Stainless Steel

Stainless steel!

Stainless Steel!

Some basic knowledge:

3 types of stainless steel quality; AISI 316, AISI 317 and Duplex 2205.

The compsition varies with regards to content of Chromium (Cr), Nickel (Ni) and Molybdenum (Mo).

The Molybdenum, has essential properties to resist corrosion, created by halogens, particularly chlorides (Cl) in seawater

316=Mo 2-3%,, 317=+3% and Duplex +2,5%. Normally 2,7% for 316/ Duplex and 3,5% for 317.

Stainless Steel!

Stainless steel Typical composition ( % ) L = low carbon content

Type of s/s316316L317317LChromium17.017.518.018.5Nickel11.513.014.014.5Carbon (max)0.050.030.080.03Molybdenum2.72.73.03.5

Stainless Steel!

Some basic knowledge:

The protective thin layer of chromium oxide on the steel surface, creates a resistant and protective border, between the stainless steel and the aggressive / corrosive enviroment ( cargo / tank atmosphere).

The chromium oxide layer, is normally a film of abt. 3-4 microns!

If the layer of chromium oxide is damaged, it will normally restore itself..

Simply done by exposing the sst surface to air, by ventilation!

BUT, the stainless steel surface must be clean!

Stainless Steel!

Some basic knowledge:

5 mayor condition, which might cause corrosion:

The Chromium oxide layer is absent, or partly damaged.High contents of halogens ( chlorides, fluorine or bromine) in cargo/ s.w!Presence of an electrolyte, aqueous solution, from cargo, ballast/ condensation.Anode / cathode couples in tank/ line.High temperature, creating crevice and stress cracking corrosion.

Stainless Steel!

Some basic ...

Keep tank, lines, pump and coils proper cleaned. Trace of previous cargo, combined with new cargo, might damage the thin passive layer of chromium oxide..

Regular inspection is essential to avoid severe corrosion or damage of the sst.

Keep tank and lines closed, except when it is necessarry to open them, avoid ingress of sea spary etc.

Use plastic shoe cover, when entering the tank

Stainless Steel!

Some basic...

If discolouration, rust spot, scratches are observed, take immediately corrective action.., grind scratch / rust spot, apply layer of pickling paste, finally passivate the tank by means of nitric acid solution (20% strenght!).

It is recommended on long voyage, with empty tank, keep small amount of good freshwater in the bottom, it will absorb the chloride, avoid dried chloride to cause pitting or corrosion..

After carriage of corrosive cargoes , ensure yourself there is no deposit left on the sst surface, since this might cause severe corrosion / pittings!

Stainless Steel!

THE IMPORTANCE OF UNDERSTANDING CHLORIDE (SALT) EFFECT UPON STAINLESS STEEL - Chlorides, which are presented in seawater, halogenated cargoes and mineral acids are the major source to the damaging effect of all stainless steel materials.- Chlorides cannot be removed by heat along, but must be dissolved in fresh water

Stainless Steel!

Some basic...

A common mistake; too much seawater in tank, when commencing freshwater rinse!

Therfore; ALWAYS STRIP THE TANK EMPTY OF SEAWATER, BEFORE TANK AND LINES ARE FRESHWATER RINSED!

Check out the chlorides contents, in tank after tank cleaning! Experienced several occasions, where there was still , much too high contents of chlorides, left!

Stainless Steel!

Some basic...After tank cleaning, a thorough flush / rinse of bulkheads, lines, pump and coils is extremely important. Correct list, trim etc..!!!!

If tank and lines have been cleaned for long time with hot seawater, then gradually cool the the steel surface down, before any freshwater rinse/ flush taking place!

Separate freshwater from lines and pump, which has been used for seawater, not only by valve, but permanent blind flange! To remove chloride in lines, takes time.

Passivation of Stainless Steel!

When shall passivation take place?After transportation of very corrosive cargoe !After welding and repairs !When sst surface is discoloured !When the sst surface feels rough! When the entire tank doesnt look bright and shiny !Signs of surfaces drying properties!If you are in doubt, PASSIVATE!

Passivation of Stainless Steel!

When handling nitric acid and pickling paste:.

Always use PPENitric acid to be stored in suitable container / tank. Never to be stored in plastic drums.All equipment shall be suitable and resistant.Avoid contamination nitric acid / non compatible products.After completion of passivation, all equipment to be properly cleanned, before stored away.If using flexible sst-hoses for passivation, avoid sharp bends, handle by care, extremely expensive.

Passivation of Stainless Steel!

When handling nitric acid and pickling paste:.

Some vessel use pressurized sst-tank, for passivation purpose, always release pressure, empty and clean the tank, before storing away..

Use recirculation or spraying method!

It is useless to passivate by means of injecting nirtic acid into tank by steam, the nitric acid consentration is diluted., but it works very well through lines!

Lines; cargo and vapour, to be sprayed with 20% solution, then remain closed for 2-3 hrs., with excellent result!

Stainless Steel!

Sediment of Phosphoric Acid!Extremely important to remove, since the combination with seawater, will cause severe damage to the steel!

Stainless Steel!

Trace of previous cargo!Bulkheads shall always be clean!

Stainless Steel!

Pump well!Condensation, a common place to observe chlorides!

Stainless Steel!

Crystallisation of chlorides is often observed in this area!Pittings!

Stainless Steel!

Pittings,by the hundred!

Stainless Steel!

Mechanical damage of the heating coil surface!Chromium oxide layer destroyed!

Stainless Steel!

Repaired pittings!Time consuming/ costly!Pittings by the hundred!

Stainless Steel!

Stainless Steel!

Rust in welding!Impurities!Grind, weld, polish andPassivate, by pickling paste / nitric acid!

Stainless Steel!

Clad sst tank, never been passivated! This result after transporting phosphoric acid!

Stainless Steel!

Stainless Steel!

Stainless Steel!

Stainless Steel!

Stainless Steel!

Stainless Steel!

Stainless Steel!

Stainless Steel!

Stainless Steel!

Stainless steel!

Stainless steel!

Stainless Steel!

Last words!Nobody can turn the clock, but we can take precautions for the future, by eliminate the sources of corrosion!Take immediately action, when corrosion is observed!Ensure yourself, that your colleagues and superintendant, are informed!Carefully record all observation in tank history, the repairs carried out, as well!When your vessel is gassfree, its time for planned repairs!Good luck!

*Halogens Fluorine, chlorine, bromide, iodine, astatine, Fluorine fluorine cyanide, fluoroacetic acid, fluorobenzene etcChlorine - *More molybdenum better / higher resistance to pittingChromium form very thin,passive surface film of chromium oxide,which provide the steel with its great corrosion resistance.S/S for cargo tanks is all austenitic type,with nickel content above 8%AUSTENTIC nonmagnetic alloys of iron,chromium and nickel noted for their resistance to corrosion*If you have a long voyage you can steam tanks to reducing chloride content in tanks.**N.B. Steaming is also helpful to remove chlorides from tanks - *Generally all acid mixed with seawater can cause severe damages to the steel*Tank structure report note all observation ,discoloration , pitting , rusty places etc , After cleaning inspection when tank is dry check if some cargo residues or dry salt remain in tank and were ( note position)