SS SOLID AND CLAD SUS 316 LN , SUS 317 LN
The oldest example of stainless steel is antiquated as the iron pillar of Delhi erected 1600 years ago by Indian king Kumara Gupta 1. Here the patina or protective layer oxide is not by chromium but by Phosphorous.
However the presence of high chromium / nickel rock ore mines in India have ensured that some of the old steel weapons are still intact. Even modern technology has not developed any steel which is completely stain proof or corrosion proof.
Stainless steel is the best for aggressive cargoes and acids.. Stainless steel corrodes in mineral acids, non oxidising acid solutions, reducing acids ( Sulphuric acid of >80% strength is not very aggressive , only medium rage is aggressive due to non-oxidising character ), and hot caustic whose corrosion rate increases with increased temp.
Chlorinated hydrocarbons also called halogenated hydrocarbons by themselves do not attack SS since they are non electrolytes. The dissolving effect they have on organic paint coatings is not possible on SS. It is the impurities which corrode. If the decomposition products are acidic or contain high concentration of chlorides rapid corrosion may occur.
Oxidation or hydrolysis (salt + water = acid + base ) reactions between Chorinated HC and residual water can cause corrosion. The water content of Chlorinated HC if greater than 100 ppm corrodes the tank. Oxidation is prevented by inhibitors. Obtain full specs incl impurities. This must include residual water, Chloride and Hydrogen Chloride content. Clean tanks ASAP after discharging. Remember Chlorine ( or inorganic chloride ) in fresh water does not attack SS.
Chromium metal dissolves in Hydrochloric acid and dilute Sulphuric acid at room temperature. But it does not dissolve in HNO3 because of a curious phenomenon known as passivation in which the metal becomes covered with a thin unreactive coating. Ordinary iron gets passivated by Nitric acid due to the formation of Ferroso Ferric Oxide Fe3O4 film on the surface of iron.
On ships you do not allow crew to enter a SS tank without shoe covers. The idea is not about preventing of dirtying the tank. It is about damaging an armour and contaminating it with the carbon of black sole safety shoes.
Stainless steel depends for its corrosion resistance on the formation of a passive surface film, which is composed, mainly of chromium oxide. Although this film forms spontaneously when the metal is exposed to air or to water, it is possible to damage or contaminate the surface during fabrication or during service so that the ability to form a satisfactory film is impaired. It is necessary to avoid or correct such damage if the optimum corrosion resistance of the stainless is to be obtained. It should be noted that the surface will usually repassivate if the film is damaged- if this were not so, the corrosion resistance of stainless steel would be no better than that of a coated steel. Only when the chromium content of SS is greater than 12% the chromium oxide layer (CrO2)forms.
The thickness of a well builtup oxide film is about one millionth mm on the surface of stainless steel. The unit is Angstroms which is one divided by ten million of a millimeter. Despite the thinness of this armour provided, it will provide good corrosion resistance. A tank which has been hot washed for long must be allowed to rest before wall wash allowing the Chromium Oxide layer to build up. Since passive layer consists of oxides the stainless steel has to be oxidised to maintain the positive layer. Remember the Chromium Oxide layer is invisible. The film can be destroyed by active metals like iron particles or by oxygen starvation.
A protective armor will form on Austenetic SS only if oxidising condition are present. Nitric acid is a strongly oxidising liquid. Passivating with 14% cold Nitric acid is the best way to build up the chromium oxide layer with 2 hours of contact time. Then the SS has to be rinsed with fresh DI water till the pH is 7. This solution also acts as a thorough cleaning agent for stainless steel, rapidly and selectively dissolving free surface iron while conditioning the surface for self passivation. (do not use Nitric acid greater than 20 percent strength for passivation as the iron contamination will not be removed).
Conditioning possibly involves anodic oxidation of the surface. The nitrous gases which emanate while recirc in the tanks are toxic and have a delayed effect. Rinsing clean SS in oxygen rich water or storage in open air can also do the trick but it takes too long ( few days ) to develop to the correct thickness . Steel automatically passivates by the action of mother nature. So it is advisable to put 100 litres of fresh water inside a cleaned (to WWT or WW) tank on the ballast passage. Air must be replenished regularly ( no forced ventilation is allowed ).
Passivation with Nitric acid is usually done prior loading very aggressive chemicals. Passivation is necessary after discharging even moderately aggressive cargoes like Palm fatty acids. Passive SS can be rendered active by dipping in Nitric acid heated to 75 deg c. Hence the reason for cold nitric acid recirc. In circumstances favourable to passivity ss has a potential of near noble metal. If passivity is destroyed SS is no different from ordinary iron. Nitric acid > 20% should not be used for passivation as iron contamination will not be removed. In absolute reality nitric acid removes surface contamination which would interfere with the passive oxide film redeveloping.
