Along the pre-treatment line of a hot dip galvanizing plant, conditions are difficult: workpieces have first to be cleaned in diluted hydrochloric acid to ensure that they are as clean as possible. As a result, the hydrogen chloride content of the air along the pre-treatment line is around 2 per cent. This means that the indoor crane and the components on to it are subjected to exceptional acid exposure. In the factory of the company Wiegel, plastic energy chain systems from the motion plastics specialist igus have been proving their worth for several years. They move in plastic troughs made of a chemical-resistant material, and as a result the system easily copes with the aggressive conditions.
Since 1986, Meissner Krane GmbH has been offering customer-oriented solutions and services for crane systems and hoisting technology. One focus of the company is the equipment of galvanic plants and hot dip galvanizing plants. In 2015, the company equipped the first process crane in the pre-treatment line of a hot dip galvanizing plant of the Wiegel Group with a modern energy chain system from igus. With approx. 1,600 employees at 39 locations in Germany, Austria, the Czech Republic and Slovakia, the Wiegel Group is one of the largest and most important hot dip galvanizers in Europe.
Preparation is half the job
Hot dip galvanizing is the most effective corrosion protection method that is used industrially to protect steel from corrosion in an extremely long-lasting and environmentally friendly way. By immersing the workpieces in metallic molten zinc, a zinc coating is applied. Iron-zinc alloys of different composition are formed on the steel surface. To the outside, the surface is closed by a coating of pure zinc. However, for a permanent and complete iron-zinc reaction, the workpieces must be pre-treated. By immersing in a series of pre-treatment baths, the workpieces must be clean before the actual hot dip galvanizing. To remove production residues such as oils, the parts are first cleaned in a degreasing bath and then stained. Here, other characteristic impurities such as rust and scale are removed from the surface. Both usually take place in dilute hydrochloric acid.
This is followed by two rinses in order to minimise the carryover of acid and salt residues with the galvanizing material as far as possible. In the penultimate step, the workpieces are immersed in a flux bath, which is intended for the very fine cleaning of the steel surface. Thereafter, the parts are dried before being immersed in the molten zinc.
The operating temperature of a galvanizing bath is between 440 and 460°C in most plants. The steel surface enters into a solid bond with the zinc in the molten bath by mutual diffusion, the quality of which is achieved with no other protective method. Afterwards, workpieces are cooled in the air.
Difficult conditions in the pre-treatment line
Due to the open process baths, which are completely enclosed by the surrounding environment, an HCl condensate of about two percent prevails in the air in the area of the pre-treatment line, which corresponds to a technical environment of 20 g/l hydrochloric acid. This means that it is important to use chemically-resistant components inside the pre-treatment line.
In the past, a cable festoon was used at all Wiegel locations for the energy supply of the process cranes. Cables and hoses are suspended and moved with small trolleys. A disadvantage of this technology, however, is that at the end of the travel, a cable station takes up a lot of space and for this reason the space of the pre-treatment line cannot also be completely sealed off. The rollers of the cable trolleys also fail after some time, because the C-profiles in which they are guided become corroded and sharp-edged due to the acid exposure. As a result, the rollers quickly wear out.
In 2015, the first energy chain from igus was used in the Wiegel's Lauchhammer plant. "It was not until the end of 2014 that we presented the product to Meissner Krane," recalls Christian Richter, Technical Sales Consultant responsible in the field.
"Meissner was immediately ready to test this new approach." A year later, the next application followed at the location in Feuchtwangen, where igus supplied both the E4/4 series energy chain and the new plastic trough made of the chemical-resistant igumid EG+ material. This ensures a high resistance to chemicals, so that the use of the guide trough is ideal for use in hot dip galvanizing or, for example, fertilizer production.
Lightweight and chemically resistant: guidelite plus from igus
The material igumid EG+ is characterised by a very low water absorption. This offers an increased resistance to aggressive chemicals. The material can be used in a temperature range of 0°C to + 100°C with its clearly optimised gliding properties for the guide trough and also for most series of e-chains. Since both the e-chain and the trough in the complete system consist of the chemically resistant material, they are very good gliding partners.
The complete system, consisting of chain and trough called guidelite plus EG, is now used in four Wiegel plants.
Required service life: 10 years
The first systems have been in use at Wiegel for several years now. The relatively moderate crane speed of 40 m/min does not pose a big challenge to the igus energy chain system. The company has already successfully implemented applications with speeds of approx. 10 m/s. Much more crucial here is the durability in the aggressive environment. Peter Zorn from Wiegel: "Of course we had to be bold to a certain extent and rely on the new solution from igus. Prior to this, we tested the chain made of the chemically resistant material by using it on the crane for just six to eight weeks to see if the material could cope with the environment. Now we expect a service life of at least ten years, so that the higher initial cost pays off compared to a festooning system."