(Click here to view article in digital edition)
This can result in malfunctions or in the total failure of assemblies or electronics within the expected lifetime.
These air pollutants are found in varying degrees in a range of industries. Rust, one of the results of these air pollutants, is typically only a visual flaw on the housings and screws. Components making electrical connections such as plug connectors, relay contacts and soldered joints are particularly critical. Rust formation in these areas often causes a loss of function or even total failure.
Corrosion tests have been standard practice in the telecommunications and automotive industries for many years. The corrosion test is also becoming an important quality feature for applications within the processing industries, in road construction and in wind power applications.
The aim of a corrosion test is to prove that corrosive effects do not occur or only occur below agreed limits. If the design of a power supply is done properly, the device will provide a reliable service for a long period of time. There is no need for early replacement after only a few years.
A realistic assessment of the corrosive behaviour requires that the corrosion tests are performed very closely to the actual operating conditions of the equipment. In addition to setting the corrosive gas concentration, it is important for the simulation to operate the test sample, as it would be used in practice. In continuous operation mode, the tendency towards corrosion is less pronounced. The constant heating reduces the moisture in the immediate vicinity of the corrosion-sensitive materials. Sulphur dioxide, however, requires moisture to react, and this is not typically present during continuous operation.
More meaningful results are therefore provided by a cyclical operation mode in which the test sample is switched on and off at regular intervals. The air flow arising from these cold/hot temperature fluctuations creates moisture, promotes reaction with sulphur dioxide and increases the tendency for corrosion. This cold/hot effect is particularly important in the case of power supplies, as they usually cause a hot spot in the control cabinet.
The test time is shortened by increasing the concentration of the corrosive gases. This means that the operating time of over 10 years is simulated within a test duration of just 21 days.
White rust and red rust
To be able to assess the corrosion, knowledge of the different rust types is important. White rust forms on zinc surfaces such as galvanised steel as a thin zinc oxide layer evenly distributed across the surface. Its appearance is a whitish sheen. The layer adheres to the surface and does not drop off. If wiped with a finger, it fades a little. White rust is generally non-critical and is merely a visual flaw.
White rust can be hazardous in combination with salt. Salt or salt mist combines with zinc oxide and efflorescence forms that can develop into crystal formation. Pieces of these efflorescence or crystals can break off and cause short circuits in the electronics. Therefore, Zinc-plated surfaces should not be generally used in the offshore sector or for road construction equipment where salt can be expected. In addition to the gas corrosion tests for these applications, there are special salt mist tests available.
Red rust is the classic form of corrosion on ferrous or steel materials when corrosion protection is insufficient. Red rust should be avoided, as it spreads and efflorescence can easily break off causing short circuits because the rust is conductive. The extent to which red rust occurs on zinc-plated steel sheets, cutting edges or screws depends on the quality and homogeneity of the zinc-plating or corrosion protection.
Environmental simulation testing according to IEC 60068-2-60 (method 4)
On an international level, flowing mixed gas environmental simulation tests for general applications are covered in IEC 60068-2-60. This standard is actively maintained and continuously updated. This is also necessary as air pollutants are changing over time. For example, method 4 of this standard was defined recently and represents the typical pollutant loads of the specified fields of applications from the beginning of this article.
Method 4 conforming to IEC 60068-2-60 is a 4-component gas corrosion test with flowing mixed gas. The test is carried out at exactly +25°C and a relative humidity of 75%.
Environmental simulation testing according to ISA-S71.04 (G3)
ISA S71.04 is an American standard covering corrosive gases for outdoor applications. Severity level G3 (according to ISA-S71.04) is better known internationally than the aforementioned IEC standard, and has therefore established itself as a reference in many documents. In practice, the ISA standard is out of date and no longer covers the current requirements. The last revision was made in 1988, and the ISA workgroup was dissolved after this. The high proportion of NOX in the corrosive gas composition reacts with the SO2 and causes the aggressive SO2 to be relatively non-reactive and prevents it displaying its normal characteristics. In practice, this means that the ISA test puts considerably less stress on the test specimen than the test conforming to IEC 60068-2-60 method 4.
A further deficiency in the G3 test conforming to ISA is the lack of definition of nitrogen oxide NOX. In IEC 60068-2-60 this is defined as NO2, which is classified as one of the most aggressive nitrogen oxides.
Preventative measures in product design
The degree of sensitivity of a device or assembly in reacting to corrosive gases can be significantly influenced by the design, the selected components and the manufacturing process.
• Certain materials, such as silver for soldering, accelerate the reaction with corrosive gases. The use of such materials should be avoided in the design as much as possible.
• Only printed circuit boards of high quality and good tinning on the copper traces should be used.
• The contact pins of plug connectors should be protected against corrosive gases using generously proportioned and fitted housings.
• Coatings and sealing made from silicone are deceptive. Silicone and silicone coatings are gas-transparent and only provide little protection against corrosive gases.
• Components such as potentiometers or relays should be used in sealed versions.
• Metal parts, especially screws, should be chosen with the best corrosion protection.
How useful is a conformal coating?
With power supplies that have a conformal coating, normally only the assembled printed circuit boards have the coating applied, not the entire device. Printed circuit boards are typically insensitive to corrosion, even without conformal coating, provided the manufacturing process and the materials used are of high quality.
If the corrosion occurs mainly on the housing, there is a risk that red rust particles may break off and penetrate the interior of the housing. This can result in bridging circuit traces. For this reason, in the use of ultra-fine conductor path structures, it can be advantageous to apply conformal coating to avoid short circuits due to loose rust particles.
PULS avoids the use of ultra-fine conductor path structures in its power supplies and is why PULS devices do not normally require a conformal coating to protect against corrosive gases.
Corrosion tests on PULS devices
PULS periodically performs environmental simulation tests according to ISA S71.04 Severity Level G3 as well as in line with the more stringent criteria of IEC 60068-2-60 method 4. The tests are designed to simulate a service life of at least 10 years in the specified environment and are carried out on at least 10 samples both in cyclical operation and in powered off mode. The devices are evaluated after the tests by experienced personnel using high-resolution microscopes. In addition, the evaluation of returned devices following many years of use in the field from these types of environments confirms their suitability for these application areas. It also takes years of observation and recurring tests to acquire the knowledge to create a corrosion resistant design.
However, corrosion tests naturally result in corrosion forming on the devices. If this did not occur, the tests would have to be questioned. This normally occurs on housing parts or screws. This corrosion, assuming it does not impair the reliable and safe functioning of the device, is classified as a “visual flaw” which does not prevent usage in these types of environment.
Suitability for use in corrosive atmospheres, is a feature in a large number of PULS devices. The Severity Level G3 according to ISA S71.04 can be confirmed for all devices in the product series DIMENSION, MiniLine and PIANO. In addition, many devices also conform to method 4 of IEC 60068-2-60. The suitability of a specific device for use in corrosive atmospheres is confirmed in a manufacturer‘s declaration.
This quality feature gives PULS power supplies clear added value for users who have to protect electronic components against corrosive gases in their applications.
For more information, visit www.pulspower.com
Print this page | E-mail this page
Download a copy of our digital magazine