Stainless Steel, Does it corrode?

A metal that is very near and dear to most boaters hearts, is the boater bling, stainless steel.  Someone had asked me today why his stainless steel hardware had rust stains, my answer was, stainless steel corrodes.  He was shocked to here this.  Many believe that this metal does not corrode.  While it does not corrode like it's softer cousin, carbon steel, it still has the ability to discolor and also corrode. 

So, if you have been asking "Why are my zincs disappearing?",  the next topic on Corrosion will help you find out why but also how to help protect your precious boat from many of the different threats.

Stainless Steel (here comes the blah, blah, blah) is defined as a steel alloy with a minimum of 10.5 - 11% chromium content by mass.  Stainless steel does not stain, corrode, or rust as easily as ordinary steel, but it is not stain-proof.  It is also called corrosion-resistant steel.  Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion and blocks corrosion from spreading into the metal's internal structure.

The stainless steel alloys with AISI (American Iron and Steel Institute) numbers of 300 or above contain more nickel than those with numbers below 300, and have better seawater resistance. These 300-series alloys are very corrosion resistant, and are used for architectural applications, boat topside fittings, and household goods such as sinks and silverware. The 300-series alloys will usually show no appreciable corrosion in fresh water or sea water atmosphere (this does not include saltwater submersion).

Stainless steels get its corrosion resistance by the formation of a very thin surface film, called the passive film, which forms on the surface in the presence of oxygen. Therefore, stainless steel usually has poor corrosion resistance in low-oxygen environments, such as under water, in mud, or in tight places, called crevices.  This is particularly true in seawater, where the chlorides from the salt will attack and destroy the passive film faster than it can reform without oxygen present.  All of the stainless steels except the best of the specialty alloys will suffer from pitting or crevice corrosion when immersed in seawater.  

As a sailor, I can give numerous examples of stainless steel corroding in crevices or places that oxygen cannot reach.  After removing numerous chain plates that penetrate the hull or decks of sailboats in every instance, there was corrosion on the surface that goes through the deck.  Often the corrosion was to the point of cracking or breaking the fitting.  All other parts of the chain plate were shiny and clean with the exception of the small section just below the top of the deck, where oxygen had been depleted. 

When a stainless steel component is exposed to salt water, it becomes susceptible to corrosion due to the lack of oxygen and the addition of chlorides in the water.  In most cases, stainless steel will be destroyed from the inside out.  This is because the corrosion starts inside the crevice where it can’t be seen, then proceeds inside the metal where there is no oxygen, sometimes hollowing out the part or giving it the appearance of Swiss cheese.

One of the best 300-series stainless steels is type 316. Even this alloy will, if unprotected,  will start corroding if located in moist saltwater areas or any tight crevice, starved for oxygen.  If water flows fast past stainless steel, more oxygen is delivered to the stainless steel and it corrodes less. For this reason, stainless steels have been successfully used for impeller blades and propellers. These need to be protected from corrosion when there is no flow.

Passivation
Passivation is the spontaneous formation of a hard non-reactive surface film that inhibits further corrosion.  When stainless steel is passivated, it is put into a bath of an oxidizing acid, such as nitric acid.  It was observed that when stainless steels were first treated with an oxidizing acid, they would later appear to corrode less than if they had not been treated. It was thought that the oxidizing acid somehow thickened the passive film on the stainless steel to make the steel more corrosion resistant. Therefore, the treatment was called passivation. We now know that this treatment doesn’t affect the passive film in a way that lasts very long in water. 

Why does stainless steels appear to corrode less after passivation?  The oxidizing acid treatment is essentially a cleaning process that removes small particles of iron and other impurities that have gotten on the surface of the stainless steel during the rolling process, or are in the structure of the stainless steel itself and happen to be protruding from the surface. These particles corrode in waters that normally don’t corrode stainless steels, leaving behind rust or other corrosion products that are readily visible.  It looks like the stainless steel is corroding when, in fact, it is only the surface particles that corrode.  Cleaning these particles off with the acid treatment means that they will not later corrode and leave behind ugly rust spots.  It therefore seems that the stainless steel is corroding less. Some people believe that surface particle corrosion can start pitting corrosion, but controlled tests show that pitting will still happen even if all of these particles are removed.