Even ordinary iron can be rendered passive by nitric acid due to the formation of Ferroso-ferric oxide layer ( Fe3O4).
Palladium chloride test for active surfaces:
Passive steel does not liberate Hydrogen form Sulphuric acid. Palladium can absorb 900 times its own volume of Hydrogen by Adsorbsion and Occlusion. Hydrogen gas reduces Palladium Chloride PdCl2 easily to form black Palladium metal and Hydrogen Chloride.
H2 + PdCl2 = Pd + 2 HCl
This is the reason that Sulphuric acid or HCl must be mixed with palladium chloride if the black precipitate of metal must be seen by the naked eye. Passive SS does not liberate hydrogen. Palladium Chloride results can be tested using a mass spectrometer too.
Rinse PdCl2 with DI water from a squirter as the black spot is active.
If free iron ( not oxidised iron ) is present on the SS surface the iron is oxidised by Palladium Chloride ( acid reagent solution ) to iron oxide.
4PdCl2 + 4 H2O + 3 Fe = 4 Pd + 8 HCl + Fe304
Palladium chloride comes in a small bottle with a dropper. The contact time should be 3 min minimum. The shelf life is one year if you keep it in the fridge in a coloured bottle. Unitor sells it. Palladium chloride crystals or deliquescent powder is soluble in water or alcohol.
Salt spray test: passivation is evaluated by appearance of rust spots.
Copper sulphate test: a galvanic type test in which the more noble copper plays upon less noble iron partcles but not on SS. Patches of copper sulphate shows a poorly passivated surface. Passivated parts are immersed in a Copper Sulfate solution for five minutes, rinsed and visually examined. Any copper (pink) color indicates the presence of free iron and the test is considered unacceptable.
Ferroxyl test: causes coloured iron compounds to appear as specks.
Passiprobe electric test: a mildly corrosive acetate buffer absorbed in a porous test pad i brought in contact with the suspect active SS. The reference passiprobe also contacts the porous medium completing the electrical current for measurement. Surface free iron will cause the passive reading value to go higher in the non noble direction. Remember free iron is not oxidised.
Rust in minute particles can stay suspended in viscous heavy liquids like MEG which can fail a first foot. Rust can accelerate polymerisation.
Passivation Oxyliser meter test:
The passivation meter sets up a temporary corrosion cell. The anode is the stainless steel surface and the cathode is the reference electrode. The electrolyte is a suitable solution for the application and is applied dropwise to a piece of absorbent paper, which is then placed on the steel surface.
The principle is based on the electrochemical rest potential. This is one of the most direct ways to measure passivity. It is faster and more accurate than palladium chloride. The millivolt scale is from 0 to 100, in passivity units.
It is used to check the presence of free iron.
Measuring temperature is 15 to 25C
The surface to be tested must be free of rust , colours and stains.
Connect the black reference electrode and the red anode clamp to the oxiliser.
Remove the plastic soaker bottle from the tip of the reference electrode and clean tip properly with a tissue.
Connect the red clamp with SS
Moisten a filter paper strip with the fluid for SS. The paper should be wetted properly. Place this paper on the SS.
Place the tip of the reference electrode on the paper.
The tip must make good contact with the fluid.
Read off on the LCD display—wait 10 seconds for reading to stabilise.
<60 low passivity-- and reducing >60 good passivity.—and increasing
Steady values upto 90 are possible.
If after 30 seconds the passivity is reducing—the passivity is not good.
If the surface contains too much free iron—negative values can be measured.
If you grind a piece of SS with scotch brite, the surface will be activated and value<30 which will slowly increase can be measured If you scratch the SS with a iron nail <10 and reducing can be measured. The ref electrode should be immersed in 3.8 molar potassium chloride solution.—there is an enclosed soaker bottle filled with KCl for this. Powered by 9V battery.
Stainless steel is a general name given to a whole family of high alloyed corrosion resistant steels in which chromium is the main alloying element. It is important that you know the exact type of stainless steel 316L , 316LN etc you have inside your chemical tankers tanks for clad s/steel, solid s/steel ,steam coils , ladders , supports ,pump shell etc. It is widely accepted that under oxidising conditions for steel with a chromium content of at least 12%, an impervious and largely invisible oxide film forms on the surface which is self healing acts as a barrier between the metal and its environment and this effectively reduces corrosion rate. In reducing conditions the film however breaks and the air formed film will not be repaired and corrosion will be enhanced in damaged areas. Chromium improves hardness and wear resistance.
The inclusion of Nickel as alloying element improves the strength and corrosion resistance.
