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Aluminium corrosion is the gradual decay of aluminium molecules into its oxides that degrades its physical and chemical properties. It means that while nascent aluminium will react with oxygen and water in the environment, the resulting compound will form a layer on the surface protecting the material underneath from further corrosion. This non-reactive oxide layer sticks well to the surface and does not flake off easily, similarly to stainless steel. Unlike deliberate processes like laser etching , aluminium anodising , or brightening, corrosion is a slow process and will occur over many months or years.
What makes aluminium unique though is that there are many different types of corrosion pathways. Understanding these different corrosion phenomena is the first step in applying control measures to reduce or completely prevent their occurrence. The most common form of aluminium corrosion. Atmospheric corrosion of aluminium occurs as a result of exposure to natural elements. Atmospheric corrosion can be divided into three subcategories. These are dry, wet and damp, depending on the moisture levels of the service environment.
As the moisture content can change quite a bit depending on your geographical location, some regions will observe greater corrosion than others.
Other environmental factors that affect the extent of atmospheric corrosion are wind direction, temperature and precipitation changes. Concentration and variety of pollutants in the air, closeness to large water bodies, etc. Atmospheric corrosion may be exacerbated if the design does not allow for drainage of moisture. Creating pockets of water for rain and condensation, for example, are harmful design flaws. Galvanic corrosion, also known as dissimilar metal corrosion can affect aluminium when it is physically or through an electrolyte connected to a noble metal.
The noble metal can be any metal that has lesser reactivity compared to aluminium. Reactivity of a metal depends on its position in the electrochemical series. The severity of corrosion will be worse if the other metal is further away from aluminium in the electrochemical series. The intensity of corrosion is highest at the intersection, where the two metals meet, and reduces as we go further away from this interface.
For example, if aluminium and brass are in contact or even close to each other and placed in seawater, a galvanic cell is formed. Then the aluminium part will corrode as it acts as the anode positive terminal. This can be a problem in boats where brass fittings may be close to aluminium fittings while they are both immersed in seawater.
The electrons flow from aluminium to brass through the seawater. This type of galvanic cell may be inadvertently formed in other service environments and lead to galvanic corrosion. Galvanic corrosion can be much quicker than normal atmospheric corrosion. Pitting corrosion is a surface corrosion phenomenon of aluminium metal characterised by small holes pits on the surface. Usually, these pits do not affect the strength of the product. Rather, it is an aesthetic issue but can lead to failure if surface appearance is critical.
Pitting corrosion generally occurs in regions where salt is present in the atmosphere, as the presence of chloride anions is responsible for it.
Sulphate salts can also cause pitting corrosion to some extent. The worst case of pitting corrosion is observed in the presence of alkaline and acidic salts. The existence of surface defects at grain boundaries and second phase particles is a precursor to pitting corrosion. Crevice corrosion is a form of localised corrosion process in materials.
Overlapping materials or unintentional design mistakes can lead to the formation of crevices. As a result, collecting seawater into those pockets can lead to crevice corrosion. Even a small gap between a bolt and the structure is enough for this type of corrosion to begin.
As time passes, aluminium from the material dissolves and precipitates into the seawater. This ionic aluminium absorbs oxygen from the surrounding air and hydroxide ions from the electrolyte, forming aluminium hydroxide. This oxygen reduction makes the crevice acidic in the presence of chlorides which accelerates the rate of corrosion. When it comes to aluminium, the grain boundary is electrochemically different compared to the alloy microstructure.
This causes an electrochemical potential set up between the two and an exchange of electrons takes place. There are multiple variations of intergranular corrosion based on thermochemical treatments and metallic structures.
It is also found to different degrees in different series of aluminium alloys. The 6xxx series alloys, for instance, are relatively less susceptible to this type of aluminium corrosion. The anodic path will vary with different alloy systems. While in the 2xxx series it appears as a narrow band on either side of the grain boundary , in the 5xxx series it is manifested as a continuous path along the grain boundary.
Like pitting corrosion, intergranular corrosion begins from a pit. However, it propagates far quicker along susceptible grain boundaries.
