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The following article appeared in the Summer 1999 issue of the "Old" Underwater Magazine. Dr Harvey P. Hicks, PhD. was a regular contributor to Underwater Magazine and a column in each issue called The Corrosion Column. This article addresses the problems with Stainless Steel.
UnderWater Magazine Article reprint: Summer 1999
"Corrosion Control: Galvanic Corrosion and Stainless Steel"
By - Harvey P. Hack, PhD.
Galvanic corrosion is the most frequent cause of unexpected corrosion failures in seawater. It has caused
failures of ship fittings and deckhouse structures, fasteners, hull plating, propellers, shafts, valves,
condensers, and piping. In sea atmospheres, galvanic corrosion causes failures of roofing, gutters, and
car trim. The reason that galvanic corrosion causes so many failures is that it can occur any time that two
different metals are in electrical contact in seawater. Since most structures and devices are made of more
than one kind of metal, this diversity of materials is common and frequently overlooked in corrosion
prevention activities. Let’s look at why this type of corrosion happens, and how to identify it.
Metals in seawater corrode by releasing metal ions into the water around them. This happens at different
rates for different metals, but in all cases the metal must first lose one or more electrons for it to be able
to go into solution in the water. These electrons travel to another part of the wetted metal surface and
react with something in the water, usually dissolved oxygen. The balance between the reaction where
metal ions go into the water (the anodic reaction) and the reaction that uses up the electrons generated
(the cathodic reaction) causes the metal to sit in a specific narrow range of voltages. This voltage range
can be measured, and ma y be different for each metal in each type of water. When the voltages for each
metal in a specific type of water such as seawater are all collected into one place, this collection of
voltages is called a galvanic series. A galvanic series can be as simple as a list of metals in order of their
voltages, or as complicated as a graph with voltage ranges.
The position of a metal in the galvanic series does not say how fast it will corrode, but it does say
something about what happens to it if it is electrically connected to another metal in water. Any time
metals with different voltages are electrically connected in seawater, a current will want to flow between
them until they have the same voltage. This is the way a battery works.
The metal that donates electrons to this current flow, the one that has a more negative voltage to begin
with, will have its corrosion rate increased and is called the anode. The other metal, which has a more
positive voltage to begin with, receives electrons and will have its corrosion rate reduced. It’s called the
cathode. The more negative metal anode is said to undergo galvanic corrosion, while the more positive
metal cathode is said to experience cathodic protection. So, to prevent galvanic corrosion the metals
must either be at the same voltage before they are coupled, not be placed in electrical contact, or not be
immersed in an electrically conductive water like seawater.
Designers who want to prevent corrosion usually like to make structures and devices out of corrosion
resistant materials. However, they may not consider the interaction between the different materials that
they choose. For example, some aluminum alloys do not corrode very fast in seawater, and are used for
boat hulls. Some bronze alloys also do not corrode ve ry fast in seawater, and are used for propellers. As
long as the propeller does not come in electrical contact with the hull, everything works well. But if the
two come in contact through a bearing, gearing, or the boat engine itself, the galvanic series tells us what
will happen. The aluminum is very negative compared to the bronze, so the electrical contact will cause
the aluminum hull to be an anode and its corrosion rate to increase, causing heavy pitting and eventual
failure of the aluminum hull.
The galvanic series tells us that the more negative metal will corrode more quickly when electrically
coupled in seawater, but not how fast. Two metals far apart in the series will not necessarily experience
more corrosion than two metals close together. Finding the rate of corrosion in a galvanic couple
requires knowledge of polarization, the ability of a metal to change voltage while accepting or giving up
a certain amount of electrons. A metal that polarizes easily, that changes voltage quickly with a small
amount of current, will not cause much corrosion of metals coupled to it. It also will not have much
increase in corrosion when it is the anode in a couple. An example of a metal that polarizes easily in
seawater is titanium. Metals that are harder to polarize, such that it is hard to change their voltage when
current is applied, will cause or experience a lot of galvanic corrosion, depending on the other metal in
the couple. Examples of metals that are hard to polarize include copper alloys and some aluminum
alloys. So, a piece of aluminum will corrode faster if it is coupled to hard-to-polarize copper than it will
if coupled to easy-to-polarize titanium in seawater, even though the voltage of the titanium is farther
away from aluminum than the voltage for copper.
