Forms of Corrosion

     Corrosion is costly!  If you doubt this, then you probably have never been bitten by the "corrosion bug".  Imagine specifying Titanium for 10 brand new heat exchangers or reactors and later realizing that the processing stream has fairly high concentrations of flourine ions.  The Titanium will be destroyed in weeks and you'll have wasted hundreds of thousands of dollars.  There you stand in front of your supervisor, you'll get that sick feeling in your stomach....that's the "corrosion bug"!  I'd like to think that things like this don't happen, but I've heard my share of horror stories.  If you'd like to avoid a situation like this, I've got two words for you.....FLUID ANALYSIS.  A fluid analysis can save you pain, embarassment, and in some cases your job.  But if you think about, there' really no excuse for not having one done considering the impact that a material of construction decision can have.   With this in mind, I thought that it may be a good idea to review some of the most basic forms of corrosion.

Uniform Attack

  Uniform attack is a form of electrochemical corrosion that occurs with equal intensity of the entire surface of the metal.  Iron rusts when exposed to air and water, and silver tarnishes due to exposure to air.  Pontentially very risky, this type of corrosion is very easy to predict and is usually associated with "common sense" when making material decisions.

Galvanic Corrosion

     Galvanic corrosion is a little more difficult to keep track of in the industrial world.  You'll notice below that simply adding a screw of the wrong material can have severe consequences.  Galvanic corrosion occurs when two metals having different composition are electrically coupled in the presence of an electrolyte.  The more reactive metal will experience severe corrosion while the more noble metal will be quite well protected.  Perhaps the most infamous examples of this type of corrosion are combinations such as steel and brass or copper and steel.   Typically the steel will corrode the area near the brass or copper, even in a water environment and especially in a seawater environment.  Probably the most common way of avoiding galvanic corrosion is to electrically attach a third, anodic metal to the other two.  This is referred to as cathodic protection.

Crevice Corrosion

Another form of electrochemical corrosion is crevice corrosion.  Crevice corrosion is a consequence of concentration differences of ions or dissolved gases in an electrolytic solution.  A solution became trapped between a pipe and the flange on the left.  The stagnant liquid in the crevice eventually had a lowered dissolved oxygen concentration and crevice corrosion took over and destroyed the flange.  In the absence of oxygen, the metal and/or it's passive layer begin to oxidize.  To prevent crevice corrosion, one should use welds rather than rivets or bolted joints whenever possible.  Also consider nonabsorbing gaskets.  Remove accumulated deposits frequently and design containment vessels to avoid stagnant areas as much as possible.

Pitting

Pitting, just as it sounds, is used to describe the formation of small pits on the surface of a metal or alloy.   Pitting is suspected to occur in much the same way crevice corrosion does, but on a flat surface.  A small imperfection in the metal is thought to begin the process, then a "snowball" effect takes place.  Pitting can go on undetected for extended periods of time, until a failure occurs.  A textbook example of pitting would be to subject stainless steel to a chloride containing stream such as seawater.   Pitting would overrun the stainless steel in a matter of weeks due to it's very poor resistance to chlorides, which are notorious for their ability to initiate pitting corrosion.  Alloy blends with more than 2% Molybdenum show better resistance to pitting attack.  Titanium is usually the material of choice if chlorides are the main corrosion concern.  (Pd stabilized forms of Ti are also used for more extreme cases).

Intergranular Corrosion

Occuring along grain boundaries for some alloys, intergranular corrosion can be a real danger in the right environment.  On the left, a piece of stainless steel (especially suspectible to intergranular corrosion) has seen severe corrosion just an inch from a weld.  The heating of some materials causes chromium carbide to form from the chromium and the carbon in the metals.  This leaves a chromium deficient boundary just shy of the where the metal was heated for welding.  To avoid this problem, the material can be subjected to high temperatures to redissolve the chromium carbide particles.  Low carbon materials can also be used to minimize the formation of chromium carbide.  Finally, the material can be alloyed with another material such as Titanium which forms carbides more readily so that the chromium remains in place.

Selective Leaching

When one element or constituent of a metal is selectively corroded out of a material it is referred to as selective leaching.  The most common example is the dezincification of brass.  On the right, nickel has be corroded out of a copper-nickel alloy exposed to stagnant seawater.   After leaching has occurred, the mechanical properties of the metal are obviously impaired and some metal will begin to crack.

Erosion-Corrosion

Erosion-corrosion arises from a combination of chemical attack and the physical abrasion as a consequence of the fluid motion.  Virtually all alloy or metals are susceptible to some type of erosion-corrosion as this type of corrosion is very dependent on the fluid.   Materials that rely on a passive layer are especially sensitive to erosion-corrosion.  Once the passive layer has been removed, the bare metal surface is exposed to the corrosive material.  If the passive layer cannot be regenerated quickly enough, significant damage can be seen.  Fluids that contain suspended solids are often times responsible for erosion-corrosion.  The best way to limit erosion-corrosion is to design systems that will maintain a low fluid velocity and to minimize sudden line size changes and elbows.  The photo above shows erosion-corrosion of a copper-nickel tube in a seawater surface.  An imperfection on the tube surface probably cause an eddy current which provided a perfect location for erosion-corrosion.

Stress Corrosion

Stess corrosion can result from the combination of an applied tensile stress and a corrosive environment.  In fact, some materials only become susceptible to corrosion in a given environment once a tensile stress is applied.  Once the stress cracks begin, they easily propagate throughout the material, which in turn allows additional corrosion and cracking to take place.   The tensile stress is usually the result of expansions and contractions that are caused by violent temperature changes or thermal cycles.  The best defense against stress corrosion is to limit the magnitude and/or frequency of the tensile stress.

References:

Callister, William D., Materials Science and Engineering, 3rd Ed., Wiley, New York, 1985

InterCorr.com website

NASA website