Corrosion is one of the most underestimated causes of premature rolling bearing failure. While load rating, speed, temperature, lubrication and sealing are usually considered as part of bearing selection, environmental exposure is often addressed only after the first signs of damage appear.
In applications exposed to moisture, cleaning agents, acids, alkalis, salt water or aggressive atmospheres, corrosion resistance becomes a decisive design criterion. The key technical question is therefore:
How can high corrosion resistance be achieved without significantly compromising the mechanical performance and economic efficiency of the bearing?
With CORRO-SHIELD, APA offers a surface coating concept that addresses precisely this challenge. The coating combines the mechanical advantages of proven rolling bearing steel with a highly corrosion-resistant galvanic surface treatment, creating a technically and economically attractive alternative to conventional stainless steel bearings.
The Engineering Conflict: Corrosion Resistance Versus Load Capacity
In corrosive environments, stainless steels are often selected as bearing materials. This is understandable, since stainless steels generally offer better corrosion resistance than conventional rolling bearing steels such as 100Cr6 / 1.3505 / AISI 52100.
However, the term “stainless” can be misleading in bearing technology. Stainless steels can also corrode. More importantly, rolling bearings require a very specific mechanical property profile. To achieve full mechanical load capacity, both bearing rings and rolling elements typically need a minimum hardness of approximately 58 HRC.
This creates a material conflict.
Austenitic stainless steels offer comparatively strong corrosion resistance, but their use in highly loaded bearing rings and rolling elements is limited because they are only surface-hardenable by carburising or nitriding. In many cases, such hardening processes can reduce corrosion resistance. Martensitic stainless steels are more suitable for hardening, but they do not always provide the corrosion resistance required in aggressive environments.
As a result, stainless steel bearings often represent a compromise between corrosion resistance, load carrying capacity, service life, availability and cost.
CORRO-SHIELD follows a different engineering approach: instead of replacing the entire bearing material with stainless steel, it uses a mechanically robust bearing steel and protects it through a specialised surface coating.
The Technical Concept Behind CORRO-SHIELD
CORRO-SHIELD is a galvanic surface treatment for metallic surfaces. Its purpose is to significantly increase the corrosion resistance of rolling bearings without negatively influencing their static or dynamic load ratings.
The advantage lies in the separation of functions: the base material provides the required bearing hardness, fatigue strength and load capacity, while the surface coating provides protection against corrosive media.
This means that bearing steels such as GCr15, comparable to 100Cr6 / 1.3505 / AISI 52100, can retain their mechanical performance while gaining substantially improved corrosion protection.
CORRO-SHIELD therefore addresses three central requirements in industrial bearing applications:
- corrosion resistance
- mechanical performance
- cost efficiency compared with many stainless steel bearing solutions
Salt Spray Test According to DIN EN ISO 9227 NSS
To evaluate the performance of CORRO-SHIELD, coated bearings were tested in a neutral salt spray environment according to DIN EN ISO 9227 NSS.
The test compared CORRO-SHIELD-coated bearings directly with uncoated standard steel bearings and stainless steel bearings.
Tested bearing sizes:
| Bearing type | Dimensions |
| 6002 | 15 × 32 × 9 mm |
| 6204 | 20 × 47 × 14 mm |
Test parameters:
| Parameter | Value |
| Standard | DIN EN ISO 9227 NSS |
| NaCl concentration | 5% |
| pH value | 6.5–7.2 |
| Chamber temperature | 35 °C |
| Test duration | 480 h |
| Condensate volume | 1–2 ml/h on 80 cm² |
The following material variants were compared:
| Variant | Material / Design |
| Standard bearing steel | GCr15, comparable to 100Cr6 / 1.3505 / AISI 52100 |
| Stainless steel bearing | 3Cr13, comparable to AISI 420 |
| Stainless steel bearing | 9Cr18, comparable to AISI 440 |
| Coated bearing | GCr15 + CORRO-SHIELD |
Test Results: A Clear Difference After Only a Short Exposure Time
The results show a clear technical differentiation between standard bearing steel, stainless steel variants and CORRO-SHIELD.
After 1 hour, no visible corrosion was observed on any of the test specimens.
After 3 hours, the uncoated standard bearing already showed slight corrosion, while the stainless steel bearings and the CORRO-SHIELD-coated bearings showed no visible corrosion.
