D 1735

D 2247

B 368

G 85

G 87

Chamber Requirements

Insulated cabinet made with inert materials or illert coating on the interior Salt solution reservoir ASTM D 1193, Type IV (On water supply Humidifying tower One or more fogging towen with atomizer and baffles Compressed air supply Bottom drain Exhaust Heating apparatus for humidifying tower and chamber exposure zone Tcmperarure displa~ for wet and dry bulb temperatures and hurrudifying tower Appropriate ,auges and safety apparatus Test piece supports B 117 chamber without any traces of salt B 117 salt solution reservoir is used for ASTM 0 1193. Type m (Ol) water B 117 chamber

~~~:::~~~~ ~o~~rOf ASTM 0 1193. Type IV water 00 bottom or large internal water taIW in chamber exposure woe Hoating apparatus for water aDd bottom 12 in. of chamber exposure tone Method A, no humidifying tower Method B. equipped to bubble humidified air through air manifold submerged in bottom of exposure wne water Humidifying tower, only if using bubble method B 117 chamber Add bypass on humidifying tower .

Annex 1

Annex 2 and Annex 3

Annex 4

Annex 5

B 117 Ownber

Modified or purpose built B 117 type test chamber with appropriate cyclic timing controls aDd automatic mechanical apparatus to execute required exposure zone temperature and atmosphere changes,

Modified or purpoae built B 117 type test chamber with appropriate cyclic timing controls and automatic mechanical apparatus to execute required oltposure wne temperature and atmosphere changes,

502 supply and dispersion method. Flow meter Modifi_ed or purpo8e built B 117 type test chamber with

app(opriate cyclic timing controls and-automatic mechanical apparatus to execute required exposure woe temperature and atmosphere changes,

Modified or purpose built B 117 type test chamber with appropriate cyclic timing controls and automatic mochanlca1 apparatus to ex~ required exposure zone temperature and atmosphere changes.

No fogging appararus or humidifying tower Internal capadty of 300 L Supply of sulfur dioxide and inlet rube 00 108s than 50 mm from floor of chamber Means to reliove exces. presaure Heat. externally on floor and lower wall, ExternaJ heater for water Metering apparatus Located away from pollution. direct sunlight. and draft

much wider variety of materials, finishes, and applications. ASTM G8S, Annex 5, is known to be par­ticularly useful for testi ng both: architectural coatings and industri­al maintenance paints as well as other applications.

ASTM B117 - Standard Practice for Operating Salt Spray (Fog) Apparatus The ASTM B1l7 Standard is prima­rily used to test the corrosion resist­ance of organic and inorganic coat­ings on metals. Examples a re paint or plated finishes on metal items and exposure of prod ucts to "marine" or "near-shore') environ­ments. This standard will also be the base against which this article wit! describe other corrosion tests. The Bl1 7 Standard is a "static" (con­stant) condi tion, continuous test) preferably run in multiples of 24-hour periods, The test pieces are

TYPES OF TESTS In this case, ASTM B1l 7 is described first . The other ASTM Corrosion Test Standards discussed are described in terms of how they vary from the basic B 117 Standard. Only those aspects of these standards, which are different fro m the B1l7 standard) will be discussed .

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featu refocus

Fig. 2. Typical water·jacketed-type corrosion test chamber, mostly used for static condit ion tests.

Fig. 3. Typical large-size, drive-in-type corrosion test chamber.

exposed to a salt fog atmosphere from atomized solution made up of 5% sodi um chloride and 95% ASTM 01193 Type [V water by mass and held to purity standards specifYing chat it contain: not more than 0.3% by mass total impurities. Halides (Bromide, Flu oride, and Iodide) other than Chloride, shall constitute less than 0.1% by mass of the salt content. Copper content shall be less than 0.3ppm by mass. Sodium Chloride that has had anti-caking agents added shall not be used because sllch agents may acr as CO[ ­

ros ion inhibiw[s. This solution is then atom ized in to the chamber exposure zone, which is maintained at a temperature of35 +/-2°C (95 +/ - 3 ° F) and a relative humidity of at least 95%, creating a fog rhar has a condensate collection rate of 1.0 to2.0 mljhr per 80 m2 of horizontal

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collection area. To maintain these conditions, the cham­ber is heated and best main­tained with a wet bottom condit ion in the exposure zone. The equipment need­ed to maintain these test co ndition s will meet the requirements described in Table Il and be made such that all component parts that come in contact with the expo­sure zone, water, or solutions are made of, or coated with, inert mate­rials. Typical tesr chamber compo­nents and operational connections are illustrated in Fig.!.

