Engineering Testing
Automotive Corrosion Testing of Appliances

by Harold D. Hilton, Atlas Material Testing Technology LLC, Cynthia Meade, National Exposure Testing, and Kevin A. Smith, Auto Technology Company

To determine the best materials for major home appliances, some manufacturers routinely use a corrosion test developed in the U.S. automotive industry. They test individual components, partial assemblies, randomly selected production pieces, finished goods, and competitors’ models.

How does an automotive test apply to an appliance? Because appliances, which are made, shipped, warehoused, and installed around the world, are beset by the same problems caused by weathering and industrial pollution that result in vehicle corrosion. The corrosion test used is the GM9540P Accelerated Corrosion Test, an advanced cyclic method developed by General Motors (available from www.global.ihs.com and other online sources).

Until the auto industry got involved, however, there was no “universal lab corrosion test.” That is due to the many independently variable conditions that create synergistic reactions, the sum of which result in material degradation, including corrosion.

To be useful, lab corrosion tests must accurately predict the probability of corrosion in the real world, thereby allowing more rapid selection of materials, protective/decorative coatings, and packaging. It is also important that a lab test creates the same corrosion by-products and surface defects that occur in end-use. A test method, technique, or equipment that does not provide a close match to “end use” will yield false results.

GM9540P is useful because it replicates many of the worst-case environments. Manufacturers merely program the realistic corrosion test into an advanced lab cabinet and wait for the results. Since those results are more believable, materials are quickly screened, which yields a quicker time to market.

	Figure 1. Advanced corrosion test cabinet with installed options for GM9540P Operation. The cabinet features multiple, directional spray nozzles with independent on/off valves and automatic cover lifters achieve full ambient conditions “in cycle.”

Corrosion Testing

When corrosion testing was first codified, it typically involved placing samples into an unchanging environment of saturated (95 to 100 percent) relative humidity (RH), with a condensing fog made from a 5- to 20-percent salt solution at a temperature of 35°C to 40ºC. The sample was exposed to the conditions for a specified length of time, such as 500 or 1,000 hours, and occasionally up to 2,500 hours.

A variation was 100-percent RH from water without salt or other chemicals, also known as a Humidity Test. In the early days, the intent was to determine the probability of failure due to corrosion, to rank the rate-of-spread of corrosion, or simply categorize paint or coating as passing or failing. However, since the real world does not have unchanging conditions of saturated humidity with condensing fog that has 5-percent concentrated salt (let alone 20 percent) at 35ºC, no correlation to service life could be made, although many labs attempted to do so.

To predict service life for stresses that cause corrosion and to find the most economical means to achieve minimally acceptable product performance, nearly every condition that a product will encounter in its end-use environment should be replicated by the test method, not just salt. Equally critical are the extremes of conditions—high temperature versus low, wet versus dry—and the transition time between the extremes.

That’s where the automotive industry came in. It tested individual components, assemblies, and entire vehicles. It conducted parking lot surveys. Furthermore, they tested a wide variety of materials and coatings and subjected expensive vehicles to enormous ovens, freezers, artificial sunlight chambers, and the unrelenting extremes of weather. Tests that did not match real-world results were discarded. Auto manufacturers, steel suppliers, coatings specialists, and industry associations spent years performing seemingly endless permutations of test conditions.

There are two main types of lab corrosion tests that have been developed throughout the past 70 years. The popularity of cyclic testing comes from the improved reliability of results that can be correlated with actual use. Cyclic refers to the repeated changes in test conditions, ideally to replicate the changing environments where the product will be used. Rapid changes in the test condition further serve to accelerate material degradation, which can give months or even years of information in an accelerated period of time.

The time savings is critical—when coatings and other protective systems are developed and tested under accelerated conditions, design engineers and others who specify materials achieve a competitive advantage, bringing superior-performing products to market more quickly.

Table 1. Operating Cycles of Basic and Advanced Cyclic Cabinets

GM9540P

An advanced corrosion cabinet can automatically perform all four parts of GM9540P:

• High Humidity with Water Fog
• Direct Spray with Complex Electrolyte
• Dry-off at 60ºC, less than 30-percent RH
• Lab Ambient at 25ºC, 40- to 50-percent RH

It is an advantage to the lab in both time and effort when its corrosion cabinet can automatically perform Direct Spray and Lab Ambient conditions, and also automatically switch between all parts, which include:

Spray: Look for multiple, directionally adjustable nozzles for complete coverage. Each nozzle should have an independent shut-off valve. Nozzles that are not needed can be turned off to save electrolyte (see Figure 1).

Complex Electrolyte: Sodium Chloride 0.9 percent, Calcium Chloride 0.1 percent, Sodium Bicarbonate 0.25 percent, with a pH of 6 to 9. Note: to prevent an undesirable precipitate, either the calcium chloride or the sodium bicarbonate should be separately added to water and then mixed with the other solution.

