Corrosion Testing Methods

Innovative Testing Strategies for Enhanced Corrosion Resistance in Hydraulic Components

March 27, 2025

The corrosion resistance of steel components and industrial products is of central importance for application and customer benefit. It will gain even more significance in the future to pursue sustainability goals. Hydraulic components with increased service life help minimize resource consumption, reduce environmental impact, and lower costs. In recent years, many different surface innovations have been presented to enhance corrosion resistance; however, they have not always met expectations in application and practice. One reason for this is the proper selection of testing methods to measure corrosion resistance. In particular, the widely used salt spray test according to ISO 9227 is considered not particularly suitable for characterizing corrosion performance in reality. However, this does not mean that the salt spray test is no longer valid; rather, it is often misapplied.

It is helpful to distinguish between two fundamental aspects, namely tailoring to customer applications through research and development, and quality monitoring in production. Different tools are needed for both.

Testing Methods for Application-Oriented Surface Development

In research and development, the salt spray test according to ISO 9227 [1] has had a bad reputation as an industrial standard for corrosion testing for some time. The reasons for this are manifold; on the one hand, the salt spray test does not represent real conditions. The salt concentration is very high at 5% (seawater has only about 3.5% salt content), which leads to a significant acceleration of corrosion at the expense of correlation with field results [2]. Furthermore, the parts are permanently moist, which complicates the formation of passivating layers (hydroxides, oxides, carbonates) and thus reduces corrosion resistance. The artificial corrosion mechanisms make it impossible to compare different surface treatment systems regarding their real performance. For example, the corrosion results of pure zinc systems are comparatively worse than those observed in reality. In contrast, the combination of zinc with alloying elements performs significantly better in the salt spray test than is typically observed in most applications. The central concern of research and development—to develop, compare, and optimize new surface treatment systems—is therefore hardly feasible with the salt spray test.

Due to these limitations, misjudgments and misdevelopments can occur in practice. One example is the increase in corrosion performance through the use of a topcoat. In the salt spray test, there is a strong increase in corrosion resistance, which is less pronounced in reality. One reason is that temperature fluctuations, which are not represented in the salt spray test, have a more significant impact on the polymer-based topcoat in application. The typical climate change test with a variance in temperature and humidity accounts for this circumstance. Otherwise, the concept of weathering in a closed chamber with a final visual evaluation is comparable to the established salt spray test (see Figure 1). The tests can be an important aid in designing new surfaces so that their corrosion resistance is optimized based on application-relevant parameters rather than artificial conditions like those present in the salt spray chamber.

Parker has utilized a wide range of climate change tests in the development of its ToughShield Plus surface. A typical mobile hydraulic application can be evaluated, for example, with a test according to ISO 16701. Here, temperature and humidity fluctuate in different cycles between 50-90% and 35-45 °C, respectively (see Figure 2). The parts are sprayed with an artificial slightly acidic salt rain (1 wt%, pH=4) only twice a week and are not permanently wet as in the salt spray chamber. This cycle is adapted to the conditions of a Central European climate with moderate salt exposure (e.g., through winter salt spreading or coastal climate) and offers a good correlation with real conditions [2]. Figure 3 shows pipe fittings with a conventional zinc surface treatment and the Parker ToughShield Plus surface after a corrosion test in the salt spray test and a climate change test according to ISO 16701. The corrosion appearances differ significantly. In the case of conventional zinc plating, the part from the salt spray test is completely covered with red corrosion, while the part from the climate change test shows corrosion appearances at exposed areas such as corners and edges. Particularly, deformed surfaces in the area of the crimp nut corrode significantly. This corrosion behavior corresponds much more to field and application experience. In the case of the Parker ToughShield Plus surface, no visible corrosion appearances can be detected even after 12 weeks of testing, corresponding to several years of field use.

