Time for a re­think on zinc?

So com­mon is the use of zinc in an­odes that Amer­i­cans sim­ply call them ‘zincs’. But that could be about to change, Rob Melotti re­ports

Practical Boat Owner - - Contents -

Does it make eco­nomic sense to swap zinc an­odes with alu­minium?

The ris­ing price of zinc has led an­ode man­u­fac­tur­ers and cor­ro­sion ex­perts to ad­vise coastal and off­shore boat own­ers to fit alu­minium an­odes in­stead.

MGDuff, the mar­ket leader in sup­ply­ing an­odes to the UK and Europe, has an­nounced that due to com­mod­ity prices, alu­minium an­odes are now 20% cheaper than the zinc al­ter­na­tives.

A spokesper­son for the Ital­ian firm Tec­noseal agreed with the as­sess­ment as does Nigel Calder, au­thor of Boa­towner’s Me­chan­i­cal and Electrical Guide. “Given the op­por­tu­nity I ad­vise peo­ple to switch to alu­minium,” he said.

“Alu­minium an­odes have been on the mar­ket for the last 30 years or more, but since the price of zinc has dou­bled in the last two years, alu­minium is now cheaper,” says MG Duff com­mer­cial di­rec­tor James Beale. “And it’s the bet­ter an­ode ma­te­rial: it has a higher driv­ing volt­age so, un­like zinc, it works in salt wa­ter and brack­ish – and has a higher amp-hour per kg rate mean­ing it lasts longer too. An­other plus is that alu­minium an­odes are more en­vi­ron­men­tally friendly as, un­like zincs, they don’t con­tain cad­mium.”

Wider us­age

Princess Yachts has been us­ing alu­minium for a num­ber of years, but ac­cord­ing to Beale, Sun­seeker switched to alu­minium re­cently as well. The three most com­mon an­ode ma­te­ri­als are zinc, alu­minium and mag­ne­sium. For fresh wa­ter, which is the least con­duc­tive medium, the most re­ac­tive me­tal, mag­ne­sium, is re­quired. Alu­minium is the next most re­ac­tive and was al­ways spec­i­fied for brack­ish wa­ter and of­ten for com­mer­cial ap­pli­ca­tions in salt wa­ter. Zinc was al­ways the cheap­est ma­te­rial and ef­fec­tive enough for sea wa­ter, how­ever re­cent price in­creases have changed the equa­tion.

“For the vast ma­jor­ity of our zinc an­odes we make an alu­minium equiv­a­lent,” says MGDuff sales di­rec­tor Toby Bid­dle­combe. “The only rea­son peo­ple have been us­ing zinc in the past is the price. In the com­mer­cial world we’ve been us­ing alu­minium in sea wa­ter for over 30 years. We’ve spent a lot of time talk­ing to dis­trib­u­tors ex­plain­ing the alu­minium op­tion and it is slowly feed­ing down to the chan­d­leries. The lat­est cat­a­logue will in­clude plenty of ad­di­tional alu­minium mod­els, es­pe­cially in the shaft an­ode ranges where there hasn’t pre­vi­ously been the de­mand. There will be a por­tion of the mar­ket that sticks with zinc and a por­tion that is open to change and un­der­stands why we are of­fer­ing alu­minium now. For mail-or­der chan­d­leries, ship­ping alu­minium is much cheaper than zinc be­cause it’s so much lighter.”

I asked whether alu­minium an­odes in high salin­ity con­di­tions could ac­tu­ally over-pro­tect the me­tal com­po­nents un­der the wa­ter. Both rep­re­sen­ta­tives of MGDuff were adamant that it could not hap­pen and Nigel Calder agreed: “I haven’t heard of that be­ing an is­sue. The volt­age is al­most iden­ti­cal to zinc. If you get over-pro­tec­tion on a fi­bre­glass boat all it’ll do is lift a lit­tle bit of the paint off the cath­ode, so it’s not go­ing to do any phys­i­cal dam­age.

“The key thing is you can’t mix zinc and alu­minium in the same sys­tem... al­though you can have zincs in the engine (in the wa­ter-cool­ing sys­tem for ex­am­ple) and alu­minium on the hull be­cause from a cor­ro­sion per­spec­tive it’s two dif­fer­ent bod­ies of wa­ter.”

Degra­da­tion of a zinc an­ode shows it’s work­ing to pro­tect other met­als on a boat

This shaft an­ode has been do­ing its job

Alu­minium an­odes are likely to be more widely used

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