The addition of Molybednum up to 3.5 % will reduce the susceptibility to pitting attack, which could result from the presence of chlorides. Molybdenum improves hardness and reduced enlargement of grain structure. On a parcel tanker it is important to note which tanks have steam coils of higher molybdenum content for cargoes like heated caustic. Stress corrosion occurs in heating coils. Higher mo grades are more resistant to pitting. Molebdenum is soluble in hot concentrations of nitric acid.
Stainless steel is classified according to their crystal structure which is defined in terms of composition and heat treatment into following groups: Martensitic. Ferritic. Austenitic.
When carbon distributes itself evenly in the iron the material is called austenite. It is named after Sir WC Roberts Austen. The austentic stainless steel is generally used in the construction of chemical tankers, since they are compatible with a wide range of cargoes. The austentic grades have good workability and comparatively good weldability. Material formation of austenite group is like a cube. The boundary of the cube is corroded by any carbionic trouble., grain corrosion. Austenitic stainless steel is non magnetic and non heat treatable. The surface of SS is not so bright.
The Molybdenum free grades are 304, 304L and 321 while the molybdneum containing grades are 316, 316L, 316LN or 317 and 317L. The austenatic steel generally consists of C: 0.02 to 0.10 %, Cr: 18 to 25%, Ni: 8 - 22 5, Mo: 0-4%. The notation “n” means it has more nickel for better resistance.
304---Chromium/ 19% Nickel/ 10% Molybdenum/ nil Carbon / 0.08% 316l--- Chromium / 17.5% Nickel / 13% Molybdenum/ 2.7 % Carbon/ 0.03% 317l--- Ch/ 18.5% Ni/ 14.5% Mo/ 3.5% C/ 0.03% 254smo---Ch/ 20% Ni/ 18% Mo/ 6.5% C/ 0.02% Al-6XN--Ch/ 22% Ni/ 25% Mo/ 7% C/ 0.03%
The thermal expansion of stainless steel is higher (1.8 times) compared to mild steel while heat conductivity is lower (40%) and electrical resistivity is greater. There are great possibilities of distorsion and warping due to longer cooling time of SS. Therefore whenever possible clamps and jigs should be used to keep the pieces in line until they have cooled.
Certain specific agents such as halides causes a local breakdown of the passive film on the steel surface. This highly localised breakdown followed by electrochemical action results in pitting which rapidly propagates into steel. Chlorides are particularly conducive towards this form of attack even when present in minute quantity. Pitting hole is like a bulb and is difficult to detect without a dye penetrant.
Collection of solid sludges on the surface of stainless steel may also result in pitting. When sludge is known to be present in certain type of cargoes, means of circulating the cargo should be provided e.g Phosphoric acid.
Crevice corrosion occurs with chemicals containing chlorides and other halogen ions. Rapid corrosion will occur in narrow crevices / geometric irregularities in the metal surface where oxygen cannot gain access. It is essential to design tanks with this in mind. All welds are smooth finish and all spatter is removed. Different electric potential can exist on metal surface with anode rapidly corroding and perforating. Stress corrosion may be defined as the fracture of a component by the conjoint action of a tensile stress in a corrosive environment. The failure of this is confined to environments containing either Sodium Hydroxide, Sulphides, Halides and mainly Chlorides. Chlorides are particularly conducive to this form of attack and cracking, which is transgranular is found most frequently in the ph range of 3 to 8. Cracking associated with caustic solutions may be either trans or granular and is usually confined to solutions of high conc and with temperatures in excess of 60C. Causic embrittlement is the corrosion resulting in cracking of steel stressed beyond its yield point due to localised concentrations of hydroxide ions breaking down the cohesion between the ferrite grains. If the steel is heated in the temperature range of 500 to 850C the carbon may precipitates as Chromium Carbide at the grain boundaries. These areas become denuded of chromium and the protective oxide film is prevented from forming. Steel in this condition is termed as "sensitised" and on exposure to a corrosive environment will suffer localised attack at the grain boundaries. During welding some regions in the parent metal near the weld will reach temperatures in the sensitised range, and subsequent exposure to a corrosive environment the steel will be susceptible to this form of attack. This is also known as weld decay. This can be generally avoided by using steel with carbon content below 0.03% max, and addition of elements with a stronger affinity for carbon than chromium like titanium or niobium.
Solid stainless steel: the tanks are built of full stainless steel plates. Usually the corrugated forward and aft bulkheads are made of solid steel.
Clad steel: the term clad steel refers to a mild steel base plate to which a thinner layer ( 3mm ) of a stainless steel is continously and integrally bonded. In this way cost of vessel could be reduced. The mild steel side is used in water ballast tanks and the difficulties of bimettalic corrosion in connection with the use of solid stainless steel are avoided. The clad steel is formed by: Roll cladding. Explosive bonding.
