Stainless Steel and Grades

Posted By: Mlcreek

Stainless Steel and Grades - 08/03/16 02:50 PM

To All,

Saw this article in Boat US discussing different grades of stainless steel.

Not So Stainless Steel

By Alison Mazon

Why do they call it stainless if it still rusts?

Rustly stainless steel fasteners

A couple of boat show attendees stopped at my booth and looked at the section of a sailboat bow and foredeck we had on display. My astute associate had rescued it from a boat that was being scrapped due to years of neglect. A number of the fasteners securing cleats, anchor-roller bracket, pulpit mounts, stem fitting, and so on, were heavily corroded especially on the underside of the foredeck. One turned to the other and said with great certainty, "Those bolts aren't stainless steel," and off they walked. Unfortunately, that encounter is emblematic of the level of misunderstanding about stainless steel and corrosion.

In fact, the fasteners used by that sailboat manufacturer were likely a good quality 300 series stainless steel. So, why were the fasteners on this bow section, and so many other pleasure boats, so heavily corroded? The answer is as hidden as the source of the corrosion itself. First, we must learn a little about what makes stainless steel stainless.

The science of stainless steel is incredibly complex, and a deep understanding is a formidable task. Fortunately, the average boat owner doesn't need to know much beyond the basics to be able to prevent or to recognize and correct most stainless-steel corrosion. Most marine-grade stainless used on production boats is from the 300 series. These stainless steels are suitable for a wide range of marine applications. Type 304 is a good, multipurpose steel. The Gateway Arch in St. Louis is clad with 304. Type 316 and 316L have a slightly higher nickel content and added molybdenum to improve their corrosion resistance over 304 especially with regard to pitting and corrosion in chloride environments. Type 316L has a lower carbon content than 304 and 316 to avoid carbide precipitation in welds.

The single most important fact about stainless steel that all boat owners should know is that the chromium in the steel combines with oxygen to form an invisible surface layer of chromium oxide that prevents further corrosion from spreading into the metal's internal structure. According to Anne Marie Helmenstine, Ph.D., "This protective film will self-repair if damaged, if sufficient oxygen is present. Stainless steels have poor corrosion resistance in low-oxygen and poor circulation environments. In seawater, chlorides from the salt can attack the passive film more quickly than it can be repaired in a low oxygen environment."

With this basic knowledge, what makes decent quality stainless steel corrode aboard your boat? For simplicity, let's assume that whenever the word stainless is used, we are referring to a grade-300 series such as 304, 316, or 316L, as these are typically very suitable for a majority of marine exterior applications. Also, in the spirit of simplicity, we will use the general term "corrosion" to represent the many different types of corrosion that can attack stainless steel. Differentiating between them can require a metallurgist and usually isn't necessary for our purposes.

Stainless steel is "normally" corrosion resistant if the correct grade is selected, properly fabricated and finished, and correctly installed in the appropriate application. Most U.S. production boats were built with appropriate grade stainless steel and are reasonably well assembled. So, what caused the fasteners on our section of bow to corrode so heavily? The short answer is the same as the reason this vessel was cut up and sent to a landfill: lack of maintenance.

Corroded stainless steel cleat fastenersThese stainless cleat fasteners corroded over time when seawater leaked through the deck. (Photo: Alison Mazon)

The knowledge of what causes stainless steel to corrode is one part of the equation. Knowledge of basic vessel construction of decks, cabin tops, and **** (horizontal surfaces) is the other crucial part. The horizontal surfaces on most production boats are a composite or sandwich consisting of an inner and outer layer of fiberglass with an inner core of plywood, end-grain balsa, or foam. The upside of this type of construction is that it adds rigidity with relatively light weight; plus it provides excellent thermal and acoustic insulation. The downside is that the core can absorb water if the caulk used to seal the deck fittings breaks down. Since very few boatbuilders isolate this core from the fasteners, it will absorb moisture if there are any leaks. And, it holds the moisture in contact with the fasteners just like a sponge.

