Corrosion Test Types for Plated Parts and Assemblies
Rust can wreak havoc on metal, so corrosion testing is a common practice for most companies that use metal heavily in their manufacturing processes.
Over the years, a number of testing protocols have been developed, including cyclic corrosion testing, salt spray testing, humidity testing, Kesternich chamber testing and specialized tests designed to simulate the specific conditions of a given coating (e.g. chrome, nickel, copper, etc.).
Industries using electroplating
Electroplating is a commonly used tool in various industrial applications. Below are examples of some of these industries.
Automotive Plating
Most people who own a car have experienced rust, and automakers are always looking for new and better ways to protect their products. Popular among automakers today is a zinc-nickel plating solution that inhibits rust on exterior and interior parts, including catalytic converters and plastic parts.
Gold plating is frequently used in the electronics industry because it provides the level of electrical conductivity and corrosion resistance required for electronic products, including semiconductors and connectors. Copper plating is less expensive than gold plating for circuit boards and other components.
Palladium-nickel and palladium-cobalt alloys provide effective protection for cell phones, PC batteries and other electronic devices.
Plating for medical equipment
Among other benefits, electroplating is used to improve the biocompatibility and corrosion resistance of medical devices. In SPC, precious metals such as gold, silver, and platinum are used for electroplating of medical devices.
Tin is commonly used to coat tungsten and lead components in MRI devices. Titanium components used in joint replacements are also coated.
Aerospace Metal Plating
Titanium's high strength-to-weight ratio makes it a commonly used fabrication material in the aerospace industry. Nickel and copper plating provide exceptional corrosion and wear protection. Copper plating is also available for heat treating applications.
Testing methods and misunderstandings of electroplating parts
A common misconception associated with phosphate coatings is that they can be the ultimate protection against corrosion. They are effective in enhancing the performance of organic coatings such as paints and oils. They also improve the rust protection of waxes and drawing compounds. That's not to say that phosphates don't offer any corrosion protection at all - just not to the level of protection offered by other coatings.
Coating failure can take up to six months to occur, by which time it is too late to take protective measures. For this reason, accelerated test methods can be used to assess the relative corrosion effects over days or weeks.
But beware - the accelerated corrosion test is only an indicator of the onset of corrosion and may not be reliable under certain conditions. Why? Every corrosion test involves many variables, any one of which can affect the results if the test is not controlled.
For example, it is not uncommon to see wildly different results on parts of the same coating system that have been coated with different paints. More specifically, using the humidity test as an example, one paint might go bad in 24 hours while another might last 2000 hours. And that's without even accounting for variables like differences in paint application methods.
Therefore, it is important to control the production painting process. Here are five keys to determining the performance level of a painted part:
the time it takes to bake the paint
Paint temperature setting
The viscosity of the paint being baked
Paint thickness applied to metal
Post-bake time to allow paint to cure
Temperature and baking time are especially important. Curing paint consists of chemical reactions that result in a change in the form of the paint. If the proper temperature and bake time are not applied, the desired end result - a high quality paint finish - will be compromised. Likewise, viscosity changes with changes in atmospheric pressure and humidity, and differences in viscosity can affect the spreadability and coverage quality of a paint—thicker coats offer better protection than thinner coats.
Cure time is also important. This is the time it takes for the paint to polymerize and produce the final product.
humidity test
Humidity testing is one of the reliable tests for measuring corrosion resistance because it is related to field performance and has no controllable variables. However, the results are relative because there are too many variables to control.
A typical humidity test cabinet is airtight and moisture-proof, and has a top that slopes to one side. The sloped design directs condensation collected on the top to run down the sides of the cabinet instead of raining on the components being tested.
In the humidity test, the temperature was set at 100 degrees Fahrenheit, with an acceptable change of 2 degrees plus or minus, and 100% relative humidity. Moisture can enter the cabinet through misting. It can also be added by circulating a small fan in a cabinet with a tank filled with distilled (or deionized) water. These are known as recirculating humidity cabinets.
More common, however, are cabinets that contain a water reservoir but do not circulate moisture. These condensation cabinets cause moisture to break down earlier because they collect more water than circulation cabinets. In most cases, the panels to be tested are placed on shelves inside the cabinet. The panels are placed at an angle of 15° to 30° from the vertical, so the panels receive an equal amount of condensation—at least on one side. Then observe whether the upward side fails.
Off-vertical angles are a critical requirement that is often overlooked, leading to irreproducible results. Another variable that should be noted is condensation. It is not equally present in all areas of the cabinet. The amount of condensation varies with the construction of each unit.
Salt spray test
There are so many variables associated with salt spray testing that the results are not even as reliable as humidity testing. The overall appearance is similar, except that the salt spray cabinet is larger than the humidity cabinet. For salt spray cabinets, there is a tank filled with water around the perimeter of the top lip, and the cover prevents the salt spray from escaping. It works like this:
Water is added to the saturation tower, then conveyed by air and blown out through nozzles located inside the tank. The nozzle is mounted on the siphon valve which draws the saline solution from the reservoir.
The salt solution then mixes with the incoming water-saturated air to create a salt spray in the cabinet. The mist is collected in a flask and tested according to the desired concentration.
Initially, a test using a 20% saline solution was considered adequate, but a 5% solution is a more stringent test and has become the industry standard test for metal finishing companies. This is because the chemical activity of salt increases with the amount of water.
When performing a salt spray test, make sure the salt is iodine-free and produces the proper pH when mixed with distilled or deionized water. Some time ago, the ASTM specification was revised to include automatic water level control and to relocate the atomizing nozzles to the tower located in the center of the salt spray cabinet to improve salt spray distribution. Previously, the nozzles were located near the bottom and at one end of the cabinet.
The saline solution in the reservoir should be replaced every 48 hours, and the cabinet should be operated continuously at 95 degrees plus or minus 2 degrees. Clearly, even before considering the variables associated with preparing and viewing the panels, there are many factors that can affect the performance of the cabinet: water level, air pressure, pH, location of the misting nozzles and their cleanliness, frequency of saline solution refreshment, used Type of salt, concentration of salt solution and cabinet temperature.
Therefore, the cycle test correlates better with outdoor conditions than the conventional salt spray test. They are ideal for evaluating galvanic and crevice corrosion, etc.
Cyclic corrosion testing is a more realistic method of performing salt spray testing than traditional steady state testing. Because actual atmospheric exposure often includes both wet and dry conditions, it makes sense to align accelerated laboratory testing with these conditions. The study shows that the corrosion rate obtained by the cyclic corrosion test is consistent with the outdoor conditions.
Cyclic corrosion testing can be used to evaluate various corrosion types, but is particularly effective for galvanic and crevice corrosion.
Immersion test
A soak test involves submerging a painted panel in water for a period of time, looking for early signs of blistering, similar to the humidity test described earlier. A common method is to immerse the panels in distilled or deionized water kept at 95 to 100 degrees Fahrenheit. Some changes include:
Use tap water, although distilled and deionized water have better penetration, resulting in a harsher test environment.
Submerge only half of the panel and watch how the panel meets the water.
Change the water temperature from 70 degrees to about 212 degrees.
Soak the panels in water or solvents mixed with various chemicals.
Outdoor Exposure Test
This is representative testing, but may take up to two years to complete. This is because the paint coating can fully cure - under normal conditions of variation in temperature, humidity and precipitation.