Cutting: Cold cutting: solid stainless steel plates are often cold sheared to size. Hot cutting: Flame cutting with flux or iron powder injection Plasma arc cutting. The oxy- acetylene flame cannot be used due to the formation of a highly refractory chromium oxide at the interface. But with fluxing agents injected into the cut the smothering oxide is rapidly removed and fresh surfaces are continuously exposed. Clad steel may be mechanically sheared to size, but this should be carried out with clad side uppermost so that any burr will be on the carbon steel side. When flame cutting it should be done from the carbon steel side. Note: after gas cutting the edges should machined or grounded to clean all scale prior welding. Plasma arc can cut SS upto 10 cms thick with temperatures 11000 deg c to 28000 deg c.
Welding: A correct procedure and correct consumables should be used for welding of stainless steel to prevent weld joints to crack. It is important that the heat input during welding is kept low to ensure a small heat affected zone and exposure to the sensitising temperature range for the shortest possible time. Spatter and rough surfaces can result in local breakdown in passivity resulting in pitting. Precautions should be taken before welding commences to prevent any spatter formed from adhering to the surface adjacent to the weld by employing masking tape or by applying suitable temporary coating. The welds should be smooth free from porosity, inclusions and cavities and carried by competent welders.
Welding of solid stainless steels: For butt welds the vee preparation has been used successfully in the construction of ships tanks for both flat and vertical positions. Some times for welding of thick plates copper backing bars are used. The copper backing bars extracts heat during welding and also gives profile to the root bead. In order to avoid distortions for thicker plates double V preparation with separate runs deposited alternately one on each side of the joint is used.
Welding of clad steel: Welding of clad steel plates generally involves separate treatment procedures for steel base and cladding material. The weld deposit for the backing plate must at least match the strength of the carbon steel parent material. The usual practice is to choose an electrode, which would be used, if the cladding was not present. If this carbon steel weld metal comes in contact with the corrosion resistant cladding, the weld material will be diluted by the alloy and the deposit is likely to be hard and brittle. This problem is offset by using of filler materials having alloy contents greater than those of the parent stainless steel. Generally the first weld made from carbon steel backing side should penetrate to the stainless cladding but not into this layer. The backing side should be completed before chipping back to clean metal on the clad side. Sufficient metal should be removed to allow at least two runs of alloy weld metal to be deposited. The first two runs are made by using filler materials having alloy contents greater than those of the parent stainless steel. For subsequent runs should be made with a view to ensuring that the deposited metal matches the composition of the clad material and that the properties are not impaired. For thin gauge material the total plate thickness may be welded with electrodes comparable in composition with cladding material.
Generally all the fusion welding process like manual arc welding with covered electrodes, metal inert gas or tungsten inert gas welding technique could be used for welding stainless steel joints. Submerged arc welding is suitable for thickness greater than 10 mm. The saw process gives high heat input hence it is essential that the parent steel grade has a very low carbon content. Light gauge electrodes and a low welding current should be used. With multipass welding the interpass temperature should be less than 100C in order to reduce the possibility of precipitation of grain boundary carbides. Many of the processes used in tank construction affect the passivity of the surface. The metal surface near welds oxidises during the welding operation and the oxide films produced are less protective than the normal film.
The commonest types of surface contamination which can lead to localised pitting in corrosive environments are: Embedding of matter such as iron and iron dust in to the surface. This causes pitting, pores in welds and rough surfaces. Entrapment of slag weld metal. Weld spatter- particles of slag and weld metal (from the welds in the tank itself or from structures adjacent to the tank adhering to the metal surface. Coloured oxide films near welds - these result in a dechromised surface beneath the film. Splashes of paint etc which may stick to the surface with formation of crevices.
Care during fabrication: Contamination of the surface by iron, rust etc can be minimised by: Keeping the surface of stainless steel covered. Placing sheets of hardboard over the ss when it is being shaped by rolling. Coating surface with anti spatter before welding, which are readily removed after fabrication. Stainless steel wire brushes for deslagging etc should be used. A set of cutting tools / hand grinders + wheels should be separately kept exclusively for stainless steel work they should be suitably colour coded. As far as possible the handling of carbon steel and stainless steel on the same work tables and in same work shops should be avoided. The stainless steel work place should be separately demarcated. Do not put any carbon steel / rusted equipment on the surface during fabrication. The carbon steel tools to be avoided on the surface. Men engaged on stainless steel work should not be permitted to use steel shot footwear.
As far as possible handling of stainless steel and carbon steel on the same working surfaces and in the same work areas should be avoided. Also prefabrication of stainless steel should be preferably take place in areas which are free from carbon steel dust. If grit blasting is applied then only clean sand or grit, uncontaminated by iron or rust should be used. On no account the stainless steel surface shouldbe shot blasted. Handling equipment such as slings, hooks and lift trucks forks should be protected with clean wood, cloth or plastic buffers to reduce contact with iron surfaces.