The larger the wetted surface area of the cathode, the worse will be the corrosion on the anode. For
example, steel corrosion will be increased by contact with copper, according to the galvanic series. A
steel fastener used to hold a copper plate will corrode quickly, because there is a large area of copper
and a small area of steel. However, a copper fastener will not cause much increase in corrosion of a steel
plate because its area is so small compared to the steel. This effect was first discovered by Sir Humphry
Davy when he was exploring attaching copper plates to ship bottoms to prevent barnacle growth. This
leads to an interesting rule of thumb: always paint the cathode. To slow down galvanic corrosion on the
anode, you can paint the cathode (which is not corroding) to decrease its wetted surface area. Painting
the anode will only increase its corrosion rate at defects in the paint.
Recognizing galvanic corrosion is not always easy. If a metal normally corrodes by pitting, it will just
pit faster when it’s the anode in a galvanic couple. If it normally corrodes uniformly, it will do so more
quickly when coupled. So galvanic corrosion can’t be recognized by the form the corrosion attack takes.
Sometimes galvanic corrosion can be recognized because it is usually worse close to the cathode that is
causing it. In the copper fastener case above, the steel will corrode more quickly close to the fastener
than far from it. Galvanic corrosion will usually be worse near joints between dissimilar metals. But the
best way to recognize galvanic corrosion is to know the order of metals in the galvanic series and look
for the more positive metals in the vicinity of the corrosion failure. If they are there, they likely
contributed to the problem.
Corrosion of Stainless Steels
Aside from steel, stainless steels are the most common construction metals. There are many different
types of stainless steels, divided into five major categories by crystal structure type. The austenitic
stainless steel alloys, with AISI numbers from 200 to 399, are usually nonmagnetic. The alloys with
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 atmosphere. The 400-series
ferritic and the martensitic alloys are usua lly magnetic, stronger, and less corrosion resistant than the
austenitic alloys. They are used for knife blades and certain hand tools. These alloys will sometimes
suffer from mild surface rusting when exposed to fresh water or sea atmosphere. Duplex and
precipitation hardenable stainless steels are specialty alloys. Some are very strong and not very
corrosion resistant, such as 17-4PH, and others have intermediate strength and corrosion resistance
between the austenitic and the ferritic or martensitic alloys. There are some specialty alloys that are very
corrosion resistant because they add more special elements to the alloy, and are consequently somewhat
more expensive than standard grades, such as the austenitic 6XN.
Stainless steels get their 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 steels usually
have poor corrosion resistance in low-oxygen environments, such as under deposits, in mud, or in tight
places, called crevices, where structures or hardware are attached. This is particularly true in seawater,
where the chlorides from the salt will attack and destroy the passive film faster than it can reform in low
oxygen areas. All of the stainless steels except the best of the specialty alloys will suffer from pitting or
crevice corrosion when immersed in seawater. One of the best 300-series stainless steels is type 316.
Even this alloy will, if unprotected, start corroding under soft washers, in o-ring grooves, or any other
tight crevice area in as little as one day, and it is not unusual to have penetration of a tenth of an inch in
a crevice area after only 30 days in seawater. If water flows fast past a 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.
Painting stainless steels usually does not stop the crevice corrosion; it will occur any place where there
is a scratch or nick in the paint. For this reason, I usually recommend against using any stainless steel
except certain specialty alloys in seawater for more than a few hours at a time. There is a strong
tendency to use in seawater the same materials that work well in fresh water or sea atmosphere, so that
types 303, 304, and 316 stainless steel are often used for undersea applications. They will also usually
fail if the exposure is long enough, unless they are in continuous solid electrical contact with a material
that will provide them with cathodic protection such as steel or aluminum. As soon as the electrical
contact is broken, the steel will corrode.