Ambient Conditions: A cabinet should have automatic cover lifters to ensure absolute ambient temperature and humidity conditions. If the cabinet manufacturer blows lab air into their cabinet, only the temperature is returned to an ambient level; the RH may stay near 100 percent. The combination of high RH and the “wind tunnel” effect may cause abnormal corrosion rates, resulting in false “fails.”

Table 2. Types of Corrosion Test Methods

Details of Cyclic Corrosion Testing

Historically, the logistics of cyclic testing was labor intensive: a lab technician placed samples in a testing cabinet that had a specific condition such as corrosive fog. When the prescribed time elapsed, the technician moved the samples to the next condition that simulated a different environmental condition. That second condition could have been created in an oven (dry, rapid heating), a lab bench (drip dry and ambient temperature), a sink (direct spray, immersion), and so on.

This type of staff-dependent sample movement is no longer necessary. Modern cyclic cabinets, however, bring multiple environments to the sample. This provides repeatable test conditions, reduced operator error, time savings, improved reproducibility between tests and between labs, and superior accuracy of results. Depending on installed options, an advanced cabinet can create up to 14 environmental cycles in any order, replicating the variety of end-use conditions (see Table 1).

An advanced cabinet, sometimes with installed optional equipment, can also reduce the need for multiple cabinets to perform different tasks, resulting in significant savings of vital resources: budget, floor space, and operator time. Further, an advanced cabinet can perform virtually all basic cyclic and traditional corrosion tests, and provide for future testing needs.

Each manufacturer offers options (usually factory-installed) or accessories to expand testing capabilities or to increase operator efficiency. Typical options for an advanced lab cabinet include:

Adjustable Humidity—Automatically controls relative humidity at the set point; used to prolong the transition time between wet and dry.

Direct Spray—Provides a means to directly spray samples with a steady “rain” of the specified solution.

Immersion System—Automatically transfers solution into the exposure zone where samples are flooded/immersed.

Automatic Retracting RH Probe—Upon signal from the cabinet controls, inserts and retracts the sensitive RH probe (thin-film dielectric), protecting it from corrosive atmospheres.

Air Operated Cover Lifters—Automatically lifts the cover when a test method calls for ambient conditions—an advantage if this condition is required when an operator is not present, such as evenings, weekends, and holidays.

Software, PC-Based—The most powerful and versatile of all controlling and recording options—Pentium™ computer controls and records all functions; set points, readings, multi-pen trending, input/output status for components, alarms; the operator can monitor and change conditions inside the cabinet; can allow remote troubleshooting thereby reducing service costs.

Mixing system for Salt Solution Reservoir—When activated by the operator, it injects air into the solution reservoir, which thoroughly mixes the soluble chemicals with the DI water.

External Condensate Collection Package—Includes all necessary equipment to check the condensation rate without opening the cover, a convenience.

Gas Injection—Allows certain gasses, used to simulate industrial pollution, to be injected into the condensing fog.

Very High Temperature operation—Allows tests to be conducted at elevated temperatures in the Dry Cycle, up to 90ºC/194ºF.

Very Low Temperature operation—Allows tests to be conducted at greatly reduced temperatures in the Dry Cycle, down to -30ºC/-22ºF.

Extra Solution Tank—A second reservoir to provide for longer unattended testing, or to supply a second electrolyte to become fog or used as direct spray.

Next-Generation Tests

SAE, ASTM, ISO, and technical committees in many standards organizations have developed corrosion tests to replicate the effects of actual outdoor exposure (see Table 2). As good as GM9540P is at testing automobiles and major appliances, there are new tests in place and on the horizon. The most important of those, from the Society of Automotive Engineers (SAE) and the Auto/Steel Partnership, was created following nearly 20 years of research by a diverse group of corrosion testing specialists, engineers, and statisticians. In 1998, after widespread peer review, they came up with an advanced cyclic test codified as SAE J2334.

Since this test was first published, it has been refined and re-published. The SAE J2334 test has been shown to be a reliable accelerated laboratory corrosion test and is in widespread use throughout the automotive industry.

Summary

Comparison of corrosion cabinets and test methods for major home appliances is not difficult when features and specifications are compared. The number, shape, and size of appliances or components to be tested indicate the necessary cabinet size in terms of exposure volume or testing plane. When cabinet size is determined, narrow the search to those with the proper manner of making and distributing fog. Then, find the cabinets with the required operating cycles for current and future needs. Finally, determine the best combination of floor space usage, length of unattended operation, available options and accessories, appearance, technical service after installation, and manufacturer reputation to determine overall value and cost.