Choosing an accelerated corrosion test

control analysis, and testing CHOOSING AN ACCELERATED CORROSION TEST FRANK ALTMAYER SCIENTIFICCONTROL LABORATORIESINC.,CHICAGO;www.sc[web.com

Accelerated corrosion tests are typically used to determine if a coating/substrate combination has been produced to yield a satisfactory service based on historical data from previous testing and field exposures of similar coating/substrate combinations. The intent is to find out, in a relatively short amount of time, what the appearance or performance of the product will be after several years of service. Real-life exposures are complicated events that may involve several factors including geometric configuration, porosity/adherence of corrosion product, soiling, abrasion, frequency of cleaning, cleaning procedures, cleaning chemicals, sun exposure, and temperature variations. Because of this, it is critical that the accelerated test chosen simulates "real-life" corrosion mechanisms as much as possible. The following guidelines were prepared to assist in choosing the best accelerated corrosion test for a given application.

CORROSION MECHANISMS Coated metallic products are subjected to two basic corrosion mechanisms during their service life: (1) electrochemical (galvanic) and (2) chemical attack.

Electrochemical (Galvanic) Electrochemical corrosion can be caused by dissimilar metals contacting an electrolyte. This is the common "battery" effect. Detrimental galvanic corrosion effects occur when the substrate is electrochemically more active than the protective coating, or when the corrosive environment contains a metal that is less active than the coating and substrate. The electrolyte (water, salt solution, acid, etc.) must be in contact with both metals for this mechanism to occur. Examples of the beneficial use of this corrosion mechanism include galvanized or electroplated zinc over steel, where the zinc, being electrochemically more active than steel, will corrode in preference to the steel when exposed to a corrosive environment (electrolyte). This protection is extended even if large scratches are present through the zinc and into the steel. Another example is duplex nickel. Nickel containing sulfur (~0.02%) from the addition of brightening agents is electrochemically more active than nickel without sulfur. A two-layer system, consisting of semibright nickel (no sulfur) followed by bright nickel, yields a galvanic couple wherein the bright nickel corrodes in preference to the semibright, providing galvanic protection to the second nickel layer, thereby delaying corrosion of the basis metal (and failure). The electrochemical mechanism can also involve a difference in the quantity of oxygen contacting the surface of the exposed specimen in the presence of an electrolyte. Two dissimilar metals are not required. The area of the specimen that is oxygen deficient becomes anodic to the area that contacts the larger quantity of oxygen. The anodic area dissolves into the electrolyte, leaving a corrosion pit. Hydroxides (alkali) are deposited at the cathodic (oxygen-rich) area. The physical variations of the oxygen concentration cell mechanism are given various names. 504

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Crevice corrosion: When specimens with complicated geometric shapes are allowed to contact corrosive liquids (water, salt solution), the sharply recessed areas (pores) in the surface of the specimen contact less oxygen than the remaining surface due to differences in oxygen diffusion. The metal inside the pores becomes anodic to the bulk. An example would be the crevice underneath the head of a bolt or screw when these are tightened to a nut or other surface. Sandwichcorrosion:The gap between joining flat surfaces becomes anodic to the exterior surface. The corrosion products tend to push the joined surfaces apart. An example is the gap between riveted aluminum sheets comprising an aircraft wing or body panel. Poulticecorrosion:Accumulated soil on a corrodible surface acts as a "poultice," holding thousands of pockets of electrolyte (water, salt, etc.) onto the surface. Differences in oxygen concentration then act to corrode the surface. An example is the dirt that accumulates on the underside and wheel wells of an automobile. Filiform corrosion: This type of oxygen cell corrosion is peculiar to organic coatings (paints, lacquers, etc.) that are subjected to chipping. The chipped area contacts more oxygen than the metal covered by coating. As the covered metal that is some distance from the chip corrodes and precipitates hydroxides under the paint, a wormlike appearance in the coating develops as the coating is lifted off the base metal. The "worm" traces take straight lines under constant temperature and twist under variant temperature.

ChemicalAttack The chemical nature of acids and certain chemicals is that they attack (dissolve) metals. The performance of a coating in resisting attack is determined by subjecting the test specimen to varying concentrations of acids, acid-forming gases, or chemical solutions. Acid attack, chemical attack, and electrochemical techniques can also be used to determine the porosity of a coating, which can sometimes be related to service life. Examples of such techniques are the ferroxyl test, electrographic printing, and the copper sulfate test (on hard chrome over steel). TEST M E T H O D S

The following are commonly performed accelerated corrosion and porosity tests. Table I summarizes applicability and denotes the most common test used on a given coating/substrate combination.

Salt SWay (ASTM B 117) This is the most widely specified corrosion test. It has broad applicability. The salt solution that is utilized closely simulates the corrosive effects of outdoor exposure on automotive hardware (except some decorative nickel-chromium applications; see CASS and Corrodkote in Table I). Test results normally are obtained in a few hours for lesser protective systems (phosphate, oil, hard chromium plating) and it may take many days (40-80) for superior systems such as tin-nickel plating, heavy galvanize, and galvanize/paint combinations. The corrosion mechanisms employed are oxygen concentration cell and galvanic effects, accelerated by use of an electrolyte with a chloride content of 5% weight (more has been shown to be unnecessary); elevated temperature; inclination of test specimen; and utilization of a fine-fog mist. 506

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100% RelativeHumidity (ASTMD 2247) This test has wide applicability for protective coatings that are exposed indoors, in sheltered areas, or in areas where water (condensation) can accumulate on the surface of the specimen. Many variations of this test are employed to more accurately reflect service conditions. These include varying humidity levels from 60 to 95% relative humidity; cycling humidity with periodic dips into corrosive liquids (ASTM G 60); cycling humidity with drying cycles for coatings on wood (ASTM D 3459); and combining humidity with severe temperature fluctuation (ASTM D 2246). The corrosion mechanism employed is oxygen concentration cell, accelerated by high humidity level, elevated temperature, and inclination of test specimen.