ASTM D1735 - Standard Practice for Testing Water Resis tance of Coatings Using Water Fog Apparatus The ASTM 01735 Standard uses a standard fog-generating chamber as used for the ASTM B117 Srandard. The primary variation s are the chamber temperature, which is held at 38°C (100°F) and the elimination of the corrosive salr (electrolyte). The fog is created by atomizing ASTM 01193 Type IV warer wirhout any salt whatsoever. This test method is used to gauge the life expectancy of coating systems by measuring their resistance to water penetration.

ASTM D2247 - Standard Practice for Testing Water Resistance of Coatings in 100% Relative H umidity The ASTM 02247 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 fai lure and degree of fai lure can be measured. A coating sysrem typically passes if there is either no sign of water-relat­ed failure after the specified test period or less than an esrablished specified amount o f failure.

For the ASTM B2247 Standard the ASTM 01193 Type TV warer is placed in the bottom of the exposure zone to a depth of 6 to 8 in . and rhen heated. The area of the exposure zone above the water is not heated

\

~

i.:-® i !

t_~Jii_~ ./ \

- '::''''::::'-'''''

Fig. 4. Humidity test chamber schematic drawing.

directly in order to cause condensa­tion on the test pieces. The water is introduced intO the chamber before the test pieces and maintained at a temperature approximately 5 to 10 °C (10- 20 ° F) higher than the temperature of the vapor above it. The major variation in the chamber is the elimination of the fogging tower. The 100% humidity results from the vaporization of the heated water in the bottom of the chamber and/or having heated humidified air pumped thro ugh the water. The humidifying tower is llsed when the oprion of having air pumped through the heated water is chosen. The exposure zone is maintained at 38° ± 2°C (100° ±4°F) and 100% rel­ative hum idity. 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 exposure zone and the water jacket filled to 12 in. above the bottom to ensure that the immer­sion heaters are submerged. It can also be rlln in larger chambers, such as walk-in or drive-in size un its (see Fig. 3), equipped with heated water tanks in the exposure zone to gener­ate the required humidity. A primary concern when conducting the ASTM 02247 resr is that rhe chamber be sufficiently insulated. Figure 4 is a schematic of a typical ASTM 02247 humidity test chamber configura­tion. To ensure that rhe ASTM 02247 test is being performed con­sistently, it is recommended that a control test piece with known dura­biliry be included in each test run.

ASTM B368 - Standard T est Method for Copper Accelerated

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featu refocus Acetic Acid-Salt Spray (Fog) Test (or CASS Test) The ASTM B368 Standard, original­ly developed by the Natio nal Association for Surface Fin.ishing (NASF), is used primarily in the development of coated metal prod­ucts, incl uding decorative coatings, which will be exposed to severe serv· ice and more corrosive environ­ments and also to evaluate the per­fo rmance and protective properties of more durable coatings which would not be affec ted by the typical ASTM B117 type salt fog test. This test method is particula rly applica­ble to the evaluation of the perfo rm­ance of decorative copper/ nickel/chromium or nick­el/ch romium coatings on steel, zinc alloys, aluminum alloys, anodized alum inum and plastics designed for

Fig. 5. Typical cyclic-type corrosion test chamber with PLC control for automatic operation of changeable atmospheres. temperatures, and durations of various steps in cyde.

severe service. This test meetS the challenges of providing a simulated service evaluation to test prod uct specifications in research and devel­opment and provide manufacturing controls for environmental changes caused by human intervention.

The B368 Standard is a variation on th e basic ASTM Bl17 Salt Spray (Fog) Test. For this variation, adjust the pH of the 5% salt solution into the range 6.0 to 7.0, and then add 0.25 grams of reagent-grade copper chloride per liter of salt solution. The test is not considered to start

\ 161 metalfinishing I November/ December 2012

until the tem perature with in the exposure zone has reached 49 ° C + /-1°C (120° +/- 2_0 F) and is then run continuously for 6 to 170 hours as agreed upon between purchaser and seller pri or to th e start of the tes t. It is required that the temperature within the exposure zone be checked twice a day only on regular business days when the chamber may be periodically opened for "short" durations (preferably less than 5 min) to work with test pieces and/ or replenish solution.

