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COIL ZIP 2 - The slider is ‘frozen’ on the coil zip and can’t be moved

The cause of this problem is salt accumulation on a metal diecast slider followed by a long period of non-use or storage. Once the protective layer on the slider becomes worn or damaged (internal wear will occur just from operating the slider) the diecast is easily corroded. (If you sail or spend a lot of time on the coast you should choose wet-weather gear with moulded-tooth zips and plastic sliders. These sliders use stainless steel for parts like their auto-locking pins and are hardly affected by salt).

Even a small amount of salt, present for a continuous period of time, will cause a metal slider to oxidise. Salt is 'hygroscopic' meaning it attracts water directly from the atmosphere. Once dampened the chloride ions are mobilised and metal corrosion begins. Like rust on iron the white metal salts that result from the diecast corrosion take up far more space than the original metal. This material expands and 'freezes' the slider in position.

Depending on how much corrosion has occurred you may be able to dissolve away the metal salts with a weak acid solution. Badly corroded sliders will need to be replaced.

Try dissolving the metal salts with vinegar first. It is best to submerge the slider in a container of vinegar (acetic acid) and leave it there for some time, checking every few minutes. Alternatively you could use hydrochloric acid (available from your hardware store). You must wear safety glasses and rubber gloves. Dilute the acid 20 parts water to 1 part acid, slowly adding the acid to the water, not the water to the acid. Weak acids do not affect the type of plastics used in zips: polyester (for the coils, tape and thread) and poly-oxy-methylene (for moulded teeth). Acid does however trouble nylon yarns so take care to avoid prolonged contact with any nylon fabric nearby the zip. If you are successful in freeing the slider using a weak acid and it appears to be working OK be sure to thoroughly rinse out the remaining acid (and salt). Once the zip is completely dry spray it with a silicone lubricant.

To remove a badly corroded slider from a zip you will need to somehow hold it firmly while a fine hacksaw or hacksaw blade is used to cut down diagonally through the bridge on the nose of the slider. You must cut on the diagonal to avoid damaging the zip coils or teeth. Once almost cut through it will be easy to break apart the top and bottom parts of the slider. (You can use side-cutter pliers or a fine screwdriver inserted into the hacksaw cut to do this). The best place to work is at a bench vise, gripping the back end of the wider, coil side of the slider in one end of the vise jaws. If you have vise-grips (locking pliers) you can use these in a similar way but you will need someone to hold them rock-steady while you saw. Once you have cracked the slider off clean away all the corrosion material using a fine metal point and stiff brush.

You can find instructions on how to replace a slider on another DIY page.
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