Deck hardware requires periodic recaulking to prevent leaks. This is especially true of hardware subject to high or sudden loads like stanchion and pulpit bases, anchor rollers, windlasses, cleats, and chainplates. Time and flex breaks the bond between the caulk and the hardware. This allows water to leak alongside the fastener where it passes through the structure. This moisture is held alongside the stainless steel where it penetrates the deck or cabin top. Over time, the oxygen naturally contained in the water is consumed, and the chromium oxide film on the fastener is eroded. The result is the stainless switches gears from passive to active. As the iron in the fastener corrodes, it expands and begins to push in all directions resulting in the rust blobs on the underside of our chunk of foredeck rather than as the result of using non-stainless steel fasteners. Boat owners often describe the staining around the base of deck hardware as a "blush" caused by saltwater. While a very light surface staining may fit this definition, in most cases it is the result of water getting in under hardware. Visible corrosion requires immediate action.

The lack of oxygen in the presence of an electrolyte (water) is only one reason stainless steel goes bad. A number of other factors can cause stainless steel to corrode or fracture. Some examples can be found on imported vessels of the 1970s and 1980s. Much of the stainless was of questionable heritage; welding was often improperly performed; installation techniques were often poor; and water leaks seem to have been engineered in. Corroded hose clamps, pulpits, hand rails, mufflers, and tanks were commonplace. A majority of them corroded in or adjacent to the welds.

Boat owners must be vigilant when purchasing stainless steel. Fasteners, clamps, valves, and other fittings are sold by a wide variety of vendors. Don't be lured in by low price or the convenience of a neighborhood store. Specify the grade stainless you want. Ask the clerk to verify the grade he is selling you. Take a small magnet with you when you shop. 300-series stainless steel will be nonmagnetic or very close to it. When checking hose clamps, test the clamp screws as well the bands. Many "stainless steel" hose clamps have plated-steel screws that will turn into rust blobs allowing the clamps to take a swan dive into the bilge. Quality costs. But, it only costs once.

Published: August 2016

Posted By: waterbug_wpb

Re: Stainless Steel and Grades - 08/04/16 02:14 PM

.... which invariably leads to the question of which sealant/bonding agent to use for the interface between the stainless hardware and the fiberglass deck?

There was an argument made a while ago about butyl tape (black or white) and it's merits of flexibility and resistance to drying/rotting. It had no adhesive abilities, but that is not necessary in situations where you've got something bolted through-deck.

But the epoxy bedding of through-deck fittings (hollow out the core material in the deck, fill with thickened epoxy, and re-drill the hole) is golden advice. Adds strength to that area as well as (according to your article) reduce chance of core moisture absorption..

Check your shroud base attachments and bridle bases at the hull Forrest. That's about the only place you might find crevice corrosion on your boat.

There seemed to be enough air circulation in the stern for the rudder bolts and beam bolts that I didn't notice any corrosion when I changed them after about 10 years..
Posted By: Ventucky Red

Re: Stainless Steel and Grades - 08/04/16 10:57 PM

T304 grade made in USA is probably the best you can go with.... unless you want to start getting exotic..

Ask a Piper owner about this! And then run......
Posted By: northsea junkie

Re: Stainless Steel and Grades - 08/05/16 06:37 PM

Mlcreek, you sound like a expert. So what can you tell me about stainless steel grade 630.

I ordered five years ago in the States a rod of this material for making ultra strong clevispins and pins for my rudderlock system.

They worked great; no corrosion at all and much more stronger then 360.
Posted By: Mlcreek

Re: Stainless Steel and Grades - 08/05/16 06:43 PM

No sir, not an expert at all in the field of metallurgy, insurance claims yes, but not steel. I simply saw the article in Boat US and gave them all credit and or disavow anything wrong. Just simply wanted to pass it along as useful information.
Posted By: waterbug_wpb

Re: Stainless Steel and Grades - 08/08/16 01:56 PM

yes, you should expand on the concept of "consequential damage" and it's relevance to a boat policy... Seems to be a hot topic.

So your shroud base has crevice corrosion, parts, and down goes your mast in pieces shredding the main and spinnaker...
Posted By: brucat

Re: Stainless Steel and Grades - 08/08/16 07:35 PM

That would seem like neglect to me ("deferred" maintenance). I'd be surprised if any of that were covered (why should our premiums be used to cover the stupidity of others, or worse, those working the system for an easy upgrade?)...