Quality Assurance through Corrosion Testing

Although climate change methods are central to the development of new surfaces, they do present challenges as quality tools. For example, corresponding test chambers are expensive to purchase and complex to operate, which often restricts their use to a few central laboratories with the appropriate infrastructure. The testing logistics are further complicated by the fact that climate change tests work with cycles instead of permanently identical conditions, thus imposing a rigid schedule for test initiation and evaluations. Furthermore, calibration and thus achieving comparability of different test chambers in most climate change tests is not or only inadequately described. Here, the salt spray test can demonstrate its advantages in quality assurance (lower effort, higher flexibility, better calibration, and comparability). However, it is necessary to carefully consider, describe, and reproducibly execute the application to limit the inherent variance to a manageable level.

First, the type, amount, and especially the timing of the sampling must be precisely defined. Corrosion resistance is not an absolute property but a volatile product characteristic. Once the parts are coated and then subjected to stresses from assembly, packaging, transport, and storage, the corrosion resistance decreases in an uncontrolled and unquantifiable manner. Quality assurance measures based on this are not effective. According to relevant standards (e.g., ISO 19598 [4] or ISO 4042 [5]), corrosion properties must therefore be tested before packaging and transport, i.e., directly after surface treatment. The common practice of taking parts from stock and testing them in the salt spray test is not compliant with standards and does not allow conclusions about surface quality. For the precise execution of the test (part preparation, such as plugging holes, method of suspension/positioning in the chamber, number of test specimens, etc.), ISO 9227 provides many valuable hints, which, however, must be tailored to the specific task.

Such tests only develop meaningfulness when corrosion appearances have been very precisely examined and documented in advance. For example, the corrosion of zinc-nickel surfaces differs significantly from unalloyed zinc surfaces in the salt spray test. Instead of the voluminous, easily recognizable white rust, zinc-nickel typically only forms an inconspicuous gray veil. This is difficult for the untrained eye to assess; therefore, appropriate reference image material must be created and trained. Compared to zinc, the base metal corrosion of zinc-nickel is also significantly slower. While zinc completely corrodes within a few days after the first appearance of red corrosion, this span is significantly longer for zinc-nickel. In some cases, initial red rust spots may even disappear. This phenomenon can be observed in the case of an extremely corrosion-stable zinc-nickel layer: The progression of corrosion of the initially red rust-filled pores is so strongly slowed that they are gradually covered by the zinc-nickel gray veil. The definitions and specifications of white and red rust for zinc-nickel are therefore not trivial and can only be established based on extensive data.

However, once all these data are known and described, the salt spray test can be used as an efficient testing method in surface quality monitoring. All platers producing Parker's ToughShield Plus surface are qualified according to a precisely defined procedure and are subject to the described process monitoring.

Balancing Innovation and Quality: The Path to Superior Corrosion Resistance

The corrosion resistance of hydraulic components is one of the central challenges for customer satisfaction, cost efficiency, and sustainability. It is essential to select the right testing methods for the respective questions. For an application-oriented evaluation and further development of surface treatment systems, climate change tests are the method of choice. The much-criticized salt spray test can play an important role in quality monitoring when correctly applied. Parker has successfully utilized this combination to not only establish the technology for the highest corrosion resistance with the ToughShield Plus surface but also to produce it in consistently high quality.

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References

[1] DIN EN ISO 9227 “Corrosion Tests in Artificial Atmospheres – Salt Spray Tests”
[2] LeBozec et al., Mater. Corro. 66 (2015), p. 215
[3] DIN EN ISO 16701 “Corrosion of Metals and Alloys - Corrosion in Artificial Atmospheres - Accelerated Corrosion Testing under Cyclic Influence of Humidity and Intermittent Spraying of a Salt Solution under Controlled Conditions”
[4] DIN EN ISO 19598 “Metallic Surface treatments – Electroplated Zinc and Zinc Alloy Surface treatments on Iron and Steel Products with Additional Cr(VI)-Free Treatments”
[5] DIN EN ISO 4042 “Fasteners – Electroplated Surface treatment Systems”