Another variation from the stan­dard salt spray (fog) test is in the mandatory verification fo r consisten­cy of test exposu re conditions. In ASTM B using "Nickel Mass Loss Coupons" rather than steel (as used in the B 11 7 Practice, Section X3 [non-mandatory]). T he B368 Standard uses corrosion test cham­ber apparatus that meets the require­mentS for the ASTM B 117 Standard and will withstand the increased tem­perature and stronger electrolyte solu tion used for this test.

ASTM G8S - Standar d Practice for Modified Salt Spray (Fog) Testing Five variations of this standard have been developed to accommodate its use to different purposes. The varia­tion that is used by interested parties in any situation must be agreed upon prior to the start of testing. Followi ng are brief explanations of each of the five individual test variations:

Annex l-Acetic-Salt Spray (Fog) Testing The Acetic Acid-Salt Sp ray (Fog) Test is llsed to test ferrou s and non­ferrous metals and inorganic and organic coatings for resistance to additional and more corrosive envi­ronments than the ASTM B117 Standard. Thi s test is designed as a continuous exposure test without interruptions. [t uses a 5 ± 1 part by mass solution of sodium chloride in 95 parts of ASTM 0 1193 Type IV water. The pH of the solution is adjusted with the addition o f glacial acetic acid to be within the range of 3.1 to 3.3. This solution is then

atomized into the exposure zone to

create a fog that has a condensate collection rate of 1.0 to2.0 m l/hr per 80 m 2 of horizontal collection area while the temperature of the expo­sure zone is being held at a constant 35 ° +/- 2 °C (95° +/- 3°F). This vari­ation IS particularly useful 111

research when evaluating the effect of altering the parameters of an electroplating process or evaluating the quality of the process on decora­tive chromium plating. It is used pri­marily for steel and zinc die-casting substrates. It should be noted by llsers that even though this test is utilized as a more severe alte rnative to the ASTM Bl17 Salt Spray (Fog) test, the type of actual corrosion produced is not necessarily of a "like-kind" to that result ing from the B 117 test on 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 humidifying tower temperature is raised ro 57°C (135°F); however, the most significant change is the wet and dry cyclic nature of the test. Thi s change in the tes t requires that the test chamber be equipped with apparatus to produce differing atmospheres for the various steps in the test cycle and timing apparatus that wi ll cause the atmosphere with­in the cham ber co 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 pu rged of humidi ty. The final cycle is 1 % h ou rs of high humidity as described by the temperature and collection rates specified. Adding the cycles to this test increases its effectiveness if evaluating how prod­ucts per fo rm 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

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featu refocus per liter of solution, in this varia­tion, is to increase its usefulness for production control of exfoliation­resistant heat treatments used in producing 2000, 5000, and 7000 series aluminum alloys. The pH is adjusted into the tange 2.8 to 3.0 and the test is performed at a tem­petature of 49 °C (120°F). When using this variation to test organic coatings on metallic substrates the test is performed at a pre-selcC[ed temperature in the range 24 to 35°C (75-95 ° F). The collection tate speci­fication for fog cycles of this test is unchanged from the B ll7 Standard at 1 to 2 mljht of operation; howev­er, 2-hr cycles are used throughout the test period. Because of the cyclic nature of this tcst, prior to starting the test a separate 16-pOllf salt fog test is necessary (0 establish and ver­ify proper condensate collection rates. The rest chamber m ust be equipped with apparatus and C011-

troIs that will cycle the exposure zone repeatedly through a lJ2 -hr spray step then 1 Ih hours of soak time at 98% relative humidity (see Fig 5).

