Thursday, 15 November 2018
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Outwitting corrosion

Product development can be inspired by very different philosophies. At times low manufacturing costs take pride of place - leading to the product performance limits being framed as narrowly as possible, the reduction of product quality to a minimum, the acceptance shorter product life cycles and shifting parts of the investment costs to later operating costs.

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Another philosophy considers the overall service lifetime costs and concentrates on lasting customer benefits, operating costs and environmental properties. A new hot dip coating for sheet metal adheres to the second philosophy.

Newly developed zinc-magnesium hot dip coating

A newly developed zinc-magnesium hot dip coating recently became available from the steel industry for steel sheeting, which could be of interest in lift building. Compared to conventional hot galvanised materials, this one is above all distinguished by its very greatly improved corrosion properties and chemical resistance. It offers clearly improved protection against saline media, strikingly better white rust resistance, much lower red rust corrosion and better cutting edge protection. Material tests by the manufacturer have revealed corrosion resistance up to six times greater than standard galvanisation procedures.

HandwerkThe zinc-magnesium sheet metal has good malleability, can be welded like hot galvanised sheet metal, painted just like electrolytically galvanised surfaces and is hence very suitable for subsequent paint coats. Its manufacturers emphasise the lasting contribution to the environment from the economic employment of the raw material zinc in this process.

Crash barriers and parking systems are familiar areas of use for this material. Meiller has been looking into this material for almost a year. Providing an improvement in customer benefits above all for those operators who see the service life of a lift and its components in a realistic 15 - 25 year range, combined with an improvement in the environmental balance sheet, fits in well with the company's philosophy.

Paint treatment as decisive criterion

Initially trial model series using zinc-magnesium sheet were produced in order to evaluate its production properties. It turned out that there was an improvement in processing with shears, blanking and laser systems compared to electrolytic and hot galvanised sheet metal, since there was hardly any burr. Edge, point and projection welding as well as bonding are no problem.

However, the decisive criterion for this new material is the paint treatment, since whether and how these surfaces can finally be painted without preliminary priming or grounding is a decisive factor for customer acceptance. For this purpose processed components were painted and powder-coated without any pretreatment. The subsequent tests of the surface quality by grid cuts produced very good results.

Comparisons with powder-coated sheet metal

Zinc-magnesium and hot galvanised components were subjected to a standardised test procedure, the NSS salt spray test according to EN ISO 9227 for a period of 240 hours, to test the corrosion properties compared to conventional galvanised standard sheet metal.

HandwerkThe result confirmed the manufacturer’s information: after 240 hours the sheet metal had no red rust and only moderate, no conspicuously voluminous, white rust. No red rust could be found on the cut edges thanks to the cathodic protective effect of the new hot dip coating. There were not even any signs detectable on the welding points on the door leaf after 280 hours.

Parallel to this comparisons with powder-coated sheet metal were carried out. To test the corrosion-resistance of the paintwork, standard conventional door components in the design described were likewise subjected to the NSS test according to EN ISO 9227. The first red rust could already be identified after only 24 hours. The first white rust was evident on the galvanised parts after 24 hours and the first red rust after 120 hours.

Clear product improvement

The basic problem with this surface coating is simply that the individual panels are first joined and then undergo surface treatment. As a result there are untreated spots that are correspondingly susceptible to rust. By contrast, zinc-magnesium coated parts enjoy "all-round” protection against corrosion and on cut and blanking edges and as a result do not form any concealed rust sources after joining.

Meiller is convinced that zinc-magnesium coated sheet metal represents a clear product improvement, which can greatly extend the service life of lift doors. This surface ought in particular to provide those operators with added value who run their lifts in the public sphere and in an atmosphere that is not always completely dry, and who in particular see the service life of a lift and its components in a realistic range of 15 - 25 years, especially since Meiller will adopt neutral pricing for all sliding doors based on this material from April 2016.

Wolfgang Nothaft

www.meiller-aufzugtueren.de

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