Corrosion

  • 141. Top Tips Tuesday - Corrosion Control

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    Time flies when you are enjoying yourself! There I was getting kind of used to ‘semi retirement’ just three days work, mainly in the sailloft sometimes working with rigs and bliss, four days tinkering with my own project, when a call came through, "Can you drop everything, go down South this week and do some pattern making on a National Trust historic building, it’s just two days work and don’t forget that you're flying out Sunday afternoon to fit two more of the UV protective sunshields that your sail loft manufactured to another National Trust property". Mount Stewart on the shores of Strangford Loch, the ancestral home of Lord Londonderry, is being ‘fitted’ with a series of sunshields to prevent UV from damaging priceless oil paintings in the galleries below. So when Claire, one of our IT experts, asked when she could expect my draft Top Tips today she was met with an oh...... sugar lumps or worse!

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    Anyway here goes, this time of year it’s worth checking out the mast and boom for any signs of corrosion behind a stainless fitting, be it still standing or removed, likewise alloy stanchions and their bases especially if a stainless bolt or split pin has been used to secure the post to the socket. Correct procedure is to always use a barrier between dissimilar metals such as aluminium and stainless. Tefgel or Duralac are my first choices, however I have also used Forespar Lanocote with success.

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    If you lift out in the winter and then remove the mast, make sure you remove both the standing and running rigging. So many times have I seen stainless standing rigging casually wrapped round a horizontal spar which can result in a ‘line’ of corrosion down the mast! As for wrapping the spar in polythene, its not a good idea. Instead wash down with fresh water then apply (assuming spar is alloy) a protective coat of either Hempel Alu-Protect or Yachticon metal polish. If its lying on a set of trestles or a mast rack its easy to check for signs of corrosion behind stainless fittings. If there are telltale signs of corrosion remove the fitting and if its only mild, clean the surface apply the barrier and make sure your replace and remember, if fastened with rivets use monel not alloy! If the corrosion is more severe, check with the mast manufacturer as to their suggested course of action.

  • 91. Top Tips Tuesday - Protect Your Threads - Galling

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    Last Tuesday in my TOP TIPS TUESDAY blog the headline I used was ‘Stainless Doesn’t Rust (or does it)' I then went on to talk in the article about crevice corrosion, and I also stated that crevices can form under welds, WRONG* (but more about that later!)

    Galling is the term used when two surfaces in contact seize up as a result of cold welding. The problem (also known as adhesive wear) is most common in materials such as stainless steel and we in the marine trade come across it occasionally where you have stainless rigging screw body** and a stainless stud or fork. To help prevent galling make sure that the two surfaces are clean and free from any contamination, do NOT use a mild steel brush to help you clean the threads; consider the Shurhold Detailing Brush which has stainless steel bristles and a squirt of WD40. My recommendation after cleaning is to use a lubricant such as Lanocote or Selden Rigging screw oil to lubricate the threads.

    ** most good turnbuckles these days either have a chrome plated phosphor bronze body, or if the body is stainless there is a bronze threaded insert both ends which prevents galling however, threads should still be clean before adjusting.

    Incidentally when mooching round our local boatyard a couple of years ago I came across these very simple homemade covers (made from old bits of hose pipe) for protecting the threads and help keep them clean.

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    Some skippers of course leave their mast up when lifting out, me, I prefer my mast to be down and then you get the chance to check everything out, rigging, nav lights, mast head equipment etc and of course all the fastenings used to hold the fittings in place.
    As a consequence of last weeks article I did get a response from an ‘avid reader’, a retired metallurgist, who wrote and I quote "Your summary of the significant subject of crevice corrosion is fine - that is it is caused by a lack of oxygen in the crevice leading to a breakdown of the oxide film (chromium oxide) that gives stainless steel its corrosion resistant properties (see below image showing the results of crevice corrosion).
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    * However you also mention corrosion associated with welds. This is commonly known as weld decay and is brought about by the heat of the welding process causing the chromium present in stainless steel to combine with any carbon present to form chromium carbide - thus depleting the steel in the vicinity of the weld of its essential chromium. (Lose the chromium and you say goodbye to any corrosion resistance). This problem can be overcome, at a cost, by adding titanium to the alloy. Titanium has a greater affinity for carbon than does chromium, hence mopping up the carbon to form titanium carbide and thereby leaving the chromium behind to do its corrosion resisting job". Thank’s to Peter Baylis for putting me right!

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