Saturday, April 10, 2010
NITRIC AND CITRIC ACID PASSIVATION ON STAINLESS STEEL CHEMICAL TANKERS-- CAPT AJIT VADAKAYIL
PASSIVATION -- CITRIC VS NITRIC ACIDS-- Capt Ajit Vadakayil
Passivation is a process to build up a layer of Chromium oxide armour on Austenetic Stainless steel of ships cargo tanks.
This mono-molecular oxide layer is about 10 angstroms ( one divided by 10 million-- millimeters ) is invisible , chemically unreactive, self repairing and uni-potentialises the Stainless steel surface. The action is by oxygen and water on a clean surface.
It is our policy to passivate the SUS 316L and SUS 316LN austenitic SS tanks with 14% by volume cold Nitric acid . It is also our policy not to include hydrofluoric acid in a passivation solution. Hydro-flouric acid is a pickler which removes chromium and nickel along with heat hardened scale, it NOT a passivator.
The concentrated acid you get on board in 200 litre drums is usually 68%. Contact time for Nitric acid is 2 hours. Then rinse with Fresh water of low chlorides as obtained from ships fresh water generator tille the pH value is 7. After that allow the tank to dry by natural ventilation for about 12 hours. Before recirc acid , the tank must be totally degreased and clean of all NVM matters.
The usual aggressive cargoes which require passivation to de done is after dischg of wet Phosphoric acid ( fertilizer grade ) , Hot Caustic soda solution, Sulphuric acid, Epichlorohydrin etc. Prolonged washing with very hot salt water after dischg of Palm oil fatty acids etc can also cause local failure of the passive layer. Passivation is also done when free iron , rust and other NVM deposits can adversely affect the metallurgical or sanitary condition of stability of the surface.
In USA the Nitric acid must be carried in stainless steel drums. Sometimes the USCG has been known to give a non conformity if the Nitric acid on board is stored in Plastic drums, never mind how reputed and standard the manufacturing company is.
When you read the ships Charter party find out who is going to pay for Nitric acid and for what all cargoes. Returning empty SS drums can be a very cost intensive process. This is in addition to the rent paid for keeping the drums on board till you land it and it reaches the owners of the drum. This is not as simple or economical as returning empty bottles of oxygen and acetylene.
This is where passivation with Citric acid comes into the picture. It must be very clear that Citric acid passivation is only the second best. EMS wants Nitric acid passivation as far as possible. Only when the cost are excessive to the ships budget ( not charterers account ) then Citric acid passivation may be considered, provided the process is done the right way. Or if you have to load FOSFA cargoes , where you cannot declare in writing that the passivation was done with FOSFA banned Nitric acid.
Passivating with Citric acid is a misnomer by itself, as Citric acid is not an oxidizing chemical like Nitric acid. It just removes the free iron like Nitric acid. The building up of the Chromic acid layer is now done ONLY by the magic action of oxygen and water of mother nature. Recirc with 7% Citric acid by weight , contact time required is only one hour. Both these parameters are half that of that of Nitric acid. Rinse till pH 7 with ships FWG chloride free water But the natural venting time to get the proper angstrom thickness of CrO2 must be at least 24 hours which is double that of Nitric acid.
PROFORMA:--- Item/ Nitric acid pure/ Citric acid formula
Strength of acid solution to recirculate/ 14% by volume/ 7% by weight
Acid contact time/ 2 hours/ 1 hour
Water to use for rinsing/ FW generator water FW generator water
Rinsing till pH / 7/ 7
Steaming tank/ No /For 30 minutes, to sweat out acid from deep pore penetrations
Ventilation/ Natural for 12 hours/ Natural for 24 hours
Chelating action ( removal of free-iron , exogeneous iron, ferrous contaminants)/ Yes/ Yes, faster
Oxidising action on SS/ Yes/ No—BIG DISADVANTAGE
FOSFA banned/ Yes/ No
Cost effective/ No/ Yes
Degreasing required prior recirc of acid/ Yes/ Yes
Slop retention / Changes colour (yellow)- decomposition due to sunlight/ No colour change
Toxic fumes/ Yes/ No
Penetration into SS pittings and pores/ Good/ Very good
Flash corrosion attack/ No/ Yes—if the acid does not contain corrosion inhibitors or wetting agents
Smog/ NOx vapours/ No
Safe on humans/ No/ Yes
Biodegradable/ No/ Yes
Type/ Inorganic/ Organic
Solubility in water/ Fully soluble/ Fully soluble
Re-use of solution/ Yes/ Longer use andlonger lasting
Accelerators –NOT TO BE USED ON OUR SHIPS--/ Sodium di chromate(oxidizer booster )in the passivation solution for faster build up of chromium oxide layer/ Very light 100 ppm solution of Hydrogen peroxide ( powerful oxidizer ) after rinsing till pH 7, to cut down the excessive ventilation time
Negative action of recirc acid on a UN- degreased surface/ Burns grease into the
pores and pittings/ Gas bubbles which affect natural passivation
Oxidisation of nickel to prevent uncontrolled tarnish/ Yes/ No
Passivation of ordinary iron plate/ Yes/ No
Removal of heat hardened scale/ Yes— pickling along with Hydroflouric acid/ No
AS YOU CAN SEE CITRIC ACID SCORES OVER NITRIC ACID BY BEING A FASTER CHELATOR ( CHEMICAL DISSOLUTION ) , BUT HAS A HUGE DISADVANTAGE, WHICH CAN HARDLY BE WISHED AWAY OR DOWNPLAYED BY ENVIRONMENTALIST LOBBIES—IT IS NOT OXIDISING REPEAT IT IS NOT OXIDISNG. HOWEVER THE USE OF A WEAK HYDROGEN PEROXIDE SOLUTION , AS A POST PASSIVATION RINSE CAN ELIMINATE THIS PROBLEM.
Hydrogen peroxide can oxidize SS but it cannot remove free iron. A strong solution used inadvertently can react with free iron left behind after improper chelation and affect the chromium metal.
Ships SS tanks are Austenitic in the 300 series like 316L , 316LN and 317L . What is good for Martensitic or Ferritic grades or other series does not hold good for Austenetic grades.
Citric acid is the same acid found in oranges and citrus fruits. It is used commonly in household cleaners and disinfectors.
The US EPA has recogonised the safety of Citric acid to the environment. It efficiently removes free iron from the surface of Austenetic SS and thus can be used in lesser concentrations than Nitric acid. Since it has very ,low reactivity with other metals of Austenitic SS, the slops can be retained for longer periods .
Due to low reactivity of Citric acid with metals other than free iron it is generally safe to leave the slops on board for longer time than Nitric acid.
When SS is buffed or ground with non-metallic discs, more chromium is exposed which reacts with oxygen and water to form the chromium oxide armour. If some particles of carbon steel is embedded in a rough surface then the self repairing cannot take place. Instead corrosion will take place when the metal is wetted or exposed to a corrosive environment ( galvanic couple between 2 dissimilar metals in an electrolyte ).
CAPT AJIT VADAKAYIL ( 28 years of command )