Exfoliation corrosion is a special type of intergranular corrosion found in aluminium alloys that have marked directional structures. This is predominantly evident in aluminium products that have undergone hot or cold rolling processes. It occurs along elongated grain boundaries in the microstructure. The term exfoliation comes from the fact that the corrosion product is more voluminous and gives the impression of lifting from the material surface.
This type of aluminium corrosion expands above the surface as well as sideways building up stresses in the product. In turn, this causes a wedging action initially at the surface before it migrates into the bulk of the product. Severe delamination takes place and the material weakens. Surface degradation results like pitting, flaking, and blistering can occur. The 2xxx, 5xxx, and 7xxx series are more prone to exfoliation corrosion due to their highly directional grain structures.
This makes the grain boundaries far more sensitive to intergranular corrosion. The susceptibility to exfoliation corrosion can be modified by using heat treatment methods to redistribute the precipitates. When corrosion takes place almost uniformly on an aluminium product surface, it is uniform or general corrosion. This type of corrosion can happen with products constantly exposed to a highly acidic or alkaline medium.
It may also occur in the presence of high electrochemical potential while the product is in an electrolyte. A typical example is rusting of an aluminium plate in acidic solution. Uniform corrosion is the result of the continuous shifting of anode and cathode regions in contact with the electrolyte which manifests as a uniform corrosive attack on the surface.
In high and low pH solutions, the oxide layer is also unstable and does not protect the metal underneath. The thickness of the material reduces and it will eventually dissolve completely. The attack is not completely uniform and there will exist peaks and valleys. The absence of small deep corroded areas is enough to term it as a general corrosion example.
Deposition corrosion occurs when a dissimilar metal gets deposited on the aluminium surface leading to serious localised corrosion. Imagine water flowing through a copper tubing. When the water flows through, it picks up copper ions. These copper ions are now in a solution. The ability to paint on and leave overnight makes the job of scraping the hull next day far easier than dry scraping. And finally, soda blasting is the least labour-intensive � but was extremely slow.
A bigger compressor would sort that out. If you can afford it, paying someone else to blast it off might not be a bad idea! You may be lucky this year. So sand off all this cement until none is left. If you can organise it, cleaning off the hull like this is actually a job best done soon after the boat comes out of the water.
Old antifouling and marine growth is easier to remove before it hardens out of the water over winter � and it saves time in the springtime. Once a steel keel has started to corrode and become pitted it can be hard work to restore its smooth finish.
The trick is to remove the paint first. Then you can begin to tackle any rust. Use a wire brush, angle grinder, drill attachment or soda blasting to begin to take the keel finish back to bright metal.
Apply a rust converter such as Fertan to get rid of the rust. An area most likely to show signs of corrosion is the keel root. On many boats this is marked by rusty dribbles, although in many cases this is just an irritation rather than being symptomatic of a deeper problem. If, as is most likely, the rust is just cosmetic, you can take the visible bits of keel back to bright metal before priming them with rust- remover. You can them prime them with epoxy primer to stop any water ingress, before sealing the gap with Sikaflex or another flexible underwater sealant.
Before you antifoul, consider a further coat of a tie coat primer like Primocon before you paint on the antifouling. The gap between the horizontal red dotted line and the pale cream section leached area is where the paint has eroded when the boat moves. The leached area is the part of the antifouling film which has lost all its biocide through leaching, but because the boat has not been used enough, the dead paint has not been eroded away. This area dries outand becomes brittle and unstable.
You can see a small crack in the film on the left side next to the yellow section which highlights how brittle the film can become. If the wet sanding is not done, it is likely that the old antifouling would flake off on the roller as the new paint is applied or the new paint will fall off underneath the water when the boat goes back in � neither of which are desirable!
You need to find out what will work best on your type of boat and which will be most effective in the area you keep her.
This erosion releases biocides in a controlled manner, providing a longer and more consistent performance throughout the season.
It also helps to reduce long-term build-up of antifouling. High-speed vessels, boats on drying moorings and racing boats require a hard film antifouling rather than an eroding one. This stops the increased pressure on the hull, whether from high speed, silt and mud or scrubbing, from removing the paint film too quickly. These antifoulings work because the biocide leaches out of the paint without the paint film eroding away. Another variety with a dedicated following is copper-based paints. These use particles of copper suspended in an epoxy coating and can last up to 10 years, saving you the annual ritual of antifouling.
