Patented batch galvanizing process offers excellent corrosion protection

Hot-dip galvanizing is an established process for increasing mechanical resistance, e.g. to stone chipping, as well as corrosion resistance. Further merits of the process are high reliability, environmentally-friendliness and ease of recycling. In conventional batch galvanizing however, zinc coatings with a thickness of 60 - 80 µm have a negative effect on component weight. A high level of reworking must also be carried out due to threads and holes. Alternative methods which offer thinner coatings often use hexavalent chromium, the use of which will be banned for automotive parts from July 2007 under an EU directive (2000/53/EG).

The positive characteristics of the 5% aluminum-zinc alloy in continuous strip galvanizing were taken as the starting point for developing the MICROZINQ® D4 process for batch galvanizing. In contrast to conventional batch galvanizing, this patented process with its superior characteristics involves a new flux composition and an adapted process chain. The average coating thickness is 8 - 20 µm. Compared to the standard galvanizing procedure (ISO 1461), the higher aluminum concentration makes the active protection mechanisms four times more effective under long-term exposure. The excellent corrosion resistance characteristics of MICROZINQ® D4 are corroborated by numerous tests made by independent institutes.

- Over 1,000 h in the salt spray test (DIN 50021) before red rust begins to form.

- More than 8 cycles in the cyclic corrosion test (VDA 621-415) before red rust begins to form.

- More than 45 cycles in the Kesternich test (DIN 50018-KFW 0.2) before red rust begins to form.

- No contact corrosion with aluminum and stainless steel after 45 cycles in the Kesternich test.

- Very efficient and durable cathodic corrosion protection.

- No cracks in the bending test according to DIN 50111.

- High adhesion (30.3 N/mm2) according to DIN EN 24624.

The process offers considerable weight and cost saving potential, particularly for structural components consisting of two or more joined parts. For example, when seaming is used (Fig. A), the soldering effect of the zinc can produce up to 40% increases in stiffness. As component stiffness is a fundamental design criterion, particularly for axle components, it is possible to partially dispense with additional reinforcements or reduce the material thickness.

Additionally, several service life tests were carried out with welded components which were dip-coated and D4 galvanized for comparison purposes. The galvanized components had an approx. 25% longer service life. The cause of this is not entirely known as our laboratory investigations are not yet complete.

It can however basically be assumed that the MICROZINQ® D4 process offers considerable benefits in combination with specific joining techniques.

Contact:
ThyssenKrupp Automotive AG
Viktor Braun
Tel.: +49 / 234 / 919 - 60 12
Fax: +49 / 234 / 919 - 60 14
e-mail: viktor.braun@tka.thyssenkrupp.com