Marine Corrosion 101
Two local companies, All Marine Spares and Marine Protection Systems, are spreading the word about the latest advances in corrosion protection.
For the past year or so, two local suppliers have been touring the country giving presentations to the industry on cathodic protection and corrosion prevention for boats. It’s a hugely important topic, not least because of the consequences of getting it wrong and the impact it can have in terms of poor boating performance, increased running costs and expensive repair work. Corrosion prevention and remediation takes up thousands of man-hours each year and is a major cost-factor in many boating activities. Making sure that boats are properly protected against the ravages of the marine environment is critical on many different levels – economic, environmental and even from a safety standpoint.
And yet, despite its importance, many boaters and the service providers looking after them know very little about how cathodic protection works and how they can best safeguard their boats. A lot of current thinking on this topic is rooted in historical practices which are no longer considered the most effective or economical methods of protecting boats. In recent years, corrosion protection technology has undergone some significant advances but, in many cases, this shift has yet to make itself felt throughout the industry. The risk is that local marine businesses are continuing to fall behind.
To help address this issue, Jason Mitchell at All Marine Spares (AMS) and Brian and Jessica Gatt at Marine Protection Systems (MPS) have embarked on an education program to help inform the local boating community, both boaters and service providers, about the different types of corrosion and what can be done to manage them. They’ve been out talking to groups at boatyards, marinas and sailing clubs as well as presenting at boat shows as part of a campaign to educate users and spread the word about what boaters should be doing to combat corrosion.
Both companies possess a lot of knowledge in this field derived from many years’ experience in manufacturing and supplying anti-corrosion technology for a range of applications. MPS supplies the Maddox range of anodes as well as other corrosion protection systems such as a galvanic isolator called the Electrolysis Blocker designed to protect boats from damaging DC currents. AMS supplies the Martyr range of anodes manufactured by Canada Metal Pacific (CMP) as well as distributing the Propspeed foul release coating system. Together, they have access to a huge amount of useful information covering corrosion protection and anti-foul technology on all types of vessels ranging from small tinnies up to large commercial vessels, ferries and cruise ships.
More than just metal
Cathodic protection is all about managing how the materials used to construct a boat interact with its day-to-day environment ie sitting in water. Primarily this involves an understanding of how different types of metal behave in different types of water and the impact this interaction has on other materials too, including wood. It’s not just about protecting the metallic parts of the boat – the props and shafts, trim tabs and swim platforms – but also how other elements can be affected too, such as the performance of anti-foul coatings in combating marine growth.
No doubt there are many boaters – and indeed marine service providers and even boat builders – who come across anti-foul coatings which have failed or lost adhesion rapidly and their immediate response is to blame the coating itself, not realising that its failure may be due to the type and number of anodes being used, or some other failure of the cathodic protection. Equally, there are undoubtedly many boaters who, at their annual haul-out, view the impacts of marine growth and corrosion on their pride and joy and reluctantly accept that is just part and parcel of keeping a boat in water. Many marine businesses too rely on a constant procession of boats being lifted for scrubbing and cleaning and coating, unaware that, in the worst cases, this work is avoidable.
One of the key messages being presented by AMS and MPS is that there are solutions to these problems and that it is in the long term interests of boaters, commercial operators and the industry at large to learn about and implement them.
So what are some of the changes taking place in the field of corrosion protection?
Rethink your zinc
The biggest change in marine cathodic protection which has been happening over several decades now is the shift from zinc-based to aluminium-based anodes. This is not a new development but it’s one which is gaining momentum and likely to lead to lasting changes in terms of how boats of all sizes are maintained. CMP is one of the leading manufacturers leading this change with its Martyr Premium range of aluminium anodes distributed by AMS.
The principle of cathodic protection and the humble sacrificial anode is well-established and underpinned by the known galvanic properties or nobility of different metals. When two metals are submerged in an electrolyte such as seawater while also being electrically connected by an external conductor – such as a bolt or cable– then the less noble metal will experience galvanic corrosion. There are other factors at play but the idea of the sacrificial anode is that, being less noble, it will corrode at a faster rate than the protected metal.
Traditionally, zinc has been used as a base metal for sacrificial anodes because of the position it occupies in the galvanic series. It is one of the least noble metals which means it can be used as an anode in combination with many of the metals found in boats such as brass and bronze. The problem with zinc, however, is that in order to function effectively as an anode, it needs to be mixed with other metals in minute quantities, including cadmium, a known carcinogenic and environmental hazard. As the anode breaks down in the water – as intended – the cadmium is released into the waterway, potentially poisoning the marine environment and posing a health risk. In the US, several jurisdictions have already moved to ban the use of zinc anodes for this reason and although there have been no moves by environmental authorities to do the same locally, it’s probably only a matter of time.
Increasingly aluminium anodes are being promoted as a replacement for zinc, and not just because aluminium is a non-toxic metal. Aluminium is also lighter, which means less weight on the boat as well as making the anodes easier to transport and store. Aluminium anodes have also been shown to perform better, lasting up to 50% longer than zinc anodes of comparable size in most types of water, from brackish to saltwater. These days they also cost less to produce than zinc anodes (which is not to say you can’t buy cheap zinc anodes but – buyer beware - they are unlikely to be made to the correct specifications).
