troubleshooting, testing, & analysis

CHOOSING AN ACCELERATED

CORROSION TEST

FRANK ALTMAYER

SCIENTIFIC CONTROL LABORATORIES INC., CHICAGO; WWW.SCLWEB.COM

Accelerated corrosion tests are typically used to determine if a coating/substratecombination has been produced to yield a satisfactory service based on historicaldata from previous testing and field exposures of similar coating/substratecombinations. 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 yearsof service.Real-life exposures are complicated events that may involve several factorsincluding 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 theaccelerated test chosen simulates “real-life” corrosion mechanisms as much aspossible. The following guidelines were prepared to assist in choosing the bestaccelerated corrosion test for a given application.

CORROSION MECHANISMS

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

Electrochemical (Galvanic)

Electrochemical corrosion can be caused by dissimilar metals contacting anelectrolyte. This is the common “battery” effect. Detrimental galvanic corrosioneffects occur when the substrate is electrochemically more active than theprotective coating, or when the corrosive environment contains a metal that isless 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 galvanizedor electroplated zinc over steel, where the zinc, being electrochemicallymore active than steel, will corrode in preference to the steel when exposed toa corrosive environment (electrolyte). This protection is extended even if largescratches are present through the zinc and into the steel. Another example isduplex nickel. Nickel containing sulfur (~0.02%) from the addition of brighteningagents is electrochemically more active than nickel without sulfur. Atwo-layer system, consisting of semibright nickel (no sulfur) followed by brightnickel, yields a galvanic couple wherein the bright nickel corrodes in preferenceto 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 quantityof oxygen contacting the surface of the exposed specimen in the presence of anelectrolyte. Two dissimilar metals are not required. The area of the specimen thatis oxygen deficient becomes anodic to the area that contacts the larger quantityof 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 givenvarious names.Crevice corrosion: When specimens with complicated geometric shapes areallowed to contact corrosive liquids (water, salt solution), the sharply recessedareas (pores) in the surface of the specimen contact less oxygen than the remainingsurface due to differences in oxygen diffusion. The metal inside the poresbecomes anodic to the bulk. An example would be the crevice underneath thehead of a bolt or screw when these are tightened to a nut or other surface.Sandwich corrosion: The gap between joining flat surfaces becomes anodic tothe exterior surface. The corrosion products tend to push the joined surfacesapart. An example is the gap between riveted aluminum sheets comprising anaircraft wing or body panel.Poultice corrosion: 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 exampleis the dirt that accumulates on the underside and wheel wells of an automobile.Filiform corrosion: This type ofoxygen cell corrosion is peculiar to organiccoatings (paints, lacquers, etc.) that are subjected to chipping. The chippedarea contacts more oxygen than the metal covered by coating. As the coveredmetal that is some distance from the chip corrodes and precipitates hydroxidesunder the paint, a wormlike appearance in the coating develops as the coatingis lifted off the base metal. The “worm” traces take straight lines under constanttemperature and twist under variant temperature.

Chemical Attack

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 subjectingthe test specimen to varying concentrations of acids, acid-forming gases,or chemical solutions.Acid attack, chemical attack, and electrochemical techniques can also be usedto determine the porosity of a coating, which can sometimes be related to servicelife. Examples of such techniques are the ferroxyl test, electrographic printing,and the copper sulfate test (on hard chrome over steel).

TEST METHODS

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

Salt Spray (ASTM B 117)

This is the most widely specified corrosion test. It has broad applicability. Thesalt solution that is utilized closely simulates the corrosive effects of outdoorexposure on automotive hardware (except some decorative nickel-chromiumapplications; 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; inclinationof test specimen; and utilization of a fine-fog mist.

100% Relative Humidity (ASTM D 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 thesurface of the specimen.Many variations of this test are employed to more accurately reflect serviceconditions. These include varying humidity levels from 60 to 95% relativehumidity; cycling humidity with periodic dips into corrosive liquids (ASTM G60); cyclinghumidity 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, acceleratedby high humidity level, elevated temperature, and inclination of test specimen.

ASTM B 287 (Acetic Acid-Salt Spray)

This test is intended for coating systems that provide excellent corrosion-protectionresults in long-term salt spray (B 117) exposure. This test is approximatelytwice as severe as the salt fog (B 117) test, although this may vary significantlywith each application.The corrosion mechanism employed is oxygen concentration cell acceleratedby operation at lowered pH (3.1-3.3 vs. 6.5-7.2 for the salt fog test); use of anelectrolyte with a chloride content of 5% weight (more has been shown to beunnecessary); elevated temperature; inclination of test specimen; and utilizationof a fine-fog mist.

Copper Accelerated Salt Spray (CASS) (ASTM B 368)

This test was developed for use on copper-nickel-chromium coatings over ferrousand nonferrous substrates.The oxygen concentration cell corrosion mechanism is accelerated by operationat lowered pH (3.1-3.3 vs. 6.5-7.2 for the salt fog test); use of an electrolytewith 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.

Corrodkote (ASTM B 380)

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

FACT (ASTM B 538)

This test is limited to anodized aluminum coatings and is basically a porositytest used to obtain rapid corrosion protection results based on the porositylevel found. Substantial service background is needed to correlate test data withservice 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. Apotential is applied between a platinum anode and the test specimen (cathode).As hydrogen is discharged from the cathodic sites (pores), alkalinity is developed,attacking the coating and decreasing the cell voltage. The decreasing cellvoltageis integrated over time, yielding a comparative result.

Weatherometer (ASTM G 23)

This test is utilized to evaluate the performance of paint and lacquer systemsunder simulated outdoor exposure. The test yields data on the resistance of thecoating system to condensation effects (rain) and the stability of the pigment inthe paint (colorfastness) when exposed to sunlight. Intense ultraviolet radiationfrom twin carbon arc lamps and variant humidity levels (cycling from approximately70 to 100% relative humidity) provide long-range test results in a shorttime frame (100-2,000 hr).

Lactic Acid

Lacquered brass- and copper-based alloys are tested for porosity and resistanceto tarnishing by everyday handling (perspiration), using this test. Although

not ASTM standardized, the procedure is gaining industrial acceptance. Theprincipal 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 120OF.

3. The item is suspended in the air above 100 ml of 30% acetic acid in an airtightchamber (desiccator). An acceptable substitute is the vapor producedby 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 appearanceof green (nickel) corrosion, loss of adherence of the organic coating, ortarnishing of the brass or copper plate.

Sulfur Dioxide/Kesternich (ASTM B 605)

A few coatings are so stable that normal corrosion-resistance testing resultsin 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 specificationDIN 50018. These procedures normally are limited to tin-nickel and similarinert coatings, or to coatings where a quick detection of a fault or surfaceimperfection is required.Both procedures use sulfur dioxide gas at elevated temperatures and humiditylevels, forming sulfurous acid on the surface of the test specimen.

Triple Spot (ASTM A 309)

Galvanized steel is protected from corrosion by the sheer thickness of the zinccoating. An estimate of the corrosion resistance can, therefore, be obtained bymeasuring the average thickness of the zinc coating on the steel. The thicknessis measured using the “weight-area method” and an inhibited acid for stripping.This indirect method for measuring corrosion resistance of galvanically protectedsteel is limited to thick coatings (galvanize). Relatively thin coatings, suchas electroplate, are not as easily correlated to corrosion performance, becauselocal variation in thickness will yield service failures that are not “predicted” bythis method.

Electrographic and Chemical Tests for Porosity

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