Final cleaning: Although careful attention to the procedures described above will minimise surface contamination, it is impossible to avoid this completely and some final surface treatment, normally involving acid pickling and washing is necessary.this procedure is often referred to as 'passivation' implying that some form of protective, film is developed on the metal surface. In reality, however, this treatment is to remove surface contamination which would interfere with the passive film normally developed on the stainless steels. Degrease tank before passivation or nitric acid will burn the grease into the SS surface.
The pickling is done by: Applying pickling paste. These pastes are based on Nitric acid or Nitric / Hydrofluoric acid mixtures and are applied to the areas of the tank where surface contaminatation is evident. E.g. Coloured oxide films near welds. To wash down the tank surfaces with a mixture of Nitric acid (6 to 15 % by volume) and Hydrofluoric acid 1/2 - 1 1/2% by volume) or somewhat stronger solution of nitric acid (10 - 20% by volume). Although Nitric/ Hydrofluoric acid is faster than Nitric acid care must be taken to ensure that it does not over etch the steel surface.
Another typical pickling paste combination is Nitric acid 14%, Hydroflouric acid 4.8% ,Ammonium BiFlouride 5%, tackiness increment agent 20% and other non ionic surface active stuff. Remember Pickling and Passivation are as different as sugar and salt. Do not confuse one with the other. Picklings strips –passivation armours. Pickling removed hard oxides formed by weld heat. It can be compared to the Johnsons liquid strippper and polish you apply in ships alleyways. Pickling involves metal removal and dulls the visual brightness of SS. Sulphuric acid , phosphoric and hydrochloric acid can be used for pickling—which is removal of impurities.
Following is the procedure generally used for final cleaning of tanks: Remove weld slag by wire brushing / grinding. The uneven weld profile is ground smooth contour. Remove weld discolouration with abrasive stainless steel wire wool, pickling paste or local swabbing with a pickling solution. Wash the tank with a strong detergent solution to remove grease, swarf, dust, paints, oil etc. The application of heat to the tank will expedite this process. Clean tank surface with a 6 - 15 % nitric acid solution with or without hydrofluoric acid and continue cleaning until all visible scale is removed. Remove as much acid from the tank as possible; wash down throughly with hot water containing a detergent and empty. If the above procedures are correctly carried out a clean iron free surface should be obtained on the tank. You can reuse pickling solutions. If the pickling liquids iron content is >5000 ppm renew the soln.
Over pickling must be avoided. Uniform removal of scale with acid pickling depends on the acid used, acid concentration, solution temperature and contact time. Continuous exposure to pickling solutions for more than 30 mins is not recommended. The item should be drained and rinsed after 30 mins and examined to check the effectiveness of the treatment. Acid cleaning is not generally effective for removal of oils, greases and waxes. The surface should be cleaned to remove oils and greases before acid cleaning. After Sulphuric acid discharge brown discolouration is removed by pickling gel and then again repassivating.
If any repairs or welding have to be carried out on the tank after it has gone in to service than the cleaning and passivation procedure should be repeated, although this need only be over a local area.
Note: although the corrosion resistance of stainless steel improves somewhat as the degree of surface of finish improves, it is unlikely that any practical finishing process, for example hand grinding and disc polishing will result in a significant improvement over the performance of a hot rolled annealed and pickled surface which has been properly cleaned after fabrication. The use of extensive grinding can result in significant loss of thickness, which may be of particular significance in clad steel tanks. The value of grinding and polishing apart from smoothing weld profiles and removing obvious surface defects, is doubtful.
The fresh water is the preferred cleaning medium for stainless steel and where possible it should be always be used. Due to unlimited availability of seawater, it is often used for initial tank cleaning operations. Where this done a final tank wash with fresh water must be given to avoid prolonged contact of pools of stagnant seawater with stainless steel surfaces.
Piping systems require particular attention to avoid leaving inadequately drained areas full of seawater it is not necessary to repeat the acid cleaning and passivation process unless the tank has been repaired or it has carried a particularly corrosive cargo like phosphoric acid containing high levels of chloride and fluoride impurities. The acid could leave steel surface in an active condition with pits containing corrosion products. In such circumstances Nitiric acid treatment after washing would be beneficial.
For all practical purposes the general corrosion of austentic stainless steels in sea water can be taken as zero. However the passive film on these steels is prone to local breakdown when chloride ions are present. Hence pitting is likely to occur if stainless steels are left in prolonged contact with static sea water. The attack is likely to occur particularly at crevices or under deposits where access of oxygen to the surface is restricted. Chlorides can locally attack the passive layer and prevent formation of new film. Stagnant salt water can initiate pitting within 24 hrs—you add some Sulphuric acid to it—and time is now reduced to 12 hours.