Cheaper to produce, less harmful to the environment and better performing – what’s not to like about aluminium anodes? Some sectors of the industry clearly agree. Outboard manufacturers, for instance, now use aluminium anodes as standard on all their engines. What tends to happen though is that when those anodes need replacing, zinc ones are put on instead. Why? Because that’s the way it’s always been done. Boaters go to the store or boatyard, ask for new zincs and that’s what they are given, often without being made aware there is an alternative which is cheaper, longer lasting and better for the waterways they use.
The onus is on the industry, from manufacturers through to the chandlers, brokers and dealers, to educate the public about new ways of protecting their valuable assets. Of course, there is always the cynical argument that helping boaters to better look after their vessels means there is less work to be done at haul-out, fewer expensive repairs to be carried out and fewer anodes sold. That may be true in the short term but, from a consumer’s point of view, who would you rather have service your boat: someone who is up to speed on the latest protection systems and able to give good advice to help save money, or someone who charges more for an inferior service? Ultimately it is the boaters who will decide.
Protecting wooden boats
The Maddox anodes manufactured by MPS are different again to the Martyr aluminium anodes, engineered for specific circumstances using a patented composite alloy which occupies a slightly different position in the galvanic series, closer than zinc or aluminium to the metals it is protecting. This makes them less aggressive than traditional zinc anodes and ideal for protecting the bronze and stainless steel parts on fibreglass and wooden vessels. It’s clever technology which addresses the fact that not all boats are the same and that a one-size-fits-all solution, eg zinc, is not necessarily the right approach.
For instance, at this year's Australian Wooden Boat Festival in Hobart, Brian Gatt gave a presentation on cathodic protection and wooden vessels, meeting with many owners of beautiful wooden vessels who were using zinc anodes and yet still battling corrosion issues, unaware that there are safer, more cost-effective, less damaging alternatives.
The forums held by AMS and MPS have highlighted the knowledge gap that exists in the industry around cathodic protection and the misconceptions that can arise. For instance, although aluminium anodes last longer than zinc, there should still be some degree of degradation over time – that’s how they work. Some boat owners, however, seem unaware that if the anodes are in perfect condition every time the boat is hauled, far from it being a good thing, it’s a sign that the anodes may not be working at all. At the other extreme, there are boats which are overprotected - less is more sometimes - which can cause different problems such as paint blasting around hull fittings and premature coating failure. On wooden boats, it can cause alkaline degradation and timber rot.
Some boat owners also think they can melt down old anodes to forge their own new ones, as if they were lead sinkers, unaware that, in order to work properly, anodes need to be made to very precise specifications in strictly-controlled manufacturing conditions. Unscrupulous suppliers have also been known to do the same thing using low grade scrap aluminium instead of high-grade alloys.
Part of the problem is that cathodic protection is not taught in any great detail as part of any marine training so established industry practices tend to get passed on from one generation to the next without any real understanding of what they are supposed to do. Even boat manufacturers which boast the latest designs and construction methods will still routinely install zinc anodes on multi-million dollar vessels because that’s what they are set up to do, and always have done. The consequences can be alarming and potentially very expensive; in the worst cases of electrolytic corrosion, it’s been known for metal parts to literally dissolve in water in the course of a single voyage. Not a great experience for any boat owner.
The good news is that the information is out there for those willing to look. AMS and MPS are planning to hold more presentations this year and there are opportunities for service providers, boatyards and marinas to work with them, either in-house or with boat owners. The companies will also be repeating their Anode Zone at this year’s Sanctuary Cove International Boat Show, offering specialised advice on cathodic protection for all show visitors and trade exhibitors. They have also produced some handy wall charts to help with correct anode selection and to identify common corrosion problems – contact them to get your copy.
What's your corrosion
Two of the main types of corrosion found on vessels are galvanic and electrolytic corrosion.
Galvanic corrosion is where two dissimilar metals submerged in the same electrolyte (such as seawater) are connected either mechanically or electrically. The more active (softer) metal will corrode to protect the nobler (harder) metal. A simple example of this is when a bronze propeller will corrode to protect a stainless-steel shaft. This type of corrosion can be prevented by using sacrificial anodes or impressed current systems such as MerCathode or Volvo ACP.
Electrolytic corrosion or electrolysis is caused by the presence of a stray electrical current such as faulty or damaged wiring which is leaking current, incorrectly installed electrical equipment or a shore power connection which is not isolated. Depending on the type of stray current - alternating current (AC), direct current (DC) or radio frequencies (VHF, HF) – this type of corrosion can affect coatings, resulting in premature failure and increased marine growth. It can also cause the aggressive disintegration of underwater metals, affect metallic components above the waterline, eg tea staining of stainless steel stays, increase or stop anode consumption by reversing or manipulating the effects of cathode/anode action, and cause corrosion and rapid anode consumption within cooling systems.
The extent of any corrosion may be exacerbated by a number of environmental factors including water temperature, salinity, water movement, humidity, heat, and exposure to oxygen. Protection against the corrosion may involve a combination of methods such as applying protective coatings, selecting the right anode type, adding or subtracting anode mass and adjusting the placement, and electrical protection and design considerations.
This article was first published in the March-April 2019 issue of Marine Business magazine.