{"product_id":"magnesium-bolt-on-hull-anodes","title":"Magnesium Bolt-On Hull Anodes — Freshwater","description":"\u003ch2\u003eMagnesium Bolt-On Hull Anodes — Freshwater\u003c\/h2\u003e\n\u003cp\u003eMagnesium hull anodes that bolt on rather than weld, for GRP and for steel hulls where hot work is not wanted. Includes a bolt-on disc and a button anode.\u003c\/p\u003e\n\n\u003ch3\u003eWhy magnesium in fresh water\u003c\/h3\u003e\n\u003cp\u003eFresh water is far more resistive than sea water, and zinc cannot drive enough\ncurrent through it — it polarises, skins over and stops protecting while still\nlooking perfectly serviceable on the hull. \u003cstrong\u003eMagnesium is the fresh-water\nalloy.\u003c\/strong\u003e If your vessel works the Great Lakes, the St Lawrence, or any\ninland waterway, zinc is the wrong metal no matter what the chandlery stocks.\u003c\/p\u003e\n\n\u003ch3\u003eSizes and weights\u003c\/h3\u003e\n\u003ctable\u003e\n  \u003cthead\u003e\u003ctr\u003e\n\u003cth\u003eSize (L × W × D)\u003c\/th\u003e\n\u003cth\u003eWeight\u003c\/th\u003e\n\u003cth\u003eNotes\u003c\/th\u003e\n\u003c\/tr\u003e\u003c\/thead\u003e\n  \u003ctbody\u003e\n    \u003ctr\u003e\n\u003ctd\u003e\n\u003cstrong\u003e318 × 65 × 32 mm\u003c\/strong\u003e\u003cbr\u003e\u003cspan\u003e(12.5 × 2.6 × 1.26 in)\u003c\/span\u003e\n\u003c\/td\u003e\n\u003ctd\u003e0.7 kg (1.54 lb)\u003c\/td\u003e\n\u003ctd\u003e—\u003c\/td\u003e\n\u003c\/tr\u003e\n    \u003ctr\u003e\n\u003ctd\u003e\n\u003cstrong\u003e457 × 102 × 64 mm\u003c\/strong\u003e\u003cbr\u003e\u003cspan\u003e(18.0 × 4.0 × 2.52 in)\u003c\/span\u003e\n\u003c\/td\u003e\n\u003ctd\u003e3.5 kg (7.72 lb)\u003c\/td\u003e\n\u003ctd\u003edeep section\u003c\/td\u003e\n\u003c\/tr\u003e\n    \u003ctr\u003e\n\u003ctd\u003e\n\u003cstrong\u003e229 × 229 × 25 mm\u003c\/strong\u003e\u003cbr\u003e\u003cspan\u003e(9.0 × 9.0 × 0.98 in)\u003c\/span\u003e\n\u003c\/td\u003e\n\u003ctd\u003e2.25 kg (4.96 lb)\u003c\/td\u003e\n\u003ctd\u003ebolt-on disc\u003c\/td\u003e\n\u003c\/tr\u003e\n    \u003ctr\u003e\n\u003ctd\u003e\n\u003cstrong\u003e140 × 140 × — mm\u003c\/strong\u003e\u003cbr\u003e\u003cspan\u003e(5.5 × 5.5 in)\u003c\/span\u003e\n\u003c\/td\u003e\n\u003ctd\u003eon request\u003c\/td\u003e\n\u003ctd\u003ebutton anode\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003c\/tbody\u003e\n\u003c\/table\u003e\n\u003cp\u003e\u003cem\u003eDimensions in millimetres with inches alongside. Weights are nominal and\nconfirmed at quotation.\u003c\/em\u003e\u003c\/p\u003e\n\n\u003ch3\u003eAlloy — ASTM B843\u003c\/h3\u003e\n\u003cp\u003eMagnesium anodes are cast to \u003cstrong\u003eASTM B843\u003c\/strong\u003e, which covers two\nalloys, and the choice between them matters more than most people expect:\u003c\/p\u003e\n\u003ctable\u003e\n  \u003cthead\u003e\u003ctr\u003e\n\u003cth\u003eAlloy\u003c\/th\u003e\n\u003cth\u003eOpen circuit potential\u003c\/th\u003e\n\u003cth\u003eUse\u003c\/th\u003e\n\u003c\/tr\u003e\u003c\/thead\u003e\n  \u003ctbody\u003e\n    \u003ctr\u003e\n\u003ctd\u003e\n\u003cstrong\u003eAZ63\u003c\/strong\u003e — standard potential\u003cbr\u003e(Al 5.3–6.7%, Zn 2.5–3.5%, Mn 0.15–0.7%)\u003c\/td\u003e\n        \u003ctd\u003e≈ −1.55 V\u003c\/td\u003e\n        \u003ctd\u003eThe default. Steel and GRP hulls in fresh water, and \u003cstrong\u003ethe right\n        choice near aluminium\u003c\/strong\u003e.\u003c\/td\u003e\n\u003c\/tr\u003e\n    \u003ctr\u003e\n\u003ctd\u003e\n\u003cstrong\u003eM1C\u003c\/strong\u003e — high potential\u003cbr\u003e(Mn 0.5–1.3%, Fe ≤0.03%, Ni ≤0.001%, Cu ≤0.02%)\u003c\/td\u003e\n        \u003ctd\u003e≈ −1.70 to −1.75 V\u003c\/td\u003e\n        \u003ctd\u003eHigh-resistivity water and soil, where the extra driving voltage is\n        needed to push current.