Originally Posted by pbiJim
Yes, the corrosion resistance is not as good as 316, but the mechanical properties look a lot better, at least from a static test perspective. I would like to try some & find out how badly it corrodes in a marine environment
. If the corrosion is minimal, or even tolerable, the tradeoff may be worthwhile for the possible mechanical benefits.
According to one manufacturer:
"17-4 PH Stainless Steel
with-stands corrosive attack better than any
of the standard hardenable stainless
steels and is comparable to Type 304 in
As I said before, I don't know if it would be better in the long run or not, but I would like to find out.
There seems to be room for improvement over what is currently in use.
After working for 20 years in subsea oil&gas on Subsea template design, ROVs, ROTs and pipeline repair robots, I'd say that Oil
& Gas operators are a conservative bunch. If you get it wrong, as a small company, you're out of business immediately. I can pretty much summarize that the majority of prefered materials used in the marine environment divided into two classes
1). Short term protection: I.e. Will not be in the water
too long so cathodic protection requirements, low oxygen surface cracking, H2S fatigue cracking and are minimal.
2). Long term protection: I.e. 15-20year service
lifetime warranty. Complete cathodic protection system for service
life, optimal material use and quality sub-selection (each ingot/stock would be tested), proven qualification for all load factors and environmental requirements for service life.
Remember that service life for (2) means 15-20 year on the seabed operating 100% of the time without lifting back to surface for maintenance!
For type (1) preferred materials would be:
A). Structural: St.52-3 and St-52-3N hollow section or open section. Both types painted with an EN/NS epoxy
multi coat protection system. If hollow section then the interior
would be sprayed through with "tectil" bitumen wax. This would be protected with Zinc anodes. If Aluminum
was to be used (popular with ROVs as it reduces dynamic loading at launch and recovery), then it was preferred to leave it uncoated and protect it with Magnesium anodes.
B) Mechanical : Seawater lubricated items would typically be S165M (stock rods for hydraulic cylinders) a 800-900MPa stainless, coupled with bushings of JM7-15 or JM7-20 (oiled bronze). Sometimes 4140 would be used as a substitute for S165M.
AISI 316 and AISI 316L would be very common, and occasionally Aluminium if it was guaranteed to be electrically isolated from other metallics.
POM (Polymer) was used sometimes but frowned upon as it expands in seawater (is slightly porous) so your tolerances go to hell - PEHD was preferred.
PEEK is expensive but as strong as aluminium and survives most chemical issues, and a favorite with me as a bushing material.
For type (2) [long term] preferred materials would be:
A). Structural: St.52-3 and St-52-3N open section. Painted with an EN/NS epoxy
multi coat protection system. This would be protected with heavy Zinc anodes (I'm talking anodes of many hundred kilos).
GRP would be used on large debris covers on subsea templates. Some of these would be 1/4 size of a football field and be hinged to the subsea structure anchored in the seabed. An ROV would connect a pull-wire to the hatch
mechanism and the whole hatch
would be opened to expose the subsea template system for maintenance
B) Mechanical : Seawater lubricated items would typically be AISI 316 (rarely 304), coupled with bushings of JM7-15 or JM7-20 (oiled bronze).
PEHD (High Density Polyethylene) was used more commonly for any bushings etc as it doesn't get affected by seawater. PEEK is expensive but as strong as aluminum
and survives most chemical issues.
C) Pipelines, flowlines etc (and some fasteners) would be Super Duplex, very very rarely titanium. Inconel which was coated with silver would be a popular choice for pipeline flange-seals as it's good at withstanding H2S corrosion.
As you can see, there really isn't any "aerospace" materials in use. The Aluminium used would be T-6062 (T6061 in the US), so similar to aerospace - but that was it. You'll also note that the list of materials is limited. Of course there are many more materials used, but typically they are more limited than the "usual suspects".. the key to good cathodic protection is to minimize the number of types of materials such that the whole electrical
interaction picture is simplified.
Aerospace is a completely different environment to offshore
(saltwater). Compared to marine, aerospace materials go through much larger temperature variations. However, they don't need to be as tolerant to humidity (apart from during storage). Their chemical tolerance doesn't need to be as high and their exposure to complex electrolytes is limited. That said, aerospace structural materials need to withstand much higher dynamic stresses as they are intended to operated at the higher end of their utilization factor: there isn't much material to save weight - so it ALL gets used.
Like I said before, the offshore oil
& gas industry is actually very high-end with their subsea/sub-surface technology and material science. Literally $Billions has been poured into materials research
, due to their risk-aversion and they tend not to use materials which don't stand the test of time... It would be unwise of the yachting industry to play with materials in critical structural applications without considering the hard-learned lessons from other related [and much larger] industries: there are far fewer subsea well-blowouts than their are yachts losing keels!
p.s. I would agree that well blow-outs tend to have a much greater consequence than broken keels.