Copper-based paints are significantly more expensive than standard antifoulings, but will last a great deal longer. Each manufacturer will be able to recommend a paint for your type of boat and level of fouling. For an expert view, you can download a map of local fouling levels and recommended paints here. As a general rule, what is best is a combination of where you are and what you are prepared to pay. The more you pay, the better the results regardless of where you are.
A tie-coat primer ensures the antifouling will stick properly. Most modern paints should be compatible with each other. If your existing paint is flaking, you need to prime any bare or flaky patches. Scrape away until all the flaky paint has come off, then feather the edges by wet sanding them. Generally speaking, self-eroding antifouling can be safely applied over the top of hard, but applying hard on self-eroding should be avoided.
But what actually goes into them? Here, as an example, is the contents of one popular paint, Micron Extra. It also absorbs UV which can attack the sensitive biocides, especially at the waterline. Careful selection of the extender pigments is particularly important as their particle size and shape greatly affects their performance. The evaporation of the solvents causes the paint to dry and form a film. This is the fun part. Ideally apply it in the late morning or early afternoon in calm, dry weather to avoid the risk of dampness or overnight dew.
Sometimes you may need to put a plastic sheet down to protect the ground, or wet the surrounding area to prevent dust rising onto your paint surface. How much you need to apply varies � but most paints require at least two coats. Biocides are heavy and will sink to the bottom of the tin, so stir vigorously and for longer than you think is probably necessary.
If you do, use a roller tray, with small amounts at a time. But be warned. There was an odd reaction, not unlike a stormy sky, and the stuff seemed to thicken up. Being a cheapskate I applied it to the boat PDQ. It dried looking rather like flock wallpaper. Replace the lid on the tin, too. For a really smooth finish, someone can follow you with a pad to smooth ridges and remove dribbles.
Rollers designed for emulsion will disintegrate very quickly, leaving bits of fluff all over the newly painted surface. Some people find that smaller radiator-size rollers with long handles are easier to use with such thick paint, but coverage will take longer with these. Painting under the cradle supports is a common problem for boat owners. The time-honoured solution is to paint around the support, then get the yard to move it before painting the bare areas. I keep my boat on a trailer over the winter and, impatient to get the job done, I chocked the boat up with a hefty fence post, plywood squares and wedges, before gingerly lowering and removing the pad.
I painted underneath the pad, ran out of time and wound it back up too soon. When it came to launching, the pad had stuck to the wet paint and left an imprint of itself on the antifouling.
A boot-top line really adds to the look of a boat. If your hull is always stained yellow at the end of the season, consider raising the boot top and antifouling level by a few inches. You have a number of options when it comes to paint.
Others use standard gloss paint, which looks better when dry and shiny, but will run a greater risk of fouling. The trick to getting it straight is to take your time. Try to get a stepladder or a low scaffolding at a height so the stripe to be masked is at eye level. Even better, have someone else look along the line while you stick it on. Pull out a long length of tape, and gently lower it to the hull while looking along it.
This will give you a much fairer, even line than if you stick it down at short intervals. Stand back every now and again to check. When you get to the bow, use a number of pieces of tape to keep the line straight and level as it goes around the curve. Unfortunately, no antifouling is going to keep your boat completely slime free.
So if you want a completely clean hull then choose the most appropriate antifouling and get out onto the water. Do you really need an excuse? This article was published in the April issue of Practical Boat Owner magazine. We get scrubbing to find out what effect a variety of teak cleaners have on the woodwork of a Sigma�. Exclusive: We take to the Solent to compare the performance of the new Raymarine Quantum pulse-compression radar with a same-size�.
Skip to content �. Home Practical. TAGS: antifouling practical Premium. Flaking where patches of paint have peeled off to leave ragged edges. Blisters where water is trapped under a bubble of paint � usually diagnosed by it being easy to pop. If you suspect the blisters are osmosis, get them checked out by an expert. Dry scraping 2.
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