1996 FIN – Mayer: Cheap Can Be Expensive
June 25, 1996 FIN – “Forget the words ‘cheap’ and ‘expensive,’ Dr. Hermann Mayer advised June 12 at the FasTech Asia conference in Singapore. “The cheapest fastener can become the most expensive.”
Speaking on Corrosion Resistant Fasteners, Mayer of Zug, Switzerland-based Bossard International said that fastening elements amount to only 15% of the total cost of an assembly. Eighty-five percent is related to purchasing, quality control, storage, internal transport and preparatory work for assembling.
Thus Bossard asks about the application conditions, Mayer said.
“To select all fasteners in stainless steel can be correct and economical, but it can also lead to unnecessary costs and in some cases structural failure problems,” Mayer warned.
For example, the ceiling of a Swiss indoor swimming pool collapsed, killing swimmers. Chlorine induced severe stress corrosion weakening the anchors. “After the fracture of the first bolts, the rest became too weak to bear the ceiling,” Mayer said.
“Stainless steel is not sufficiently corrosion resistant to fix heavy overhead elements in a swimming pool with its aggressive chlorine-containing, very humid and warm atmosphere. Above the ceiling there was not enough space to control the anchor bolts. There wouldn’t have been any possibility to exchange corroded reinforcing anchor bolts in the concrete ceiling.”
“Fasteners must always be equal to or better than the assembled parts,” Mayer advised.
In the swimming pool case, the engineer needed to protect the fasteners from water, use inclined, well-ventilated surfaces to minimize contact with humidity, insulate surface coatings to interrupt the flow of galvanic current between metal and water and include a corrosion resistant base material or surface coating, Mayer explained.
When simple surface corrosion attacks fasteners they usually become visibly affected. If in plain sight, fasteners can be replaced or protected from further damage.
Crevice corrosion attacks fastened joints in areas where humidity and moisture are absorbed by the capillary effect. Inside the crevice humidity cannot escape and there is a lack of oxygen.
One of Mayer’s recommendations is that bolted joints “should contact in only the minimum of assembled parts to avoid crevices.” Also flange head screws can be used instead of screws and washers.
Any type of surface coating is an additional operation, which increases costs. “It is the designer’s responsibility to check whether this operation is economical or not,” Mayer said. “In many cases stainless steel fasteners are more economical.”
Zinc plating is the most common. Zinc will corrode before the steel and thus protects the steel. “This means that zinc is offered to corrosion as a sacrifice,” Mayer explained.
Is it more economical to add a chromating process to the electroplating than to increase the thickness of zinc, he said. Chromating is a very thin additional layer which is more resistant to corrosion than pure zinc.
Nickel plating functions like a coat of paint and is more corrosion resistant than steel. However, if the nickel layer is damaged, corrosion of the steel underneath will be accelerated. Wax will close the pores and scratches in nickel plating. ©1996/2010 Fastener Industry News
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