The next few installments in this series will discuss proper material selection in the design of marine equipment and structures. The most important piece of information needed when selecting a material for marine equipment is the definition of the environment to which that material will be exposed. This is even more important than the stress that the metal will experience, since the strongest of materials will become very weak in the wrong environment.

The environment consists not only of the salinity of the seawater and the pollutant concentrations expected, but also the length of exposure, pressure, flow, mix of materials involved, and whether cathodic protection will be used. You must know the type of exposure: deep full immersion, shallow full immersion, tidal zone, splash and spray zone, atmospheric exposure close to the ocean, or atmospheric exposure farther from the ocean.

My next few columns will address only immersed exposures, not atmospheric exposures, since atmospheric exposure is a complex subject by itself.

Design Considerations
The composition of the water is important, specifically the salinity, amount of pollutants expected, amount of biological activity, dissolved oxygen, and temperature. If the device will be subject to pressure or water flow, the details of these conditions must be known. The length of exposure must be considered. Materials that work well for short exposures may not work for long exposures, while materials that work well for long exposures may not be economical for short exposures.

The maintainability of the device is an important consideration. For example, sometimes it is more economical to repair or replace a device periodically than to make it from expensive materials, while at other times it may be impossible to access a device to perform repairs once it is put in service.

The size of the device is also important. Large devices can't be made from materials that cost a lot per pound, while for small devices the cost of manufacture will far exceed the cost of raw materials.

The relationship between the purchasing and the maintenance budgets is important for materials selection. If no money is available for maintenance then reliability is important, whereas if initial cost must be low and a maintenance budget is available, then less reliable materials can be used.

It is important to consider the device's relationship to health and safety. For example, reliability is the major concern for manned vehicles, regardless of cost.

Finally, when considering materials selection, the device must be looked at as a system, not a series of individual parts. That is because galvanic corrosion, cathodic protection, and heavy metal ion corrosion all are related to the overall system that the device is part of.

Making Informed Decisions
Let's consider how the choice of primary material for a device is dependent on size. Very large structures in or near seawater are always made mostly from steel. Steel has a low cost and is easy to obtain and fabricate. Hulls of large vessels, pilings, and large fixed structures like oil platforms are all made primarily from steel. When steel is used for large structures, some corrosion is expected and must be tolerated in the design.

For this reason, threads should not be used on steel and dimensions (especially holes) should never be made to tight tolerances.

Corrosion protection measures must always be used for steel. These include paint, cathodic protection of fully immersed surfaces, and cladding for surfaces that cannot get either of the other protection schemes, such as sliding or threaded surfaces. All steel surfaces should be painted, which is also why holes cannot have tight tolerances, since the thickness of paint in holes is difficult to control.

When tight tolerances, threaded holes, sealing surfaces, or sliding surfaces are required on steel structures, they must be made from corrosion-resistant materials that are tightly bonded onto the steel. Methods of bonding corrosion resistant materials to steel include welding, cladding, and adhesive bonding.

For example, if female threads are needed in a steel structure, a hole can be made in the structure and filled with a weld of a noble metal such as Inconel 625, then the hole re-drilled into the weld metal and tapped. Alternately, a noble metal full insert with internal threads may be glued into the steel. If male threads are needed, noble metal threaded studs may be stud-welded onto the steel. Of course, the best way to attach to steel is using through-hole bolting.

As devices become smaller, other materials besides steel may be used. Aluminum is a common choice as a hull material for smaller boats and small underwater vehicles. Aluminum devices must be available for periodic inspection and maintenance, since aluminum can corrode, especially when in contact with other metals in seawater. For this reason threads should be avoided on aluminum devices, or provided by using full inserts as for steel. The use of Helicoils to provide female threads in aluminum will work only for short-term immersion or for atmospheric exposures, but will not be reliable for long-term immersion. Aluminum doesn't have to be painted in seawater, and can be used where tighter tolerances are needed, especially when seawater-resistant 5000-series alloys are used.

Even smaller devices, where immersion time is not great and maintenance is performed regularly, may be made from stainless steel. Stainless steel devices should not be immersed for longer than a week or so at a time without cathodic protection however, since they may experience crevice corrosion.

Finally, small devices that are expensive to make, have tight tolerances, and may be in seawater for extended times are usually made primarily from highly corrosion resistant materials like titanium or nickel alloys.

Questions and Answers
Q: I must bolt together two whalers (6x8 pressured-treated) underwater for continuous permanent submersion in saltwater. Having read your previous suggestions of titanium, Monel, naval bronze, etc., I am undecided as to the proper material. Also, bolts and/or threaded rods are not readily available in these sizes (I was figuring 3/4-10x14-inch). If you could give your recommendation and, if available, a source or sources of supply, it would be greatly appreciated.
S. Zingale

A: The standard material used for this application is heavily galvanized steel. These will need to be replaced every few years, depending on the amount of galvanizing. For bolts this large, the alternatives, titanium or nickel alloys, are far too expensive. I don't know about sources of supply, but contractors that install bulkheads use these.

Q: What are the potential for, and your experience with, galvanic corrosion attack between a typical 316 low carbon stainless steel, say 316L (wrought), and a cast spheroidal graphite austenitic cast iron (SG Ni- Resist) with 30 percent Ni and three percent Cr? This combination would be immersed in seawater at about 15 to 20 degrees centigrade. Our power station here in Adelaide, South Australia, has been offered this combination of materials for our replacement seawater pumps. Bronzes in the past have suffered with cavitation (a pump problem we are solving) and de-alloying and galvanic attack. The potential is for a larger surface area of 316L (pump column) to the SG Ni-Resist (impeller, suction casing and impeller bowl) A cast 316 stainless steel impeller is also a possibility.
D. Prosser

A: Low-carbon grades, as compared to standard grades, are primarily for welding, and will have little effect on corrosion if the pump is not welded. The Ni-Resist is likely to be anodic to the stainless steel. This could provide enough protection to keep the stainless steel from crevice corroding, but the small area of Ni-Resist would likely corrode very quickly. Depending on your piping material, the 316 could be cathodic to the piping, driving corrosion of pipe near the pump unless electrical isolation is used.