After 7 hours, CORRO-SHIELD still showed no visible corrosion.
After 52 hours, the difference became especially significant: the standard steel bearing and the stainless steel bearings showed strong corrosion, while the CORRO-SHIELD-coated bearings still showed no visible corrosion.
After 144 hours, CORRO-SHIELD showed only a slight colour change, with no visible corrosion of the base material.
After 480 hours, the CORRO-SHIELD specimens showed a pronounced colour change and slight corrosion of the base material in the contact zones between ball/ring and ball/cage. In sealed and greased bearings, these areas receive additional protection from the seal and lubricant.
Why These Results Matter in Practice
The test is technically significant because CORRO-SHIELD did not only outperform uncoated standard bearing steel. In this test setup, it also showed superior corrosion behaviour compared with the stainless steel bearing variants tested.
This is highly relevant for applications where stainless steel bearings are often specified by default, even though they may not be the best technical or economic solution.
Typical applications include:
- food and packaging machinery
- conveyor systems
- cleaning and washing equipment
- pumps and valves
- agricultural machinery
- marine or salt-exposed environments
- chemically exposed installations
- machinery with regular wet cleaning
- outdoor applications exposed to changing humidity
In these environments, corrosion is not only a visual problem. It changes surface structures, increases friction, damages raceways, impairs lubrication and can ultimately lead to premature bearing failure.
Improved surface protection can therefore have a direct impact on service life, maintenance intervals, process stability, machine availability and total cost of ownership.
The Critical Issue: Hydrogen Embrittlement in Galvanic Coatings
A technical discussion of galvanically coated rolling bearings must include the issue of hydrogen embrittlement.
In high-strength steels with tensile strengths above approximately 1,000 N/mm², galvanic coating processes can present a major risk. During the coating process, hydrogen may enter the base material. In hardened martensitic steels, this can lead to embrittlement and potentially to brittle fracture.
This is especially critical in rolling bearings.
Materials such as GCr15 or 100Cr6 have a high martensitic microstructure after hardening. This microstructure is essential for hardness, fatigue resistance and bearing service life. At the same time, it can be sensitive to hydrogen absorption if the coating process is not properly controlled.
For this reason, CORRO-SHIELD uses a specialised galvanic coating process for high-strength steels. The aim is to achieve excellent corrosion resistance while almost completely eliminating the risk of hydrogen embrittlement. The mechanical properties of the base material remain intact, meaning the dynamic and static load ratings of the rolling bearing are not negatively affected.
This is the technically decisive point: a coating for rolling bearings must not only protect the surface. It must also preserve the structural integrity and load-bearing capability of the component.
Economic Advantages
Cost is another important factor in bearing selection.
Stainless steel bearings are generally more expensive than standard bearings due to higher material costs and more demanding manufacturing processes. CORRO-SHIELD offers a different value proposition: the proven bearing steel remains in use, while the surface is specifically protected against corrosion.
This makes CORRO-SHIELD particularly interesting for applications where a standard bearing is not corrosion-resistant enough, but a stainless steel bearing is either technically unnecessary, economically unattractive or mechanically not ideal.
Potential economic benefits include:
- longer service life in corrosive environments
- reduced unplanned downtime
- lower replacement frequency
- improved process reliability
- potentially lower cost compared with stainless steel special solutions
- better availability based on common bearing steel platforms
Conclusion
CORRO-SHIELD from APA demonstrates how the classical conflict in bearing technology can be reduced: high corrosion resistance, high mechanical performance and attractive cost efficiency can be combined in a single bearing solution.
The salt spray test according to DIN EN ISO 9227 NSS showed a clear advantage over uncoated standard bearing steel and also over the stainless steel bearing variants tested. Particularly important is the fact that CORRO-SHIELD is applied to high-strength bearing steel while preserving the mechanical properties of the base material.
For applications exposed to moisture, salt, cleaning media or other corrosive influences, CORRO-SHIELD is a technically robust option. It allows engineers, buyers and maintenance teams to rethink corrosion protection in rolling bearings — not as a compromise between stainless steel and standard steel, but as a targeted surface engineering solution.
Source: BDT Bearing & Drive Technology GmbH, https://bdt-bearings.de/en/
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