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.

Table shows results of salt spray tests for common corrosion-protective coatings used on pressure line tubing.

To compare the performance of the various protective systems, these can be tested by applying a salt spray test (DIN EN ISO 9227), where specimens are exposed to a standardized saline, heated environment, and the time until the first white or red rust appearance is measured. Although the results cannot be converted to how long a specific coating will protect the tube in different environments, it does provide a convenient means of comparison.

Two types of chrome
A distinct and crucial difference exists between chromate and passivation layers. The chromate layer contains toxic hexavalent Chrome Cr(VI), whereas the passivation layer does not, or only to a negligibly small amount. The passivation contains trivalent Chrome Cr(III), which is not toxic. As of July 2007, the European directive on end-of-life vehicles came into effect, banning any toxic substances such as hexavalent chrome Cr(VI), as well as lead, mercury or cadmium, in cars.

Mannesmann Przisrohr offers galvanized tubes with a two-layer combination of zinc plus a Cr(VI)-containing chromate layer or a zinc plus Cr(VI)- free passivation layer. The table above lists salt spray test results of the various protection systems. Tests were carried out on straight tube segments. All protection systems meet the minimum values required by the common standards. Furthermore, the performance of the passivation exceeds that of the Cr(VI)- containing systems in corrosion protection when compared on straight tubes.

This discussion was submitted by Patrick Lagao, Ph.D., Mannesmann Przisrohr GmbH. Learn about cylinder tubing at the IFPE Technical Conference Session 18.1 on March 13, or visit www.mhptubes.de for additional information.