Q: We are contemplating fitting an impressed current cathodic protection system - CAPAC. You advised us in the past on a corrosion problem caused through welding on a steel boat while still in the water. We would like to know whether fitting this new system would also eliminate out previous problem.
D. Rawnsley

A: I have several points to make. First, impressed current cathodic protection is an excellent way to protect large ships, but is not typically economical for smaller vessels. It really depends on how difficult it is to replace sacrificial anodes and whether continuous power is available for an impressed current system.
In my experience, if a craft is large enough to be called a ship it probably will be most economical to use impressed current cathodic protection. If it is small enough to be called a boat, sacrificial anodes are usually a better choice. Of course, that is a pretty gross oversimplification. In general, either system will protect a hull adequately if properly designed, and improperly grounded welding will overwhelm either system. The corrosion caused by improperly grounded welding will stop as soon as the welding stops, so no long-term action is necessary besides repairing and repainting the damage.
Electrocatalytic, who makes CAPAC, is an excellent company for supplying impressed current ship systems. They have supplied the US Navy for many years, they have good design people, and make reliable systems. If you decide to go with an impressed current system then they are a great choice.
You may want to hire a corrosion specialist or cathodic protection specialist (some of the best are certified by NACE International, www.nace.org). If your ship already has a protection system they can check the design to determine if it is adequate, and if this is new construction they can determine the best design for a new protection system. This is not something to be taken lightly, since the design can dramatically affect maintenance costs, and is worth spending some up-front money to resolve optimally.

Q: I am a new graduate engineer and am currently designing a mating flange/ring for a submarine rescue vessel. This flange/ring will be part of the connection from the rescue sub to the main submarine. Therefore I will have to design the mating flange/ring and a test rig (framework). The rescue sub would descend and land on the rig and ring.
I am thinking of using a stainless steel (e.g. 316/316L) for the framework. Could you give examples of other materials or sources of information? The framework itself could be painted to avoid corrosion, but the mating ring will have to be flush to ensure a good seal, therefore I would need a material, maybe SS or Monel, that would be corrosion-resistant without painting, etc. Maybe cathodic protection?
I would also be concerned about galvanic corrosion. Any suggestions or good sources of research would be greatly appreciated. Probable depth of water is between 1,476 and 2,132 feet (450-650m); forces approximately one ton for mating surfaces.
M. Corben

A: I don't like to use stainless steel in seawater because of crevice corrosion. Some recent research at the LaQue Center for Corrosion Technology suggests that painting stainless steel is only effective if there are no holidays in the paint, which is not practical on a real structure of any size. Applying cathodic protection would prevent the stainless steel from corroding, provided that good electrical connection can be guaranteed to all parts of the structure (i.e., it must be all-welded), however the cathodic protection will cause the coating to lose some adhesion.
It would probably be better to follow what the US Navy does for its submarines. For large heavy structures that must take a lot of force, steel is the best choice. They use HY-80 or HY-100 for their toughness, but you could also use a high strength low alloy steel of about the same strength level. Since you will be using steel, painting and cathodic protection are mandatory, and since you will be using cathodic protection the steel should not be any stronger than 100-ksi yield to avoid hydrogen embrittlement from the protection system.
The Navy at one time used weld-overlaid Monel, or even stainless steel, on areas that are sealing or sliding surfaces and must maintain exact dimensions. These days, the weld overlay material of choice is Inconel 625, since it will be essentially immune to corrosion under these circumstances.
Galvanic corrosion should not be an issue if you use cathodically protected steel, since that is also what is used on most big submarines. If you wind up attaching to something made from more noble materials, the anodes will be used up quickly, but the short duration of contact will not result in much galvanic corrosion. Any small parts on your rig that must not corrode should be made from painted noble metals like Inconel 625 or Monel. The steel will protect them, and if they are small and painted they will not increase the demand on the cathodic protection system significantly.

Q: I am working on a procedure to passivate CRES 316 using ASM-QQ-P-35. The yard has a plant that has 33 percent phosphoric acid solution. I plan to immerse the metal for 30 minutes at 70-90 degrees F. The problem is that the engineers want a table to extrapolate times in case the winter hits and the bath drops to 40-50 degrees F. Is there a relation between time and temperature for passivating CRES? Thank you for your help.
E. Baxter

A: Neither AMS-QQ-P-35 nor either of the two ASTM standards that involve passivation of stainless steel has any treatments that are lower than 70 degrees F. Although I suspect that the passivation treatment can be conducted at lower temperatures, to do so would be a totally undocumented procedure for which there is no precedent. You would therefore be liable if the procedure doesn't work. Instead, I suggest fitting the tank with heaters.
You didn't mention the reason for passivation in the first place. Passivation is a cleaning process to remove imbedded iron particles that can cause unsightly rust spots and does not, in my opinion, affect corrosion performance of the stainless steel. There is a small group of people who believe that the rust spots caused by the imbedded iron act as initiation sites for pitting, but if you are using the stainless steel in an environment where pitting is possible you should probably be thinking about changing materials, not passivating.
I have seen stainless steel passivated so that customers will not see rust spots later and think that the stainless is corroding. I also like to passivate any stainless steel that is painted. If the article is not visible, you may not need to passivate it at all.

