White deposits on the surface of a steel tube indicate that the protective coating has corroded, but strength of the tube has not been compromised. However, red deposits (rust) indicate that the steel tube itself has corroded, and possibly has been weakened.
High-pressure line tubing often is used in corrosive environments that eventually weaken the tube. Without any protection, its full functionality cannot be guaranteed. Seamless cold-drawn steel tubes widely used as pressure lines in hydraulic or pneumatic systems from Mannesmann Przisionsrohr GmbH conform to DIN EN 10305-4, which defines technical specifications.
These tubes are used in applications such as construction, agricultural and mining equipment exposed to harsh environments. Without a surface protection, the steel can immediately react with the environment and undergo galvanic corrosion.
A common means to protect the tube is to coat the surface with a layer of zinc. This layer can be produced, for example, by a galvanizing process, and common thicknesses are about 8 to 12 μm or 12 to 15 μm. The zinc coating Corrosion protection for hydraulic pressure lines serves two purposes — it shields the steel surface from direct contact with the surrounding environment, and it acts as a sacrificial anode, meaning it corrodes before the steel does. However, eventually it can corrode enough to expose the base material to the surrounding environment. Therefore, the zinc layer itself is usually protected by a second layer. Generally, two kinds are in common use: Cr(VI)-containing layers called chromating, typically with a yellow or olive green appearance, and Cr(VI)-free layers, called passivation, typically with a silver, blue, or black appearance. These layers add about 0.5 μm to the thickness and provide an extra barrier. Sometimes a third, sealing, layer is also applied.
However, these protective layers do not last indefinitely. Mechanical strain of any kind, especially bending, may harm the outer chromate or passivation layer over time. This exposes spots or parts of the zinc layer to the environment and, under specific conditions, allows for the zinc to corrode and form white rust, a white, powdery, bulky deposit. Although white corrosion is an early indication that the protection system has been harmed, the tube itself is not yet affected. Once the zinc layer has been depleted — again, even if only at single spots or small areas — the base material of the tube is exposed, and the steel may react with the surrounding oxygen and water. The product is rust, which is a definite indicator that the base material has been compromised, and the full functionality of the pressure line can no longer be ensured.