Crevice corrosion of stainless steels happens irregularly, but when it occurs it is very destructive. For
example, if 10 stainless steel screws are put in a plate in seawater, it may be that all but one will be unattacked,
as bright and shiny as the day they were made. That one screw, however, may well have attack
over one quarter inch deep in only a few months. The attack will occur in crevices where it can’t be
seen, and will destroy the screw from the inside out. This is because the corrosion starts inside the
crevice between the screw and the metal, 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.
Even the best of stainless steels may have its corrosion resistance affected by the way it is made. For
example, 316 stainless steel is very corrosion resistant in fresh water, but when it is welded, the areas
next to the welds experience a thermal cycle that can cause that material to corrode. This is called
sensitization, and can lead to the appearance of knife- line attack next to welds. This is why certain heat
treatments should be avoided with this and similar alloys. On the other hand, a low-carbon version of
316, called 316L, will not be sensitized, and can be welded with little effect on corrosion properties.
Austenitic stainless steels can suffer from stress corrosion cracking to various degrees when fully
immersed in seawater. Stress corrosion cracking is cracking without much metal loss in the presence of
a continuous applied load in the environment. If a susceptible material fails by cracking and has
numerous side cracks besides the one causing the failure, stress corrosion cracking should be suspected.
The ferritic and duplex stainless steels usually do not have this problem.
Questions and Answers
When buying stainless steels, some companies claim that they passivate them. What is passivation, why
is it done, and does it make the stainless steel corrode less?
When a stainless steel is passivated, it is put into a bath of an oxidizing acid, such as nitric acid.
Stainless steels get their corrosion resistance from the formation of a very thin corrosion product film of
uncertain composition called the passive film. 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. The film will
stabilize at the same thickness when exposed to the same water whether or not passiviation has been
done. Then why do 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. Passivation can be a problem for parts with tight crevices that can trap the acid used. Over time,
these acids can cause crevice corrosion. For parts without crevices, passivation does have a benefit if the
stainless steel is to be given some later treatment for which a clean surface is necessary. For example, it
is prudent to use passivation before painting or plating over the stainless steel.
Some divers meticulously rinse their equipment off with fresh water after diving in salt water, and others
don’t. I haven’t seen any problems with my equipment if I forget to rinse it off once in a while. Does
this rinsing really do any good?
Yes. The chlorides in salt water cause the stainless steel and aluminum alloys that your equipment is
made from to pit or to corrode in crevices where oxygen access is limited (and where, by the way, you
can’t see it happen until it’s too late). When you take your equipment out of the water, oxygen can
usually get to all of the crevice areas, which stops any crevice corrosion. However, if a crevice is very
deep, trapped saltwater might cause corrosion to continue. Corrosion in these deep crevices will be
stopped by a fresh water rinse. Because your equipment is made from a lot of different metals, galvanic
corrosion can also be a problem as long as the different metals are covered with salt water. The lower
conductivity of fresh water reduces the amount of galvanic corrosion that can occur. Finally, the salt
deposits that form when seawater evaporates are not only ugly, but also hygroscopic, that is, they absorb
moisture from the air. Salt deposits absorb enough moisture for the surface to become wet when the
relative humidity exceeds 50-75 percent. Your equipment will start to corrode when it is sitting in the
shed and you think it is dry. This is the same reason why cars in the northeast corrode more than they do
in the south. Road salts form a layer on the car that causes the car to corrode every time the relative
humidity goes over 50 percent, even sitting in the garage. So, rinse your equipment. Take good care of
it, your life depends on it. And while you’re at it, take your car to the car wash after you’ve driven it on
salty roads and it will last longer too. UW
Dr. Harvey P. Hack, Northrop Grumman Corp., hosts a column on corrosion in each issue of
UnderWater. If you have questions, tips, or comments, write to The Corrosion Column, Underwater
Magazine, 5222 FM 1960 W, Suite 112, Houston, TX 77069 or email
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