Corrosion testing
with fatigue

It is a phenomenon specific to corrosion-fatigue in stainless steels. The friction between parts during the cycles forms slip bands on the spring surface, which mechanically destroy the passive oxide layer (Cr₂O₃) that protects the steel. Once the area of fresh metal is exposed, accelerated electrochemical corrosion is triggered, even in a seemingly inert medium. Under static conditions this does not happen because the passive layer remains intact and regenerates. This is why shot peening markedly improves results: it introduces residual compressive stresses that delay the nucleation of slip bands.

Dacromet and Geomet are thin, rigid coatings — zinc/aluminum flakes oriented in an inorganic matrix — optimized to resist chemical attack under static conditions. Under compression cycles, the coating suffers microfractures and local delamination in the contact zones between disc springs, where the pressure is highest. Those cracks expose the 51CrV4 substrate to the medium, accelerating corrosion right where fatigue concentrates stresses. In the without-stress test (block 01) these coatings score B in most media; under fatigue they remain at modest values (4,000–6,000 cycles), confirming that their best application is static or low-dynamic protection.

It depends on the regime. In the demanding 20–80% regime (wide cycles), shot peened stainless steel 1.4310 or 1.4568 offers the best combination of service life + stability — between 20,000 and 25,000 cycles. In the moderate 20–60% regime, yellow zinc plating on 51CrV4 outperforms stainless steel (292,537 vs 27,974 cycles in 1.4568 shot peened) at a clearly lower cost. For industrial production where the load cycle is predictable and moderate, zinc plating can be the most cost-effective option. If the cycle is demanding or unpredictable, shot peened stainless steel offers a greater safety margin.

3% NaCl accurately reproduces the chloride-ion concentration of seawater (≈30 g/L), and chloride ions are the dominant corrosive agent in marine and saline environments. Standardized artificial seawater solutions (ASTM D1141) add sulfates, bicarbonates, magnesium, and other minor salts that can introduce variability into the test without providing key information about the corrosion-fatigue mechanism. To validate results against specific marine standards (offshore, naval), a complementary test in a chamber with artificial seawater would be required — write to us if your application requires it.

Yes — but only under the specific conditions of the test (0.1N NaOH, moderate 20–60% regime, room temperature). NaOH is the most benign of the four media tested for zinc coatings: the zinc reacts with the NaOH to form a passive layer of zinc hydroxide that protects the substrate without dissolving quickly. Add to this the moderate regime, which limits the mechanical stresses on the coating. Extrapolating to real applications must account for: (1) temperature — at 80 °C the ratio changes drastically; (2) the simultaneous presence of chlorides or acids, which neutralize the protective effect of NaOH; (3) the frequency and amplitude of the real cycle. In CIP cleaning applications with springs under moderate load, this result is a good indicator, but it is advisable to validate it with a test at the service temperature.