Posted By: catman

Re: Stainless Steel and Grades - 08/08/16 10:05 PM

Not sure how it can be neglect when a bow or shroud tang breaks due to corrosion. I've repaired several and have had to remove the deck or cut into it to gain access. Hardly a maintenance item.
Posted By: brucat

Re: Stainless Steel and Grades - 08/09/16 01:46 PM

Sorry, I thought you meant the corrosion was visible and started from a crack that was also visible. I've had one break before I knew what to look for.

I agree, if it's corroded inside the hull material (and not showing on the exposed portion of the fitting), you're not going to avoid that without routine disassembly, which most folks would never do.

There are often (not always) warning signs: rust stains on the fiberglass near the fittings is usually obvious.

Posted By: Mlcreek

Re: Stainless Steel and Grades - 08/09/16 06:53 PM

Since we are speaking of first party coverages, you have to refer back to the policy to determine coverage. Some policies are all risk, except for named exclusions. So, yes you can be negligent in your maintenance and collect. You don't have coverage for the cause of the loss, but do for the resulting damages. Example: you need a new roof and don't replace it, water leaks in and damages structure and contents, the structure would be covered except for the (roof), but contents aren't covered. As to the boat you would need to read the policy as to causes of loss. Yes, if it were to snap off below deck with no knowledge, you would need to see what the cause of loss requirements are in the policy but other resulting damages would be covered, neither the item or cost to repair it is.
Posted By: catman

Re: Stainless Steel and Grades - 08/09/16 09:04 PM

Yea, almost everyone I've seen have busted below the surface. Knowing that I think it may help to open the area around the tang and and acid wash the rust. Might help.

Some tangs are bedded using a light weight filler that seems to absorb water. No doubt contributing to the problem.
Posted By: jkkartz1

Re: Stainless Steel and Grades - 08/09/16 09:06 PM

Most policies also exclude design and manufacturing defects.
Posted By: Mlcreek

Re: Stainless Steel and Grades - 08/10/16 12:40 AM

true, but depending on policy not resulting damages.
Posted By: brucat

Re: Stainless Steel and Grades - 08/10/16 03:48 AM

You can't fix stupid. But, apparently we can all chip in (via our premiums) and pay for it? Nice...

True story: A local Hobie sailor called his insurance company and asked them to pay for him to cut down an old tree, on the idea that it was overgrown, leaning towards the boat and house, and of unknown structural condition. Of course, they declined.

He then asked what would happen if the tree fell in a storm. They would pay not only to remove the tree, but also to fix or replace anything it hit.

How does any of that make sense?

Posted By: Mlcreek

Re: Stainless Steel and Grades - 08/10/16 12:25 PM

If the tree is struck by lightning and falls on the boat, then yes the tree is covered (and it's replacement to limits coverage). If the wind blows the tree onto the boat, the cost to remove the tree from the boat might be covered, but not the cost of tree replacement. Wind storm isn't a listed peril for tree coverage.
Posted By: waterbug_wpb

Re: Stainless Steel and Grades - 08/10/16 08:43 PM

Originally Posted by brucat
How does any of that make sense?


Life making sense.... that's a pretty tall order there, Mike smile

Like hot dogs coming in packs of 12, but buns only in packs of 8

Or how a fitted sheet is folded.

Or Chewbacca living on the planet Endor...

It does not make sense...
Posted By: catman

Re: Stainless Steel and Grades - 08/11/16 03:32 AM

If it happens on a 20 year old boat is it a defect or just old age?
Posted By: waterbug_wpb

Re: Stainless Steel and Grades - 08/11/16 03:36 PM

well, wear and tear is not a covered peril. Neither is "internal vice" or defect.

As Forrest said, it can be found that the original part/cause is not covered, but additional damage caused could be?
Posted By: catman

Re: Stainless Steel and Grades - 08/12/16 12:06 AM

Of course they don't have to write policy's on a 20 year old boat either. If they do.....
Posted By: Mlcreek

Re: Stainless Steel and Grades - 08/12/16 01:00 PM

They write policies on 20,30 year old and older cars don't they? Its all relative as to cost of coverage versus value of the boat and or value to you.
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