The addition of Molybednum to stainless steels improves their resistance to pitting and crevice corrosion.
The possibility of pitting of stainless steel is increased if the pH of the seawater is reduced from normal level of about 8. This can happen on chemical tankers which have carried acid cargoes and cleaning of such tanks. Once tank cleaning is commenced must continue without disruption to minimise contact between stainless steel and acidic sea water. The quantity of seawater used must be the maximum possible so as to minimise the drop in ph.
Once sea water cleaning is started than under no circumstances the washing should stop till the tank is cleaned. The final rinsing after seawater should be carried out with fresh water. There should be no possibility of leaving sea water for long periods static in contact with stainless steel. The sea water temperature should not be kept higher than 50C. It is important that there are no stagnant salt water pockets in pipelines. There can be no pitting if salt water flow is greater than 1.2 mtrs/ sec.
Always clean SS tanks asap after discharge even if the same chemical has to be loaded again and again.
In order to obtain the optimum corrosion resistance from stainless steel cargo tanks it is necessary to avoid damage to the tank surfaces. This can be achieved by care in design and fabrication and by treating the steel surface after fabrication in such a way to remove surface contamination.
Care in service should aim at retaining the passive layer on the steel surface which will than provide corrosion resistance throughout the life of the ship. Great care and right procedure should be used for tank cleaning especially after discharging corrosive cargoes like phosphoric acid and using sea water as initial cleaning medium.
Chemicals which attack stainless :
There is a full lloyds list available with this manual. Some corrode with impurities over the acceptable limit , some corrode due to temperatures over the acceptable limit. A few examples just for understanding are—
Benzene sulphonyl chloride
Chlorobenzene ( benzyl chloride )
Drilling brines like calcium bromide and cal chloride soln
Lignin sulphonic acid
Sodium salt solution ( sodium hydrate )
Potassium chloride (>10%)
Sodium nitrite sodium sulphate
Do not accept odd commercial name cargoes unless you know the product. Many times such things are done to beat the system for lesser customs duties, flouting Marpol laws etc. Read cargo specs carefully. Add up the percentages to form 100% . If does not add up find out what is missing. Of if some important component corrosion accelerators like chloride or flourides are missing from green/ black phosphoric specs find out why they are deliberately omitted.
Or if the moisture content of Epichlorohydrin is not mentioned find out why. For once the stainless steel has pitted you are responsible for destroying a good tank. Once pitting starts you cannot carry sensitive cargoes like potable Ethanol or HMD. Follow proper sampling procedures. Keep samples safely. If tank gets severely pitted get it analysed in an independent lab in order to challenge the false quality certificate. If the tank inspector challenges the condition of the SS cargo tank, counter challenge him by cleaning off part of the bulkhead where ever he wants with the product to be loaded to prove all is OK.
If Hypochlorite solutions are used for tank cleaning they must never be left in the slop tanks. Bleach or Sodium Hypochlorite (11 -13%) is very aggressive to SS especially at elevated temperatures.they must never be allowed to dry up on bulkheads.
Kosher Acetic acid is usually loaded only in stainless steel tanks, though it pits and causes inter granular corrosion.
Sludge sediment from Phosphoric acid made by the wet process ( Silica Quartz) accelerates corrosion. This is the reason why recirc is done.
You are not allowed to ballast stainless steel cargo tanks without taking permission from the EMS chemical operator. If it is necessary to ballast stainless steel tanks—
1) Use clean water up to 98% .
2) Raise pH value of ballast from 8 to 11 using Caustic Potash.. Say put 40 kg KOH flakes in 1000 tons sea water.
4) Short duration only.
As soon as the ballast is out rinse with fresh water.
After aggressive cargoes like Sulphuric acid preferably the initial quenching should be done with fresh water. Sulphuric acid leaves liquid top surface profile ( wedge ) of exhothermic corrosion. If it cannot be done immediately use nitrogen blanket. After dischg black/ green phosphoric prewash should preferably be with fresh water. If sufficient fresh water is not available at least the first and last wash can be with FW, with the salt not being allowed to dry up. Acidic stagnant sea water is very dangerous for SS. Weld spatter areas are corrosion initiation points. Weld spatter must always be removed by grinding and pickling. Remove weld discolouration by wire brushing and pickling with brushes.