The reason for the passivation treatment now becomes clear. It makes the stainless steel look prettier after it has been exposed to the water for a while. It actually doesn’t affect the corrosion of the stainless steel itself, however. The treatment is fairly cheap, and usually doesn’t hurt anything, so manufacturers usually go ahead and do it, just to avoid later questions about "rust" spots forming on their stainless steel.

Knowing all this, the stains that we see on the exposed  areas of our 316 stainless steel boat jewelry will most likely be surface stains, however if the stains appear to be spreading from a connection, bolt or weld, the problem is likely more severe.  The areas that we should be more concerned with are the ones that we cannot see, like the back sides of chain plates, under washers or places that fiberglass might cover, that is where you will find the real degradation.


Hopefully this will be help you find the problem areas before it becomes serious.

Corrosion - What has been happening above and below the waterline?

As boaters, we often get out on the water after a long week at the office, relishing the moments that we can spend floating with friends and family.  We untie the dock lines, motor our boats away from their comfortable slips, and feel the thrill as the boat increases speed.  As we head out to the open water, we are probably thinking of the fishing, water sports or sailing we’re about to do.  We almost certainly are not thinking, "What has been happening above and below the waterline?"

Corrosion is a process that affects the metal parts of every single boat, without exception, all the time.  Some corrosion occurs naturally by the elements: sea water, salty, moist air, and even animal waste, for example.  Other vessels and structures that share the boat’s environment can also contribute to corrosion.   We often see corrosion as it occurs above the waterline. But plenty is happening where we can’t see it:  on underwater thru-hulls, struts and prop shafts.  I personally have had a thru-hull break while underway due to the effects of corrosion, leaving me in a highly dangerous situation.  Corrosion can be prevented or minimized. Understanding corrosion can help to combat the problem before it spoils our weekend.

I have always studied or worked closely with electricity, both AC and DC circuits, but never realized the close relationship that it has with corrosion.  While studying and researching this topic I am able to see the depth and vast amounts of information that is needed to fully understand all of the aspects that this topic covers.  For this reason I will try to break this down into several posts, hoping to make it easier on both you and me.

Corrosion is the disintegration of metals due to a mechanical or electro-chemical reaction.  Many structural alloys corrode merely from exposure to oxygen and moisture in the air, but the process can be strongly affected by exposure to certain substances.  Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Corrosion occurs on exposed surfaces, as a result, methods to reduce the activity of the exposed surface, such as passivization or chromate conversion (chromic acid bath), can increase a material's corrosion resistance.  The types of corrosion that we experience in our boats can be listed as:

Cavitation of a Propeller

•  Mechanical corrosion is corrosion caused by cavitation.  There are two types of cavitation that might be seen, inertial and non-inertial.  
• Inertial cavitation is the sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical forces, such as those resulting from rotation of a marine propeller or the impeller of a centrifugal pump.  In the following example, I will use a propeller to help explain this theory.  As a propeller spins, it creates a negative pressure on some spots of the propeller blade.  If this pressure is lower than the fluid vapor pressure, the water will vaporize and will cause bubbles to be created along the propeller.  As these bubbles move along the metal surface, they will eventually collapse, causing miniature implosions on the surface of the blade which can pit or damage the metal.     Speed and pitch of the prop cause the cavitation; therefore, correct control of these factors can eliminate this type of corrosion.   Believe it or not, dolphins and tuna are adversely affected by this type of cavitation and some types of shrimp use it to assist them in hunting and feeding.
• Non-inertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field.  When these cavitation bubbles collapse, they force compressed liquid into very small volumes, thereby creating spots of high temperature and emitting shock waves.  Diesel engines suffer from cavitation due to high compression and undersized cylinder walls. Vibrations of the cylinder wall induce alternating low and high pressure in the coolant against the cylinder wall. The result is pitting of the cylinder wall, which will eventually let cooling fluid leak into the cylinder and combustion gases to leak into the coolant.  Mechanical corrosion is a selective erosion and will only attack the surface that the cavitation contacts.