ASTMB 287 (AceticAcid-SaltSpray) This test is intended for coating systems that provide excellent corrosion-protection results in long-term salt spray (B 117) exposure. This test is approximately twice as severe as the salt fog (B 117) test, although this may vary significantly with each application. The corrosion mechanism employed is oxygen concentration cell accelerated by operation at lowered pH (3.1-3.3 vs. 6.5-7.2 for the salt fog test); use of an electrolyte with a chloride content of 5% weight (more has been shown to be unnecessary); elevated temperature; inclination of test specimen; and utilization of a finefog mist.

CopperAcceleratedSalt Spray (CASS)(ASTMB 368) This test was developed for use on copper-nickel-chromium coatings over ferrous and nonferrous substrates. The oxygen concentration cell corrosion mechanism is accelerated by operation at lowered pH (3.1-3.3 vs. 6.5-7.2 for the salt fog test); use of an electrolyte with a chloride content of 5% weight (more has been shown to be unnecessary); elevated temperature; inclination of test specimen; utilization of a fine-fog mist; and the addition of cupric chloride to provide galvanic effects.

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This test was also developed for specific use on copper-nickel-chromium coatings on ferrous and nonferrous substrates. The corrosion mechanisms employed are oxygen concentration cell and galvanic effects produced by cupric and ferric ions, plus complicated chemical effects produced by the nitrate, chloride, and ammonium ions. The test utilizes a kaolin paste that holds the corrosive ions to the surface of the test in a "poultice" fashion, similar to accumulated dirt and scale on exterior automotive parts.

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B S38)

This test is limited to anodized aluminum coatings and is basically a porosity test used to obtain rapid corrosion protection results based on the porosity level found. Substantial service background is needed to correlate test data with service life for any specific application. The test consists of an electrolytic cell with salt spray (ASTM B 117) or CASS (ASTM B 368) solution as an electrolyte. The cell is placed onto the test specimen with a gasket to prevent leakage. A potential is applied between a platinum anode and the test specimen (cathode). As hydrogen is discharged from the cathodic sites (pores), alkalinity is developed, 508

attacking the coating and decreasing the cell voltage. The decreasing cell voltage is integrated over time, yielding a comparative result.

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(ASrM C 23)

This test is utilized to evaluate the performance of paint and lacquer systems under simulated outdoor exposure. The test yields data on the resistance of the coating system to condensation effects (rain) and the stability of the pigment in the paint (colorfasmess) when exposed to sunlight. Intense ultraviolet radiation from twin carbon arc lamps and variant humidity levels (cycling from approximately 70 to 100% relative humidity) provide long-range test results in a short time frame (100-2,000 hr).

Lact/cAc/d Lacquered brass- and copper-based alloys are tested for porosity and resistance to tarnishing by everyday handling (perspiration), using this test. Although not ASTM standardized, the procedure is gaining industrial acceptance. The principal mechanism is chemical attack. The procedure is as follows: 1. The item is dipped in a room-temperature solution of lactic acid (85%) saturated with sodium chloride. 2. The item is air dried for 2 hr in a convection oven at 120°F. 3. The item is suspended in the air above 100 ml of 30% acetic acid in an airtight chamber (desiccator). An acceptable substitute is the vapor produced by 100 ml of a 50% solution of acetic acid in water over a small, open dish. A typical successful exposure is 20 hr above the acetic acid without the appearance of green (nickel) corrosion, loss of adherence of the organic coating, or tarnishing of the brass or copper plate.

Sulfur Diox~e/Xestern~b (ASTMB 60S) A few coatings are so stable that normal corrosion-resistance testing results in unwieldy exposure hours. To yield results in a realistic amount of time, an acidic attack mechanism is used. Two of these tests are the sulfur dioxide (ASTM B 605) and the Kesternich (Volkswagen) test, Volkswagen specification DIN 50018. These procedures normally are limited to tin-nickel and similar inert coatings, or to coatings where a quick detection o f a fault or surface imperfection is required. Both procedures use sulfur dioxide gas at elevated temperatures and humidity levels, forming sulfurous acid on the surface of the test specimen.

Triple Spot (ASTMA 309) Galvanized steel is protected from corrosion by the sheer thickness of the zinc coating. An estimate of the corrosion resistance can, therefore, be obtained by measuring the average thickness of the zinc coating on the steel. The thickness is measured using the "weight-area method" and an inhibited acid for stripping. This indirect method for measuring corrosion resistance of galvanically protected steel is limited to thick coatings (galvanize). Relatively thin coatings, such as electroplate, are not as easily correlated to corrosion performance, because local variation in thickness will yield service failures that are not "predicted" by this method. 509

Electrograpbic and ChemicalTestsfor Porosity Pores and cracks in chromium over nickel or nickel over steel can be detected using absorbent paper soaked in chemicals that react with the substrate, such as dimethylglyoxime for nickel substrates and potassium ferricyanide for steel substrates (commonly called the ferroxyl test). The test specimen is covered by the soaked paper and, with pressure from a stainless steel cathode, a small current is passed (the test specimen is anodic). The chemicals react with the substrate at cracks and pores, thereby forming colored traces in the paper. Correction to actual service life is difficult. Pores in hard chromium deposits over steel can be detected by applying an acidified copper sulfate solution (50 g/L with a pH < 2) for approximately 30 seconds. Galvanically deposited copper will appear at areas with pores present.

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