Remember improper welding of ss by pretenders results in complete loss of the corrosion resistant properties. Do not burn ss while grinding. Use self sharpening grinding disc to avoid smearing. Ground area should be white and not blue/ brown. To prevent overheating the grinding disc must be constantly moved over a relatively large area ( use cooling air hose ) . The edges must blend in smooth without sharp edges and borders. If a rag is drawn and lint is shed the surface is too rough. When grinding remember the weld is harder than the ss plate. Always use matching electrode for molybdenum , chromium etc. Use small dia electrode, low heat input and matching amperage. To dry pits use very slight preheat. Minor pittings are ground. Use ss iron free zircon grinding disc of grain 80 ( coarse ) or 120 ( fine ). Zircon has a hardness of more than 8 on the Moh’s scale.
Major pittings are drilled out like a valley ( the is to reach the bottom of the bulb ) and spot welded and then ground smooth. Never drill out pits as a blow hole on a spot weld is a corrosion initiator which will ooze black tatoo liquid once the non inert gas backed weld gets oxidised. Pittings start at 1 mm depth and dia. At times they are hidden with tiny entrance holes to a big cavity. Dye penetrant tests are useful when you know where to look for pittings. If you do not know where to look for pittings you will never find them. Small pittings are covered by a metal flap , but the subsurface bulb cavity may be quite large . The clad steel is only 2mm to 3mm thick on chemical tankers. If the clad steel pitting has breached through , first fill up with arc and then only with tig build up.
Any grinding must be followed by pickling and passivation. Cleaning before pickling must be done with strong warm non caustic alkaline detergent. Pickling burns iron particles and millscale. Every 12 deg rise in temperature halves the pickling time. All repair jobs such as welding, grinding etc., shall be pickled. Never pickle more than 30 minutes. Over-etching of stainless steel may produces coarse surfaces. Pickling contamination which would interfere with the passive oxide film of stainless steel. Pickling reduces pitting corrosion.
In welding SS with oxyacetylene it is necessary to use a tip which is 2 sizes smaller than a normal one used for mild steel. Since heat has a tendency to remain longer in the weld zone of SS a smaller flame reduces the possibility of destroying the properties of SS. A neutral flame is very essential for welding SS. Even a slightly oxidising flame oxidises the chrome in the SS thereby reducing its corrosion resistant qualities. Since it is difficult to maintain an exact neutral flame it is better to use a slight excess of acetylene.
Specially treated columbium filler rods are essential for good welding of SS. If SS rods are not available the second best way would be to cut strips from the mother metal and use them as filler rods. Not only does the chromium in the SS oxidise easily , but the oxide formed during welding acts as an insulating barrier between the flame and the workpiece. Success in welding ss therefore depends hugely on keeping heat to a minimum. Always use SS compatible flux and last but not the least complete the weld in one pass. You do not weave the low current small dia rod weld in SS tanks. Strike the arc on an arc striking plate and then lead the arc to the weld area. This is to make the welding current constant. Weld from edge to pit to other edge to avoid gas inside the pinhole bulb.
Welding rods for say 316L must have a carbon content of <0.03 and a Mo content of> 2.4. If the welding cable is too long, the voltage drop will be too great. If the cable is too small for the current it overheats and the power is lost.
It is commonplace for ageing parcel stainless steel chemical tankers to have on board a TIG welding set a MIG set and a plasma arc cutter. Of course every stainless steel shipyard will have them.
Tig stands for tungsten inert gas. The TIG electrode is non consumable. Inert gas prevents oxidation of electrode. For joints where additional weld metal is needed a filler rod is used.
It is important to have a weld that has the same properties as the base metal. Such a weld can only be made if the molten puddle is completely protected from the atmosphere, to prevent oxygen and nitrogen from being absorbed into the molten pool ( makes weld weak and porous ). No flux is required. Bonuses being very little smoke , fumes , sparks and weld spatter. If the amperage is > 200 the ceramic cup at torch head is water cooled—otherwise it is air cooled. Argon inert gas is heavier than air and blankets weld well. DC weld has pointed electrode and AC weld has spherical electrode. DC weld causes magnetised weld. In AC machine the electrode is not touched for starting the arc, unlike DC machine. The filler rod physical properties must be similar to the base metal. On light guage metals backing is used to protect the underside of the weld from atmospheric contamination. On heavier stock the copper backup bars draw the excess heat.
MIG stands for metallic inert gas. MIG set uses a continuous consumable wire electrode which is fed through the torch at preset controlled speeds. Less time is required to train an operator ( a couple of hours, to know the transfer type ). All the welder has to do is to pull the trigger and weld. As a rule weld failures are often due to the starting and stopping of welding—since this includes slag, cold lapping and crater cracking. Spray transfer has constant spray of metal. In globular transfer the molten ball at the tip of electrode tends to grow 3 times the dia of wire before dropping down . In short cut transfer each drop touches the weld puddle before breaking away from the electrode wire. Electromagnetic force squeezes the drop from the wire. The short circuit is broken and the arc reignites. Shorting occurs 20 to 200 times per second. Shorting of arc pin points the effective heat. Intricate welds are thus possible. Argon, helium and Co2 gases are used for arc shielding austenitic stainless steel.
