Why is my stainless steel rusting?

Rust marks, rusting stainless steel and the stains that it leaves are some of the most received complaints. Its also difficult to claim from manufacturers under warranty, it usually (if claim successful) given as "goodwill", an exemption of usual warranty standards.

"Stainless" is perhaps a bad term, it should be "resistant" or "improved", I can assure you that no stainless steel on a boat is completely rustproof and a lifetime around salt water will eventually tarnish the finish, but if a quality stainless is used, it can be longer than the boats life for this to happen. In order to find out why and what can be done about it, read on.

What is stainless steel?

In its simplest form, stainless steel is an alloy of various metals that, when added to the base layer of steel (that stuff your rusty gates and railings are made of) make it shiny and rust resistant.

Chromium, nickel and molybdenum are added to steel to make it shiny and more importantly, corrosion resistant.

Grades of stainless steel

There are two main types of stainless steel used around boats, known as 304 and 316 types, in some places 304 and 316 are also known as A2 and A4 respectively.  The important difference is that 316 has molybdenum added, this makes it considerably more resistant to corrosion that 304, this is especially important for exterior fittings that will live their live exposed to the elements, such as handrails and stanchions.

If you buy an item from a chandlery or online and it does not specifically say it is made from 316 grade, assume it to be 304 or an inferior grade of stainless steel. Some grades of stainless steel are actually magnetic and are not suitable for use on boats. 304 can be used inside, as long as it is not submerged in bilge water, I would still use 316 for all screws, buts and bolts etc. and always 316 for exterior fittings.

Why does stainless steel corrode?

If I look around a boat (especially a newer one, more on this in a bit), the rusty orange stains usually have their origin in one of two places:

Rust has formed around both sides of these welds. These photos are taken from a bathing ladder. In fairness, this ladder is 17 years old, I have seen some new ones worse than this in one year.

Welded parts, passivation is the post production process of bathing or treating the stainless steel with an acid, this is especially important after welding or cutting where impurities can later cause corrosion. The passivation helps the chromium to be more resistant by removing contaminants, basically passivation makes the stainless steel unreactive to air and water.

The most common area I have seen the dreaded orange stains forming is on bathing ladders, the areas around the welds for the ladder rungs are notorious for corrosion.

Prevention of this is really down to the manufacturer, fabricator or welder. I have notice the quality of the more mass produced boats to be shocking, one season and parts can look like they had been underwater for decades, this is not limited to off brands bought online at extraordinarily cheap prices, well known brands have gone downhill in their quality control. Quality Vs bean counters, the demise of the boatbuilder

I am not a chemist or metallurgist, so cannot offer a cure for this, I can offer an important observation however. There are many different cleaners for stainless steel on the market, the most effective at removing the stains and seemingly keeping them at bay for a long time are those that contain a high acid content. The best I have ever used is a product made in Spain by Brikensa Laboratories in Spain. They advise not to use it as your habitual cleaner and obviously use gloves and eye protection, I usually use a brush to dab it into the area, then wash with plenty of fresh water.

My suspicion on this and, please if someone could confirm or deny this in the comments is that the acid in the cleaning product is passivating the stainless steel, removing any impurities and so preventing further corrosion forming?

Another solution is to stop buying rubbish quality fixtures and fittings from brand "X". The best that I routinely see come from Italy, they are not cheap and you might end up waiting a while as they produce many less than other countries, but in my experience the quality is unmatched right now.

Three examples of crevice corrosion,
  Left, inside engine room, nuts and bolts holding trim tab rams in place,
  Top right, behind a bathing platform, holding some exterior trim in place, Bottom right, stainless steel on a transom, you can see some sealer but not enough, the screw seems too small for the hole too.

Crevice corrosion, most often seen where screws or other fixings holding something in place, a typical area is the rubbing strake or band that runs around the whole of the boat to hide the join of the deck to the topside.

Crevice corrosion is formed when something corrosive (seawater) can enter in a tiny gap, such as where a screw is used, the seawater cannot then get out, the corrosive liquid stagnates and attacks our stainless steel. It usually starts with the tell tale orange stains and in severe cases can weaken the fastener sufficiently over time that it breaks.

The solutions to crevice corrosion are to not let it happen in the first place, some top tips are:

• Use the right sized fastener.

• Use an abundant amount of sealer (Sikaflex, 3M etc), rule of thumb, when you tighten down your fastener, the sealer needs to ooze out, if it doesn't, you haven't used enough.

• If possible, countersink the hole for the fastener, this shape is far more effective at making a good watertight seal than just a hole in a flat surface.

• Only use 316 grade stainless steel fasteners, they are far more resistant to corrosion.

• Make sure the boat is rinsed down with fresh water regularly.

Cleaning stainless steel

There's two aspects to cleaning stainless steel, just scheduled cleaning, maybe to restore a shine, then really going to work to remove corrosion stains. Notice, I said stains, you cannot remove corrosion, once the surface has been damaged, it cannot be restored easily, another reason to use 316 grade, the molybdenum helps to prevent the pitting so often seen.

Less is more

Every time you polish up stainless steel, a layer of the protective oxide is removed, so for heavy cleaning and restoring a shine I would recommend a metal paste polish, I prefer this method, a little goes a long way, liquids can make a mess. To keep it shiny, waxes made specifically for metals are available.

If you are working with hard water with a high mineral content, then any of the supermarket products for removing these stains are usually fine, they can contain an mild acid so be careful, for things like stanchion bases, hinges, I usually spray a bit on, then work it with a brush, before cleaning with fresh water and soap, dry properly to avoid more hard water spots appearing.

For heavier duty cleaning, I would again recommend using an abrasive paste, use fine brass wool to help restore the finish, do not use the more common steel wool, small pieces tend to break off and leave to more rust. For rust marks, the aforementioned Brikensa stainless clean, being very careful.

They don't make em like they used to

Now you know a bit more about stainless steel. I wanted to reinforce a point I have written quite a bit about before, quality and quality control. I have noticed more and more newer boats with problems with the stainless steel, whether its the providers of the stainless using the bottom of the barrel alloys or the boatbuilders demanding it in the name of profit I don´t know.

It is painful for the consumer to see their new boat already rusting, we haven't had that problem on vehicles in probably three decades. The buyer thinks they have bought an inferior quality product and the dealer could have a problem claiming it under warranty.

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