\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003c\/tbody\u003e\n\u003c\/table\u003e\n\n\u003ch3\u003eA caution on aluminium\u003c\/h3\u003e\n\u003cp\u003eMore driving voltage is not better. \u003cstrong\u003eAluminium suffers alkali attack and\nhydrogen blistering of its paint when driven more negative than about\n−1200 mV\u003c\/strong\u003e, and high-potential magnesium will do exactly that. On aluminium\nhulls, outdrives and sterndrives use the standard-potential alloy and watch the\nprotection potential. Blistering paint and unusually fast anode wastage are the\nsymptoms of over-protection, not of needing more anode.\u003c\/p\u003e\n\n\u003ch3\u003eHow magnesium compares\u003c\/h3\u003e\n\u003ctable\u003e\n  \u003cthead\u003e\u003ctr\u003e\n\u003cth\u003e\u003c\/th\u003e\n\u003cth\u003eZinc\u003c\/th\u003e\n\u003cth\u003eAluminium\u003c\/th\u003e\n\u003cth\u003eMagnesium\u003c\/th\u003e\n\u003c\/tr\u003e\u003c\/thead\u003e\n  \u003ctbody\u003e\n    \u003ctr\u003e\n\u003cth\u003eWater\u003c\/th\u003e\n\u003ctd\u003eSalt\u003c\/td\u003e\n\u003ctd\u003eSalt and brackish\u003c\/td\u003e\n\u003ctd\u003e\u003cstrong\u003eFresh only\u003c\/strong\u003e\u003c\/td\u003e\n\u003c\/tr\u003e\n    \u003ctr\u003e\n\u003cth\u003eCapacity\u003c\/th\u003e\n\u003ctd\u003e≈ 780 A·h\/kg\u003c\/td\u003e\n\u003ctd\u003e≈ 2000–2500 A·h\/kg\u003c\/td\u003e\n\u003ctd\u003e≈ 1100 A·h\/kg\u003c\/td\u003e\n\u003c\/tr\u003e\n    \u003ctr\u003e\n\u003cth\u003eRelative life, same size\u003c\/th\u003e\n\u003ctd\u003e100\u003c\/td\u003e\n\u003ctd\u003e130–150\u003c\/td\u003e\n\u003ctd\u003e≈ 30\u003c\/td\u003e\n\u003c\/tr\u003e\n    \u003ctr\u003e\n\u003cth\u003eSpecification\u003c\/th\u003e\n\u003ctd\u003eASTM B418\u003c\/td\u003e\n\u003ctd\u003eno composition standard — qualified by test\u003c\/td\u003e\n\u003ctd\u003eASTM B843\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003c\/tbody\u003e\n\u003c\/table\u003e\n\u003cp\u003eMagnesium drives harder than either of the others but spends itself doing it —\nroughly a third of the life of zinc for the same size. That is the trade, and it\nis why magnesium belongs in fresh water and nowhere else. Put it in salt and it\nwill waste in a season.\u003c\/p\u003e\n\n\u003ch3\u003eSizing and replacement\u003c\/h3\u003e\n\u003cp\u003eSizing comes from \u003cstrong\u003eABYC E-2\u003c\/strong\u003e for boats and\n\u003cstrong\u003eDNV-RP-B401\u003c\/strong\u003e for ships. Whatever the calculation says,\n\u003cstrong\u003ereplace at about 50% wasted\u003c\/strong\u003e — output falls away as surface area\nis lost.\u003c\/p\u003e\n\u003cp\u003eWrong alloy for your water? See\n\u003ca href=\"\/collections\/saltwater-anodes\"\u003esaltwater anodes\u003c\/a\u003e — zinc to ASTM B418\nType I and aluminium.\u003c\/p\u003e","brand":"Dibblee","offers":[{"title":"318 x 65 x 32 mm — 0.7 kg","offer_id":50199409754389,"sku":null,"price":0.0,"currency_code":"USD","in_stock":true},{"title":"457 x 102 x 64 mm — 3.5 kg","offer_id":50199409787157,"sku":null,"price":0.0,"currency_code":"USD","in_stock":true},{"title":"229 x 229 x 25 mm disc — 2.25 kg","offer_id":50199409819925,"sku":null,"price":0.0,"currency_code":"USD","in_stock":true},{"title":"140 mm dia button — 5-1\/2 in","offer_id":50199409852693,"sku":null,"price":0.0,"currency_code":"USD","in_stock":true}],"url":"https:\/\/www.kentershackle.com\/products\/magnesium-bolt-on-hull-anodes","provider":"Kenter Shackles","version":"1.0","type":"link"}