May 05th, 2022
Rust, especially red rust is synonymic for bad quality. Saint-Gobain engineers understand the importance of high quality, durable bearings and have developed impressive corrosion resistant bearing solutions that last. While the primary concern is reducing red rust, white rust, where possible, should also be reduced to improve resilience and therefore the life span of the bearing.
The types and causes of corrosion are varied: galvanic or contact corrosion, movement and environmental exposure to name a few influencing factors.
Galvanic corrosion occurs when metals with different electro chemical reactivities come into contact and are surrounded by an electrolyte (for example the moisture in the air). Metals with a low electro chemical reactivity, such as bronze, are therefore resistant to corrosion and oxidation and are referred to as ‘noble’. When they are placed next to non-noble metals, like steel, galvanic corrosion emerges. The rate of corrosion depends on, in addition to other factors, the difference in nobility of the metals, measured by the galvanic series, and the PH-value of the electrolyte. The environment (and therefore electrolyte) the metals are exposed is often unpredictable, for example, rain mixed with uncontrollable external environmental factors such as dirt, debris or salt. The water or moisture enables electrons to flow between the noble metal component, such as a bearing, and non-noble mating component, for example a shaft, causing galvanic corrosion to occur. This is illustrated for a steel and bronze combination in Figure 1.
Figure 1. Galvanic corrosion
Bronze mesh was a popular bearing material choice due to its corrosion resistance and although the bearing itself would not corrode, the mating metals in the assembly could suffer galvanic corrosion.
Consequentially the mating components of (for example) a hinge system; the shaft, housing, bearing and type of plating (bronze and graphite are known drivers of galvanic corrosion when in contact with (coated) steel will need to be harmonized to achieve optimum performance and reduce contact corrosion. The other cause of corrosion is movement. Pivot points are known to have the weakest resistance. This is due to the permanent movement and in some instances unavoidable gaps between the components in the hinge system. Here, the choice of bearing and bearing materials can play a huge role in the prevention of corrosion.
In summary, the greater the difference in electrochemical negativity between the metals, measured as corrosion potential Ecorr, the risk of corrosion increases.
To prevent galvanic corrosion, components need to be designed to work together, or more accurately component materials need to work together. Choosing bearing metals which are relatively close to those in the assembly on a scale of reactivity (assessed using the galvanic series), can greatly reduce the problem of galvanic corrosion.
The NORGLIDE® Bearing steel shell can be provided with an anti-corrosive coating which can be a combination of zinc plating or Cr6 free passivation and sealing, depending on what kind of protection is needed. In a few special cases, other corrosion-protection systems can be used to avoid contact corrosion. All NORGLIDE® systems comply with the European Union End-of-Life directive for automotive vehicles 2000/53/EG.
The corrosion resistance of the individual components is not necessarily identical to the life expectancy of the assembly. The composition and operating conditions, in addition to the bearing material (the metal, interlayer and anti-corrosive coating), should also be taken into account. Actual corrosion resistance should always be verified in the assembled condition.
The resistance to red and white corrosion of individual NORGLIDE® Bearings is tested to DIN EN ISO 9227 in our own salt spray chamber. In special cases the salt spray test is conducted on parts assembled in mating components.
Some NORGLIDE® materials can be reinforced using corrosion resistant metal structures (shown in Table 1) without the need for additional corrosion protection. In this case the electro-chemical potential must be taken into account to prevent contact corrosion.
NORGLIDE® Bearings can also be made with a stainless steel backing and do not need additional corrosion protection. Alternatively, NORGLIDE® Bearings can be made using a backing of aluminium which does not require further corrosion protection because it passivates itself. In both cases the electro-chemical potential must be taken into account to prevent contact corrosion in the assembled condition.
Zinc coating is the most common process to protect components against corrosion, however, to avoid contact corrosion, attention must be given to the electrochemical series. For instance, a plain bearing containing bronze will have a high electrode potential (or electrochemical corrosion potential) when paired with zinc, a material often used to coat steel housing bores. For this reason, using bronze mesh bearings with zinc plated steel housing bores is not the optimum assembly.
The steel backing layers, which can be found on a selection of our NORGLIDE® solutions, are mechanically or galvanically zinc plated resulting in corrosion resistant bearings. Utilising zinc plating in this way ensures that NORGLIDE® Bearings meet customer specifications for salt spray exposure without any red rust. A more robust alternative would be to use a metal that has a high affinity with zinc, such as aluminium, as this combination offers corrosion resistance in the harshest of conditions. Aluminium clad bearings, such as NORGLIDE® SMALC, NORGLIDE® TALC or NORGLIDE® SALC are good to use in this instance as they contain no bronze and the steel has been replaced with aluminium clad steel. In NORGLIDE® SALC, the stretched metal interlayer is made from steel and cladded with aluminium for extra corrosion resistance. NORLGIDE® SMALC and NORGLIDE® TALC solutions comprise of an aluminium-clad steel backing layer as seen in figures 2 & 3. The corrosion resistance of the aluminium-clad NORGLIDE® solutions means an impressive 1,000 + hours of salt-spray exposure with no red rust.
To ensure that customers have confidence in the anti-corrosive performance of our products, we test to international standards, specifically ISO 9227:2017 “Corrosion tests in artificial atmospheres -- Salt spray tests”.
Figure 5: A graphical representation of a stamped door hinge, showing where NORGLIDE® Bearing solutions fit (in red). The bearing is exposed to external environmental impact and demands high corrosion resistance.
By conducting various corrosive environment controlled tests, we ensure that our customers will not be met with any nasty surprises during the operating lifetime of the part or system. In addition to a salt spray test, we can also perform other common tests such as cycle condensate tests or customer specific testing if required.
Our NORGLIDE®, RENCOL® and SPRINGLIDE® products are not off-the-shelf products. Each one can be supplied in an enormous variety of geometries, materials and other configurations. They are custom-designed to meet the specific requirements of your application. We work engineer to engineer at all stages to understand and meet your needs precisely.
Interested in finding out more? Contact us to speak to an engineer about how you could use NORGLIDE® corrosion resistant bearings to solve your challenges. Use our contact form or email us at: firstname.lastname@example.org.