For more than ten years, bridle chains used to pull towed bodies (2500 lbs) have been been failing with increasing frequency. The chains' links are 3/4 " in diameter and composed of four different metals and welding fillers. Available documentation indicates that all failures were due to discontinuities in the weldments of chains' links that were caused by poor welding or excessive casting porosity. It was decided to reduce the stainless steels used to two: 31600 (316) and S20910 (Nitronic 50). The welding filler used to weld 316 was changed to low carbon 316 (316L). The 316 was used to fabricate the links and the S20910 used for other components requiring greater strength. My concern is that simply changing the 316's filler to low carbon 316 would not end the failures, particularly in light of the long history of failures.
Although using the 316L filler apears to resolve the potentially corrosive problems caused by sensitization when 316 is subjected to high temperatures produced by welding, there may still be long-term corrosion problems. Even though the low carbon of the 316L weldment drastically reduces the amount of chromium carbide produced by carbon pulling chromium out of solution to produce chromium carbide, the high temperature zone (HAZ) at the two interfaces of each link's 316L weldment with the 316 base material appears to be susceptible to corrosion. This is due to the dilution (diffusion) of 316 into the weldment at both interfaces. The percentage that 316 contributes to the weldment depends on the welding process used. The process being used, GMAW, contributes up to 20% of 316 to the weldment at each interface for a total dilution of 40% max. With the increase of 316 in the weldment, and the subsequent increase of chromium pulled out of solution, the weldment appears to be susceptible to corrosion and is the chain's "weak link".
Research shows that common industry practice is to use low carbon 316 (316L)whenever the metal is welded in order to avoid corrosion. In addition, it appears that most distributed 316 stainless steel has had its carbon reduced below .03% thus meeting 316's higher strength requirement and 316L's low carbon requirement and is designated as dual certified. The price is approximately the same as 316's. The problem is that 316 is being used as the links base metal rather than 316L. With industries using 316L in plain water applications, it seems that using 316 in a salt water environment is taking an unnecessary risk with corrosion especially in light of the past long term problems with 316. The bridle chain is typically used several hours three to six days a month.
The engineer making the decision explained that 316 was being used because in his experience he was unaware of any corrosion problems with 316 and that 316L was unnecessary. With the previous failures due to poor welding, using 316L would make it much easier for welders to avoid sensitization by using 316L. Using 316L, instead of 316, allows the stainless steel to remain inside the range of 1100 deg. F to 1300 deg F for hours before sensitizing 316L. On the other hand, if 316 doesn't cool down rapidly and remains in the range of 1100 deg. F to 1300 deg F for more than a minute or two, the metal becomes sensitized, and thus making the 316 stainless steel more susceptible to corrossion over time.
Research shows that standard industry practice is to use 316L whenever the metal is to be welded, yet it has been decided that 316 should be used in salt water! Has anyone had any experience with 316 or 316L in a salt water environment and what is your opinion about using 316 in such an environment?
Thanks in advance for your help.
I've already posted an answer on the Technical section of this Forum.
Giovanni S. Crisi
By -
Date 07-26-2005 14:25
OK, where do we start here? You have many issues here, some valid, some questionable.
First of all, 316 or 316L is usually not recommended for seawater applications. We know that a straight 316/316H is designed for service when the application is exposed to temperatures of 400C and greater. It is true that a 316L is virtually immune to sensitization if proper welding parameters are used. Also, even a straight grade of 316 is usually not affected by sensitization because the welding process doen not allow the steel to stay in the range of sensitization long enough to have detrimental effects. The kinetics of sensitization is a function of time, temperature, and the carbon content of the steel. A 316 steel with a carbon content of 0.062% C will suffer sensitization within a couple of minutes at 1200-1400 F, while a 316 steel with a carbon content of 0.03% C would not sensitize for about an hour. Either way, using a welding procedure with a proper heat input and proper interpass temperature should allow the steel to cool rapidly enough to avoid sensitization. Granted, the lower carbon steel will give you a better shot if improper techniques are used, but sensitization can be avoided even with the straight grade 316. Naturally, the higher carbon is advantageous for added strength at elevated temperatures. The 316 is susceptable to rapid corrosion when immersed in a corrosive environment, which seawater is certainly included. You stated that the bridle chain is used only 3-6 hours each month. The 316 can possibly withstand the seawater for that limited amoount of time. I will say that there are other steels out there (2205 Duplex for one) that are much more resistant to the seawater than the 316, though. You keep mentioning poor welding techniques by the welders. That can certainly contribute the loss of corrosion resistance in any steel. I don't think that changing the filler metal to a low carbon grade will solve your problems if proper techniques ae not observed. Again, it would not be my recommendation to use a 316 or 316L for seawater applications, but your chain bridle is not exposed to the seawater very long.
Hoops:
You have good advice from both mssrs Meadows and Crisi.
Regarding material selection-
SS316 is very good in marine atmospheres unless salt residue evaporates and increases concentrations , especially at a (stress, or dissimilar metal) boundary or in a crack. Below the waterline in brine is a different story. SS316 shafting and propellers are often successfully used, but not other fittings, esp when no flow. see
<http://www.underwater.com/archives/arch/uw-su99.04.shtml>
ARTICLES FROM BACK ISSUES OF UNDERWATER MAGAZINE
Article reprint - Summer 1999 Corrosion Control: Galvanic Corrosion and Stainless Steel By - Harvey P. Hack, PhD.
If the towing loads were low, the 316 might endure , since the application is in flowing (presumable oxygenated) water. But the exposure to concentrating salt residue when not in towing service could be as great a problem.
The problem of stress corrosion cracking occurs even in oxygenated flows of chloride solutions.
Regarding welding-
Poor quality welds in themselves would also be adequate to cause failure.
The unfortunate characteristic of 316 is that it will probably fail via stress corrosion cracking or pitting, and in either case will appear OK until failure unless closely inspected, since the bulk of the metal is unaffected. If you adopt Prof Crisi's advice, the standard carbon steel chain will rust and flake all over, giving some warning. If the carbon steel is in contact with the S20910 and in moist spray in the intervals between towing, it may also be subject to pitting decay(localised corrosion).
I suggest consider rinsing the components with fresh water following brine exposure ; if clean and dry they should last (possibly much) longer.
-Tom m
By -
Date 07-26-2005 19:04
Excellent information, Tom. Since we are going into great detail, please allow me to take this one step further.
The biofilm which forms rapidly on stainless steel surfaces exposed to natural seawater has a catalytic effect on the oxygen reduction reaction. This is why crevice and galvanic corrosion are more vigorous in natural seawater than in a corresponding sterile water. This biofilm loses its catalytic effect if the water is heated to a critical temperature which, depending on the microbiology of the water, lies in the region of 30-40C. If this temperature is exceeded, the corrosivity of the water is reduced considerably.
The intermitttent exposure time to the seawater should not degrade the 316 to the point that has been described. You made an excellent point in suggesting rinsing the part off with clean water after the use in the seawater, just like most of us fishermen do our outboard motors after a day on the water (salt water). As I thought I said in my previous response, I think the biggest problem is being encountered due to the "poor" welding techniques. Poor welding techniques will cause degredation of virtually any steel. In this case, I don't think the steel (316) is to blame, but the welding of the steel. Finally, the composition of the steel determines their microstructure which, in turn, affects other properties such as corrosion resistance, mechanical strength, and weldability. In order of corrosion resistance in seawater, the 654 SMO is probably the leader, followed by 254 SMO and then 2507 Super Duplex. 316 is way down the list.
Chuck