An n ex 4- Salt/S02 Spray (Fog) Test In this test ei ther a sodium chloride or synthetic sea salt solution can be used. The determination of which will be used is dependent on the product being tested and the requirements of the interested par­ties. A primary difference in this test procedute is the addition of sulfur dioxide (S02) to the exposure zone using a predetermined cycle. The requirement of the ASTM G85, Annex 4 standard is that all the cycles during the test period be equal in length. Introducing S02 into the exposure zone on a periodic basis requires addi tional apparatus. T he chamber can be equipped with a device that wi ll disburse the S02 evenly throughout the chamber without directly impinging on any test pieces. The S02 supply source attached to the chamber must have a regulator, flow meter, and timer to allow for accurate introduction of

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S02 in accordance wi th any set cycle. It is impetative to address all safety issues: equipm ent, personnel, appa­ratus, and procedures prior to test­ing with S02' since it is a very haz­ardous highly toxic gas. Note that the primary changes to the chamber a re to allow for the correct introduc­tion of the S02' All parts of the chamber that come in to contact with the S02 must be made of mate­rials that are inert to S02 gas.

Annex 5- 0 ilu te Electro lyte Cyclic Fog/D ry Test For this test the electrolyte solution contains 0.05% sodium chloride and 0.35% ammonium sulfate in 0.60% by mass of ASTM 01193 Type IV water. This solution is much more dilure than the standard salt spray (fog) test and is run using 2-hr cycle times comprised of I-hr fog at ambi­ent 24°C ±3°C and relative humidi­ty below 75%, followed by 1-hr dry off at 35 ° C. That is, the test pieces are exposed to 1 hour of spray fog at controlled "room" temperature, then 1 hour d ry-off time at 35°C (95° F). T he collected condensate shall have a pH within the range of 5.0 to 5.4. Because of the cyclic nature of this test, a separate 16-hr salt fog test is necessary to establish and verifY proper condensate collec­tion rates prior to starting the cyclic test. Because of the differences in humidity in this test method and the cyclic nature of the tes t, the chamber will need to have a separate air line and valve to allow the atom­izing air to bypass the humidifying tower and timing apparatus to con­trol the cycle times, temperature changes, spray, and airflow. This test is a modification of the British Rail uProhesion" test developed in the 1960s for th e industrial mainte­nance coatings industry. This test is best suited for the testing of paints on steel.

ASTM G87- Conducting M oist S02 Tests The ASTM G87 standard is an adap­tation of the sulfur dioxide tests used in Europe and is particularly effective in producing easily visible

Fig. 6. Custom-sized. computer-controlled cor­rosion test chamber for cyclic testing.

corrosion on metals as would be observed in an industrial or marine environment. This test requires 2 ±0.2 L (122 in3 ±12 in 3) Type N water in the bottom of the test cham­ber and does not use a fogging appa­ratus. The apparatus used to gener­ate the necessary humidity is the same used in the ASTM 02247 test. The gas introduction apparatus is the same as used in the ASTM G8S Annex 4 test. Once the test pieces are placed in the chamber 0.2, 1, or 2 L (12, 61, or 122 in3 ±12 in3) of sulfur dioxide (S02) with a purity of 99.9% (liquid phase) is introduced. Then the temperature of the chamber is raised to 40 ±oC (104 ±5.4°F) in 1 1(,

hours and remains at this level for the duration of the test cycle. One test cycle is 24 hours. A test can be run for as many cycles as are agreed upon between the purchaser and sell­er; however, the S02 and water must be changed for each 24-hr cycle.