Generally surface porosity is the direct result of atmospheric contamination. The chief cause of crater defects is due to mindlessly removing the gun and shielding gas before the puddle has solidified.
Plasma arc cutting:
Temperatures up to a mind boggling 28000 deg C is possible. Plasma cutting is used in ship yards for high speed cutting of SS. Plasma is often considered the fourth state of matter. The other three being gas, liquid and solid. Plasma results when a gas is heated to a high temp and changes into positive ions , neutral atoms and negative electrons. When matter passes from one state to another latent heat is generated. Plasma torch supplies energy to a gas to change it to plasma. When plasma changes back to a gas heat is released. In a plasma arc torch the tip of the electrode is located within the nozzle which has a very small orifice. The gas emerges in the form of a supersonic jet hotter than any flame. It melts any known metal and its velocity blasts the molten metal through the kerf. When cutting SS best results are obtained with an Argon Hydrogen or Nitrogen Hydrogen gas mixture. Cooling water is used.
A central core of extreme temp surrounded by a sheath of cool gas. The required heat for fusion is generated by an electric arc which has been intensified by the injection of a gas into the arc stream. The superheated columnal arc is concentrated into a narrow stream. This when directed on metal makes possible penetrative welds without filler rods or edge preparation.so what is the big deal between tig and plasma welding? In plasma welding the arc column is constricted for higher heat. The arc plasma actually becomes a jet of high density current. The arc gas upon striking the metal cuts or keyholes entirely through the piece producing a small hole which is carried along the weld seam.during this cutting action the melted metal in front of the arc flows around the arc column then is immediately drawn together behind the hole by surface tension and reforms into a weld bead. A heavy duty DC rectifier is used as a source of power for plasma welding. Argon is used as plasma gas and helium as shielding gas ( argon too ).
Bottom line: fresh water rinsing of stainless steel tanks after tankcleaning with salt water must be doen by tankcleaning machine and not by hand hoses.
Some cargoes like Phosphoric acid and Sulphuric acid may form brown stains in the tanks and will not go with Nitric acid passivation or Metal brite. Such discolouration may be left as it is.
REMEMBER--Once the ship has been delivered , unless repairs have been done or contaminated with carbon steel or rusted by a very corrosive cargo--pickling is not necessary.
Know the temp of all chemicals to be loaded before accepting them on board.
Salt water at >75deg c for > 2hrs washing affects the SS. So after heated Palm oil Fatty acids, the SS layer gets affected.
When inspecting a SS tank use a magnifying glass, a good digital camera, a ladder and a good torch and chalk and a mirror. For cargoes requiring inert gas—inspect the space above the cargo level.
Carriage of aggressive chemicals:--
Chlorinated HC must be stabilised so that no decomposition can occur. Temp must be less than 35C. the ppm of components must be less than
Water/ 100 ppm
Hcl/ 15 ppm
For sodium hypochlorite solutions check the resistance tables for the carriage temp and the dilution strength. It cannot be carried on 316L and the clad steel will get holed during the voyage. 316LN, 317L and 317LN can handle the cargo
Allyl chloride at 20C pits 304L to 317L the same amount. 317LN is effected less.
Benzyl chloride and Benzene Sulphonyl Chloride at 20C pits 304L, 316L, 316LN, 317L, and 317LN almost the same.
Caustic potash must be carried less than 45 deg c and less than 50% dilution
Nitric acid of >90 strength cannot be carried. Lesser strengths depend on carriage temp .
70% to 90% --max 50 deg c
<70 %--max 70 deg c Sulfic acid— Must be >96% and <35 deg c
Sodium hydroxide solution, borohydride/ sodium hydroxide solution and potassium hydroxide solution must be less than 70% and 80 deg C
N-butyraldehyde, N-valeraldehyde and iso-valeraldehyde must be carried under inert gas padding.
The heavy oxides on the heat affected zone formed by a new weld repair can be removed by hydrofluoric acid pickling.
Nitric acid can both remove free iron and oxidize (passivate) the SS surface. It will not remove oxides.
Hydrogen peroxide can oxidize the SS surface but it cannot remove free iron to create the chromium rich layer that then oxidizes very readily in air to form the passive layer.
Citric acid passivation solutions can also get high chrome oxide ratios on the surface, and good resistance to corrosion .
AND FINALLY-- Any repairs to SS tanks are only to be carried out after authorisation from us.
CAPT AJIT VADAKAYIL ( 29 YEARS IN COMMAND )