You didn't say how large the pumps were. For small to mid size pumps, an excellent material to use in seawater is titanium. It is highly resistant to cavitation and essentially immune to corrosion. The only problem is that titanium works best with electrical isolation.

Another material in this category is Inconel 625. For larger pumps you may be able to get by with a higher grade of stainess steel, such as alloy 20, if the pump is under continuous operation, which limits crevice corrosion. Seals and wear surfaces should be overlaid with Inconel 625 to prevent crevice corrosion there, and the nearby piping should be inspected frequently for galvanic corrosion caused by the pump.

Q: My company designs and supplies refrigeration machinery for fishing vessels. One of our projects requires a DX (direct expansion - refrigerant inside tubes and seawater outside) chiller. This chiller will have titanium grade-2 tubes rolled and expanded into a SS316 tubesheet. To assist the seal feature, we make shallow grooves in each tube hole, which creates four possible seals. Since titanium has a very good "memory," this is the only way to make a gas/water-tight seal without using a titanium tube sheet that features seal welding on the outside surface. I would appreciate any simple explanations that you might be able to give about the "crevice" situation of the tube inside of the hole before it makes the first seal into a groove.
J. Mournian

A: The crevice between titanium and 316 stainless steel will likely not be as severe an environment as the crevice between 316 and itself or between 316 and a non-metal. However, it doesn't take a very severe crevice to initiate crevice corrosion on 316 in seawater, especially in warm seawater. Also, the large surface area of titanium may increase the propagation of any crevice corrosion that initiates on the 316. I suggest installing cathodic protection to prevent corrosion of the tubesheet from the seawater side or using a titanium or Inconel 625 tubesheet instead of the 316. You may still be able to seal the tubes to the tubesheet the same way and avoid welding.

Q: I am currently looking at changing some sections of steam piping from mild steel to stainless steel on the deck of one of my vessels. However, one of my colleagues has commented that stainless steel is more prone to sudden failure (in way of stainless bolts) when subjected to heat in a salt laden atmosphere. Are you able to elaborate on this?
S. Palmer

A: The common austenitic (300-series) stainless steels may be subject to stress corrosion cracking if exposed to a hot environment containing chlorides. This is not a problem at ordinary immersion temperatures, but if chlorides get on these materials and if they are heated by steam, they may crack. The way around this is to use the 400-series stainless steels instead. These are less corrosion-resistant than the 300-series, but are not as susceptible to stress corrosion cracking. Another possibility for reduced maintenance is to use a thermal-sprayed aluminum coating over the steel.

Q: We are an architectural firm in Los Angeles that uses a lot of stainless steel for our projects. A few of our jobs have been on the beach where stainless is used as handrails, etc. We have the problem of flash rusting of type 316. The handrails, for instance, are welded together, bead-blasted with a fine glass bead to give a smooth finish, waxed three times and installed. About a week after installation the rusting begins. I'm just wondering if we are removing this passive film on the surface layer of the stainless when we bead-blast it? Or is this layer something that occurs naturally in the mix of the different elements?
B. Wong

A: The passive film on stainless steel reforms almost instantly if it is removed by blasting, so that is not causing the problem. The chlorides in the sea atmosphere are causing the problem, and as you have found, wax is not an effective coating over the long term. I suggest using aluminum instead of 316 stainless steel. It is not as bright and shiny and it will develop a dull white finish over time, but it does not corrode rapidly in marine atmospheres, and what corrosion does occur leaves a white corrosion product which is not as ugly as running brown rust. If you need the brightness of stainless steel, there are more highly alloyed, and therefore more expensive, stainless steels than 316. Look for materials that have at least 6 percent molybdenum in their composition and that are advertised as being resistant to marine atmospheres.

Q: I have seen several failures of stainless steel marine hardware operating around seawater. These have been in the form of swaged fittings (high residual stress) or clevises that were under constant high tensile forces. The failures have all been in the splash zone and at ambient temperatures. There was superficial corrosion associated with the failures. The fractures had the appearance of stress-corrosion cracking. Reading the literature revealed that SCC of stainless steel does not occur at temperatures under 200 degrees F. What am I missing? Are there circumstances where SCC can occur at lower temperatures? Are there other failures modes that look like SCC?
T. Toth

A: Different stainless steels have different susceptibilities to SCC. The austenitic stainless steels such as 304 and 316 typically will experience SCC, but you are right in that it usually occurs above 200 degrees F. I don't know which stainless steel the fittings are made from, but some stainless steels will experience SCC at room temperature. Also, the splash zone has some unique properties: salt can be concentrated on the surface due to wetting and drying, and direct sunlight can heat up parts. I have experience with an aluminum plate that was set out in the sun on a moderate temperature day (not even the summertime) that reached 160 degrees F just due to solar heating. So my guess is that the combination of residual stresses at the yield strength of the material with salt concentration and solar heating led to the SCC failures that you describe.

Other failure modes that can look like SCC in this material are few, but fatigue is one. Typically, fatigue is not a problem with swaged fittings since it is difficult to create cyclic loading.UW