Q: We observe severe corrosion and erosion in a spray nozzle made of Ultimet. This spray nozzle sees a high velocity fluid, acidic (pH 1-2), contains calcium chloride 60g/l, gypsum crystals, and operates at 60¡ C. It's used for gas cleaning. I think there is corrosion in addition to erosion because under the seal, where no velocity exists, there is some pitting. May I ask what you'd think of that?
B. Siret

A: Ultimet is a very corrosion-resistant material, but it is not immune. The conditions you describe, a highly acidic chloride-containing environment with high flow, entrained solids, and crevices, is very aggressive. Even Ultimet can corrode under these conditions, as you have found out.
I suggest that you consider a ceramic material for your nozzle, such as those ceramics used to make nozzles of grit blasting units.
Ceramics will not corrode and are hard enough to resist abrasion. If this is impractical and you must use a metal, you might consider Multiphase MP-35N, although even this material may corrode under these conditions. So a ceramic is likely to be your best choice.
Q: In the galvanic series, I found that stainless steel 316 (active) is more close to carbon steel. Is it better to couple stainless steel 316 (active) to carbon steel? What is the difference between stainless steel active and stainless steel passive? Can stainless steel 316 active be bought in the market?
J. Chan

A: The stainless steels normally are passive. However, when chlorides are in the environment they may become active inside of pits or crevices. So rather than there being two different kinds of material, one active and one passive, the same material may have areas on it which are active while the rest of the surface is passive.
To prevent pitting and crevice corrosion of a stainless steel galvanically you must electrically connect the stainless to a material that is more active than the active area shown on the galvanic series. Stainless steel may therefore be protected by connecting it to steel, zinc, or aluminum, with corresponding increase in corrosion of the material used to protect the stainless steel.
Since your original question was about corrosion in concrete, remember that the normal published galvanic series is for seawater exposure, not exposure in concrete. The series is liable to be quite different in concrete, with some stainless steels not experiencing an active area at all and ordinary steel becoming passive with significant changes in potential from those in seawater. You may wish to consult with a corrosion specialist for concrete if the decisions that are to be made have large financial or safety repercussions.

Q: I design underwater systems that typically consist of structural parts and fasteners made of 316 stainless steel. The systems usually consist of stainless steel plates and housings bolted together with nuts or more commonly with blind tapped holes. The fastener threads are usually lightly coated with Anti-Seize (Zinc) Paste to prevent stainless on stainless galling. Although I am well aware of the potential for crevice corrosion in stainless steels, for the many years that we have been in business, somehow we have been spared this condition. Maybe the water flow has been adequate to keep the corrosion at bay or maybe our customers rinse off the equipment after each use. I don't know.
My latest project requires the use of a titanium housing to achieve a greater system operational depth. We would like to switch the normally 316 SS housing for a titanium (6AL-4V or possibly 5AL 2.5Sn) version. The same 316 SS fasteners would be used, some of which would be inserted into blind holes tapped into the titanium. If this solution is feasible from materials point of view then it will be a fairly straightforward design and manufacturing change. I'm encouraged to see that the galvanic series for Titanium and 316 SS are very close to each other so long as there is water flow. If flow is not adequate it may be a different story.
Do you have any ideas as to why we have yet to see crevice corrosion on any of our 316 SS systems, especially when fasteners are inserted into blind tapped holes? Do you have any reservations or comments on our plans to switch a stainless steel housing for one made of titanium?
We could live with the occasional replacement of corroded SS fasteners if this is all that is likely to happen. The systems are used in seawater for two to 12 hours while exposed to a current of three knots or greater. They are then typically stored, unrinsed, in a crate.
C. Dundorf

A: There are no "good" or "bad" materials, only those that are appropriate for the application or not. My column has, perhaps unfairly, been bashing the use of most common austenitic stainless steel in seawater for some time. In the case of your 316 hardware, the exposure to seawater is too short for crevice corrosion to initiate, and you may also be getting some protection in the threads from the zinc anti-seize. So in this case the use of 316 is just fine.
Typically it will take at least a few days for crevice corrosion to initiate in a 316 stainless steel material when immersed in seawater, and usually a lot longer. The titanium alloys that you mention will not significantly affect the initiation of crevice corrosion on the stainless steel, so performance should be unaffected. They would increase crevice propagation rate once initiation begins, but that is not an issue here.
I suggest using Ti-6Al-4V ELI, which is grade 23 in most ASTM titanium specs like B265 or B348, rather than the straight Ti-6Al-4V, which is grade 5. Although slightly harder to get and slightly less strong than grade 5, use of grade 23 will give you improved fracture toughness so there will be less chance of failure by rapid crack propagation at stresses below yield.

Q: I recently read one of your articles on galvanic corrosion and became concerned about one of our pumps. We have a centrifugal pump that will be submersed in seawater (Gulf of Mexico). It will be an intermittent running condition and will be submersed when running and may be out of the water when at rest. The pump case, cover and impeller are made from 316L stainless steel. The pump shaft, ridged coupling, motor hub, bolts are made of 17-4PH. The motor support is welded to the cover on one end and a 316L plate is welded to the other end. The motor support material is a piece of 304 stainless steel spun cast pipe (16" dia.) The hydraulic motor case is made from cast steel and is bolted to the top plate of the motor support. I was hoping you could give me some insight into any possible corrosion problems we may encounter or any concerns with the material selection.
M. Grummett