Cromox Windlass Anchor Chains Duplex CXA
Cromox Windlass Anchor Chains CXA, Stainless Grade 60 Plus, AISI 318 LN Duplex.
Measures according to DIN 766 and ISO 4565, tested, calibrated, bright polished. Other sizes available on request.
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Cromox Windlass Anchor Chains CXA
Type Breaking Force lbs. d" t" b1" min. b2" max. Weight approx. lbs / ft Diameter in mm KW-CXA5/16 14,160 0.32 0.94 0.38 1.07 0.91 8 KW-CXA3/8 22,480 0.4 1.1 0.47 1.41 1.51 10 KW-CXA1/2 37,090 0.51 1.42 0.61 1.85 2.62 13
Why Use a Duplex Stainless Anchor Chain?
To answer this question, we have to go back to the basic reason of why we like stainless steel - it has resistance to corrosion. This resistance to attack is because of the chromium rich oxide film on the surface of the steel. This invisible, inert and passive film adheres to the metal and protects against corrosion. When damaged by abrasion, it is self- repairing in the presence of oxygen. Surface finish should be mentioned at this point as well- a polished surface is more corrosion resistant than a brushed finish, for example.
AISI 316L is commonly used for anchor chain. In many cases, it will perform its function well. If the chain is used intermittently and is constantly in and out of the seawater, there should be little problem in using AISI 316L. It gets more complicated if the anchor chain is left immersed in seawater for longer periods of time. Pitting corrosion is the enemy of stainless steel that spends a lot of time in seawater. Pit corrosion can lead to catastrophic failure of the chain, often at a time of least convenience. Duplex stainless resists pit corrosion more than AISI 316 stainless. This is due to the fact that AISI 318LN has a higher PREN value. AISI 316 has a PREN of around 23.1 to 28.5, while AISI 318LN has a PREN of around 30.8 to 38.1. It is generally considered that stainless steels with a PREN above 32 are resistant to seawater corrosion.
Pitting corrosion is an electrochemical oxidation-reduction process that occurs within localised holes on the surface of a metal that is coated with a passive film. It is a form of galvanic corrosion where the chromium in the passive layer is dissolved, leaving only the corrosion prone iron. In seawater, as the chromium is dissolved, the electrically driven chlorides bore into the stainless steel and create a smooth wall spherical pit. The residual solution that is left in the pit is ferric chloride, ( FeCl3). Ferric chloride is highly corrosive to stainless steel.
Duplex stainless steels also provide increased resistance against stress corrosion cracking (SCC). When tensile stress combines with a specific corrosive environment, the net effect can result in cracking failure. SCC usually starts with pitting or crevice corrosion as a precursor to the formation of a stress concentrator. By using a duplex stainless in sea water, the likelihood of pitting corrosion is greatly reduced, thereby reducing the flow on effect of such pits.
The pitting resistance equivalent number, PREN, is a measure of the relative resistance of stainless steel to pitting corrosion in an environment that contains chloride- namely sea water in this context. The elements chromium, molybdenum and nitrogen each play a crucial role and this is illustrated by the fact that the formula for the PREN value involves these three elements.
PREN = 1 x % Cr + 3.3 x % Mo + 16 x % N
Even though it is not an absolute value, it is useful for comparing stainless steels from within the same family.