• Chemical corrosion is due to presence of an electrolyte. An electrolyte is a solution that conducts electricity via electrically charged atoms, called ions. An ion is an atom that has either picked up an extra electron (negative ion) or has lost an electron (positive ion). Both mechanical and chemical corrosion occur at the surface of the metal, where it can interact with the surrounding electrolyte such as sea water.  Metals have an inherent electrical charge, which serves to hold the molecules intact. The charge can vary a little; it can have a differing potential according to temperature, salinity, aeration, and pollution of the water.  For most metals in sea water, the charge is negative. Graphite, Platinum, and Gold for example have a positive charge in ordinary sea water.  There is a progression of differing potentials (voltages) from the less noble metals (more negative), through to the more noble metals (less negative).  When two metals of a different potential are connected to each other, and immersed in an electrolyte like sea water, the metals have their charges equalized.  The less noble metal, stripped of its inherent protective charge, is unable to prevent the chloride ions in the sea water from pulling metal away. This is electrochemical corrosion.  Less noble type metals are anodic, or negative, and if connected, will sacrifice themselves to any more noble metal they are electrically in contact with while immersed in an electrolyte. The more noble metal is cathodic (either positive, or just less negative).  Like in baseball, the less noble metal is the pitcher, and the more noble metal, the catcher.

• Galvanic corrosion is what happens when two dissimilar metals are immersed in an electrolyte, and are both electrically connected and an ionic conductive path is available.  In this case, a galvanic cell is created. Essentially, this is creating a battery and the current it produces can be measured or even used. Corrosion of the more noble metal will be reduced at the expense of the less noble metal, which will corrode more rapidly.  Two dissimilar metals immersed in an electrolyte, and not connected to each other, will corrode at their own inherent natural rate. In this case, there will be no battery, and no enhanced level of either corrosion or protection.

• Stray current corrosion is when electric current escapes from its intended path, and uses our boat or our underwater fittings as its new path: a stray current.  This is a much more severe kind of corrosion. Stray currents can cause very rapid metal loss, which is limited only by the amount of current available. If there is a direct short, the corrosion rate will be extraordinary.

•  Electrolysis describes all of the electro-chemical processes, including galvanic corrosion, stray current corrosion, and also is used to refer to electro-plating, removing hair from our legs, etc.  Electrolysis is often used incorrectly to refer specifically to stray current corrosion where in fact, it can properly be used to refer to any kind of corrosion involving an electric current.  Strictly speaking, electrolysis means that chemical changes are being produced in an electrolyte, by an electric current passing through the electrolyte. In the case of galvanic corrosion there is also an electric current, one that is created by the differing inherent charges of the dissimilar metals.  Areas of a metal fitting which are actively suffering from corrosion by one form or another of electrolysis will appear brightly eroded, as though freshly sandblasted. Steel and aluminum will be a bright silvery color, for example. The same appearance will be the case for areas suffering from cavitation corrosion.

Galvanic series

The following is the galvanic series for stagnant (that is, low oxygen content) seawater. The order may change in different environments.  The metals at the bottom of this scale will deteriorate quickly if exposed to metals from the top of the scale, the greater the difference on the scale, the greater the potential that is created.  For an example, when platinum is exposed to aluminum, the aluminum will deteriorate very quickly because it is less noble than platinum.

1.       Graphite

2.       Platinum

3.       Gold

4.       Silver

5.       Titanium

6.       Stainless steel 316 (passive)

7.       Stainless Steel 304 (passive)

8.       Silicon bronze

9.       Stainless Steel 316 (active)

10.    Monel 400

11.    Phosphor bronze

12.    Admiralty brass

13.    Red brass

14.    Brass plating

15.    Yellow brass

16.    Naval brass 464

17.    Tungsten

18.    Stainless Steel 304 (active)

19.    Chromium plating

20.    Nickel (passive)

21.    Copper

22.    Nickel (active)

23.    Cast iron

24.    Steel

25.    Lead

26.    Tin

27.    Aluminum

28.    Zinc

29.    Magnesium

The table below reports the corrosion potentials or galvanic series of metals in flowing sea water at ambient temperature.  The unshaded symbols show ranges exhibited by stainless steels in acidic water, as may exist in crevices or in stagnant or low velocity or poorly aerated water where stainless steel becomes active, while the shaded areas show the potentials of stainless steel when is in passive state.