The 24-hr test cycles are executed in one of the following ways: 1. Continuous - change of water and S02 with as little disturbance as pos­sible 2. Alternating Exposute - 8-hr exposure; drying in ambien t for 16 hours with 20- 30°C (68- 86°F) and 75% relative humidity The variations differing from the B ll7 practice are that for this test method all test pieces be placed on the same hori­zontal plane and all test pieces in a given test run be of similar reactivity. NOTE: While adding corrosive gases may expedite degradation of the pro­tective properties of the coating on the steel substrate, the purpose of the test must be kept in mind. This gas introduction may be acceptable when testing materials used in an environment that may include simi­lar acids, such as roofing materials,

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featurefocus

Table m. Typical Uses aud Products Tested Under Various Test Standards

Research and Development, Quality Assurance. Marine Exposure Environments

Re&earch Studies. Quality Assurance

Research and Development, Quality Assurance

Quality A&5unmce, Development of heat treatment perimeters, and simulation of marine or near-shore environments or applications

Research and Development, Quality Assurance, and

,simulation of industrial, marine, or ncar-shore envirorunents or applications

Research and Development, Quality Assurance

Specification Acceptance, Service Evaluation. and Quality Assurance for severe iCtVice conditions

Evaluating Service Ufe for watcr.resistance coatings

Evaluating contamination of subltrate, inadequate surface preparation. and deficiencies in coatings

Research and Development, Quality Assurance

Product

Inorganic and organic coatings, aluminum. and other metals

Decorative chromium plate and materials resistant to long-tonn B 17 exposure as well as exfoliation of aluminum alloys

Electroplating and cadmium plating on steel or zinc die castings as well IIJ exfoliation of aluminum alloys

Exfoliation-resistant heat treatments on 2000, 5000, and 70Cl0 series aluminum alloys and materials, coatings, or equipment utilized in marine-type environments

Decorative electroplating. and materia]l, coatings. or equipment u,ti lized in industrial. aerpspace. and marine.type environments where corrosive pollutants. exhaust gases, or acid rain effects may be pre·sent

PalDts on steel

Anodized aluminum, decorative electroplating, design for severe service

Coatings, substrate treatments. and coating systems

Coating systems

Metals. coatiogs, and equipment used io lndustriaJ and marine-type environments where -, corrosive pollutants. exhaust gases, or acid rain effects may be present

ASTM Standard

B 117 Operating Salt Spray (Fog) Apparatus

G 8S-Annex 1 Acetic Acid Salt Spray

G 8S-Annex 2 Cyclic Acidified Salt Spray

G 8S-Annex 3 Acidified Synthetic Sea Water

G 8S-Annex 4 Salt SpraylSOz

G 85-Annex 5 Dilute Electrolyte Cyclic Fog/D'J'

B 368 Copper Accelerated Acetic Acid

o 1735 Water Fog Humidity

D 2247 100% Condensing Humidity

G 87 Moist SOz

that could be exposed to acid rai n. record ing capabili ties. Program mable logic con troller (PLC) syStems provide for increas­ingly sophisticated data acquisition, manipulation, and display (see Fig. 6). As a tesult, both test chambers with manual ly entered settings for automatic operation, and test cham­bers with increasingly sophisticated program mable controls for au to­matic temperature changes; varying levels of controlled hu midity; auto­matic changes fo r diffe rent envi ron­mental atmosphere cycles; and au to­matic data logging/accumulatio n, record keeping, and output wi th graphic data display, are available through a variety of suppliers. These com prehensive capabilities allow each laboratory to find accelerated corrosion tes ting equipment that

EQU IPMENT With con tinued developmen ts in accelerated corrosion tes ting, basic changes and im provements h ave been made to the equipment. T he basic requiremen ts for accelerated corrosion testing equipment are list­ed on Table II under the ASTM Bll? Standard. The adjustments required for each variation on th e accelerated corrosion-tes ting model are t hen listed next to the appropt iate ASTM standard. Typical applications are listed in Table III. Along wi th the basic opera ting requirements, improvements h ave been made in the materials used in th e conStruc­tion of the chambers, the hardware and software coo t roIs, and data

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meets their specific n eeds.

