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
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.
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.”
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
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
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
and others who specify materials achieve a competitive advantage, bringing
superior-performing products to market more quickly.
Conditions in Basic Cyclic Cabinets
Conditions in Advanced Cyclic Cabinets
Salt or chemical (electrolyte) fog, saturated RH
in preceding column, plus:
Water fog, saturated RH
Controllable humidity, ambient to saturated RH
Dwell, a period of rest where no action is taken
Second Electrolyte, for fog or direct spray
5. Non-condensing humidity (i.e., “moist heat”)
Very low temperature and RH, automatic
6. Direct spray (impingement), salt or other chemical
Ambient temperature and RH, automatic
High temperature, up to 70°C/160°F
Very high temperature, up to 90°C/195°F
Gas injection, including Nox, SO2, CO2, etc.
Table 1. Operating Cycles of Basic and Advanced Cyclic Cabinets
An advanced corrosion cabinet can automatically perform all four parts
- 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
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
RH and the “wind tunnel” effect may cause abnormal corrosion
rates, resulting in false “fails.”
Salt Fog and Humidity
NBR 8094, 8095
ASTM B117, B368, D1735, D2247
ISO 7253, 9227
G85 (Annexes 1,2,5), D5894
ISO 11977 part 1
IEC 60068-2-52 part 2
Prohesion™, Direct Spray
G85 (Annexes 3,4)
Nissan CCT I, II, III, IV
Very Low/High Temperature
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,
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
Mixing system for Salt Solution Reservoir—When activated by the
operator, it injects air into the solution reservoir, which thoroughly
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
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.
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.