TEST PIECE PREPARATION! CLEANING In order for any of th e above acceler­ated corrosion tests to obtain com­parable results, all test pieces must start the test in a similar condition without contaminants p resen t on the surface of interest. The ASTM tes t standards discussed above indi­cate reference standards to be used for the preparation and cleaning of tes t pieces. Pa rticular a ttention must be paid to the wide variety of materials being cleaned an d recogni­tion of diffe ring specialized matetial and coating operations. ASTM test s tandards s tate that th e cleaning method is to be dependen t on the p roduct being cleaned and then refet to ASTM D 609 Standard Practice fo r Preparation of Cold­Rolled Steel Panels for Testing Paint, Varnish, Conversion Coatings and Related Coating Prod ucts where required. This stan dard describes five cleaning and preparation meth­ods. The preparation method s described are: Procedure A­Conversion Coating; Procedure B­Vapor D egreasin g; Procedure C­Solvent Brushing; Procedu re D ­Solvent Wiping; and Procedure E­Alkaline Cleaning. Procedure A (Conversion Coating) addresses the issue of ensuring that th e test piece be protected from rusting between the manufacturing process and the corrosion test. Procedure B (Vapor Degreasing) exposes the test pieces to solvent vapors within controlled conditions to remove contaminants. Th is procedure is currently discour­aged for environmen tal reasons. Procedures C (Solvent Brushing) and D (Solvent Wiping) both use Mineral Spiri t s as t he clean ing agent. Procedure E (Alkali ne Cleaning) desctibes methods for cleaning with an alkaline cleaning agent that is appropriate fo r the product. T he cleaning method and cleaning agent chosen for any test run should be as agreed upon between the purchaser and seller. ASTM D609 refers to the water break test as a method of checking test

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featu refocus pieces for contaminants. The proce­dure is simply to run a small amount of water over the surface of the tcst piece. If it is free of oils and contami­nants the water will run over it in a sheet and off in one fall with no breaks. Finally, ASTM 0609 discuss­es the handling of cleaned test pieces 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 assis t in the evaluation of corrosion test results are: ASTM 01654- Standard Test Method fo r Evaluation of Painted or Coated Specimen Subjected to

Co rrosive Environments; ASTM 0610-Standard Test Method for Evaluating Degree of Rusting on Painted Steel Surfaces; ASTM 0714-Standard Test Method for Evaluating Oegtee of Blistering of Paints; and ASTM B537-Standard Practice for Rating of Electroplated Panels Subjected to Atmospheric Exposure. Irrespective of the title wording, the text of ASTM B537 states that it is also used to evaluate panels subjected to laboratory accelerated corrosion tests. Other standards are also avail­able for evaluation of specific materi­als and finishes.

ASTM 0 1654 is used for the eval­uation of substrates, pretreatments, coating systems, or a combination thereof. This standard is also used when the coating has been scribed prior to exposure to the corrosive environment. ASTM 01654 recom­mends a straight-shank tungsten carbide tip or equivalent scribing tool to produce a cut through the coating to the substrate of the test piece. The scribe serves as a deliber­ate failu re of the coating system prior to corrosion testing. Once the test piece has been thro ugh the selected accelerated corrosion test it is then cleaned by rinsing with a gen­tle stream of water at 45°C (llO°F). Loose coating is then removed using compressed air to blow it off or a scraper to scrape it off. The "creep-

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back)) (perpendicular distance of failure from the scribe line) of the coating is measured at uniform intervals and the rating sys tem described in the standard appl ied.

ASTM 0610 provides a series of photographic reference standards that demonstrate degrees of rust on the surface of a test piece. The rest pieces are first gently cleaned then the amount of rust observed is reported from comparison to the photograph­ic standards and the additional use of established standardized terminology from the test standard. It is important to compare only rust and nOt surfaces stained by rust.

ASTM 0714 is used to evaluate coating adhesion by observing the amount of blistering that has occurred. The test pieces are first gen­tly cleaned then compa red to the series of included photographic refer­ences and the additional use of the appropriate "standardized)) terminol­ogy indicated in the test standard. They are graded first by the size of the blisters that appear on the surface and then the number of blisters that appear per unit area.

products and materials. T hese stan­dards have been developed, evolved and refined through cooperative efforts of experts and interested par­ties from: ASTM International, industry, government, and oth er trade o rganizations worldwide.

ABOUT THE AUTHOR Ray Singleton is the Chairperson for the ASTM Cabinet Corrosion Tests (commit­tee G.D1.0S. 03) and Vice-Chairperson for ASTM Corrosion of Metals (committee G.01) and Vice-Chairperson for ASTM Laboratory Corrosion Tests (committee G.0l-05) as well as the composer/ instruc­tor for the ASTM TPT (Technical Professional Training) Corrosion Testing course for the last 14 years. He has writ­ten various magazine articles and con­tributed to industry directories on this topic over the years. Singleton was also presented with the Francis L LaQue award by ASTM in 2010 for contribu­tions corrosion testing and the field of cor­rosion technology.

CONCLUSION Th e corrOSIOn tests discussed in this article are some of the most utilized tests of this type. They also demonstrate the diversity of the tests and some of the advance­ments in acceler­ated corrOSIOn testing and com­plexity of th e subject. They also provide a solid basis for evaluating the performance of a wide variety of www.metalfinishing.com/advertisers

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