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Stainless Steel Treatment
 

There have been seemingly endless discussions about the need to chemically treat stainless steel surfaces before they are used in the application for which they are designed. An example of stainless steel use is in the cargo storage and handling facility of chemical cargo tankers. Typically these systems are chemically pickled and passivated as part of a vessel's construction process. The maintainance of the passivity (and longevity) of the steel is often achieved by regular re-passivation of the tanks and cargo piping. Below, we have described how such a job can, and has been, undertaken successfully.

It is followed by a short description of the commonest of all stainless steels used for cargo tank construction, with explanations of corrosion phenomena and how they occur.

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Stainless Steel Cargo Tanks and Piping

Typical Guidelines for SS316L Pickling & Passivation

We attempt to follow these broad guidelines. Deviations are permitted due to operational circumstances as long as the end result of the chemical cleaning/metal surface treatment is not affected. Such deviations will only be decided by experienced MTT(S)PL supervisory staff on site (unless extraordinary circumstances dictate otherwise).
A project can be divided into the following sections :
  1. A pre-inspection of the system(s) to be treated.
  2. Preparation of the MTT(S)PL supplied and vessel's own equipment necessary for the project.
  3. Preparation of a degreasing solution, followed by the application of this solution to each of the tanks and their associated piping systems.
  4. Rinsing and evacuation of the tanks prior to chemical treatment.
  5. Preparation of a pickling solution in one tank. This can usually be used for all tanks in question.
  6. The chemical pickling of all tanks and their associated piping.
  7. The rinsing of all tanks and their associated piping by means of the vessel's own tank cleaning system, using fresh water.
  8. The preparation and use of a passivation solution in one tank. This can be used for all tanks, and can be supplemented with extra oxidizing agent. After passivation, the tanks are stripped of solution.
  9. A final rinse with low chloride water of the tanks, followed by evacuation and stripping. This stage can often be omitted if the vessel has taken sufficient high quality shore-water on board for the whole process. The chloride content of this water can be checked, but in Singapore, for example, it is generally of 60ppm or even less.
  10. The inspection of the treated system(s) and determination of passivity.
  11. The completion of formalities and repatriation of personnel and equipment.

The steps listed above will be described hereunder in more detail:

1. Pre-inspection

The tanks and piping will be inspected to ensure that they are in a state conforming to readiness for the chemical surface treatment. A check will be made that any tankcleaning operations which may be necessary to remove solid particles and old cargo residues are complete, and that no incompatible materials are in the system to be wetted by corrosive media.

2. Preparation and initial testing

The MTT(S)PL tankcleaning equipment will be set up ready for the cleaning of the first tank. Fresh water will be added to the tank of a sufficient quantity to ensure recirculation through the discharge manifold, MTT(S)PL equipment and back to the tank.

The cargo pumping system will be started and cargo pump oil-flow adjusted. Circulation will be established and the correct functioning of the MTT(S)PL equipment will be ensured with the elimination of leaking connections.

3. Preparation and use of light degreasing solution

The volume of fresh water in the tank will be adjusted.

A water-based degreaser will be added. Recirculation will be re-established, and continued for a prescribed period.

The MTT(S)PL tankcleaning machines will be transferred to the next tank, and the solution transferred to that tank using first tank's cargo pump. When the first tank is empty, the MTT(S)PL equipment will be moved to the discharge manifold of the tank containing the degreasing solution.

Using the cargo pump of this tank, re-circulation will be established.

This process will be repeated in the form of transfer of solution to the next tank, movement of MTT(S)PL equipment to that tank's discharge manifold and recirculation over that tank, until all tanks have been treated by the degreasing solution.

Disposal of the used solutions is the responsibility of the client.

4. Rinsing of the tanks prior to subsequent chemical treatment

Using the vessel's own tankcleaning equipment, each tank will be washed with fresh water. 

5. Preparation of pickling solution.

The solution will be prepared in the first tank. The MTT(S)PL equipment for pickling will be transferred back to that tank's discharge manifold, with the tank cleaning machines inside the tank, ensuring full coverage.

6. Chemical Pickling

With the prepared solution, continue circulation over the first tank for a prescribed period.

The solution is transferred and re-circulated in the same way as for the degreasing phase, with the following exceptions :

a) During circulation, extra care is taken that all connections are tight and that tank openings are covered to minimize chemical spatter.

b) A sample is drawn at the beginning of the pickling of each tank to determine dissolved iron concentration.

7. The rinsing of the tanks after pickling.

After the above operation, the tanks will be virtually empty.

They are then rinsed, one at a time, using fresh water and the vessel's own tankcleaning system until all remaining acidic residue is removed. Samples of the washings are checked for pH, and rinsing continues till it is established that the pH of the washings are neutral.

At the end of this operation, the tanks will be virtually empty.

8. The preparation and use of a passivation solution

The MTT(S)PL tankcleaning equipment will be attached, if it is not already the case to the discharge manifold of the first tank . The tankcleaning machines will be lowered to the correct heights into the tank.

Fresh water is added to the tank and circulation established with the cargo pumping system.

After checking for function and tightness, an aqueous oxidizing agent is added while circulating.

The solution is transferred and passivation of all tanks is done in a similar way as for the light degreasing and pickling,

9. Final rinse of nine tanks with low chloride water ( if necessary)

After the passivation phase, above, all tanks will be virtually empty. The MTT(S)PL tankcleaning equipment will probably be attached to the discharge manifold of the final tank passivated. The tankcleaning machines will be lowered correctly into that tank.

Low-chloride water is added to the tank to which the MTT(S)PL tankcleaning equipment is attached. This is circulated for 45 minutes, following which it is discharged.

This process is repeated for all tanks . Following this, all the tanks will be stripped and their respective pipelines drained.

10. Testing for the passivity of the steel

Prior to the commencement of the chemical treatment of the tanks, a test piece of stainless steel 316L will have been placed into each tank, near to the tank top. It will be attached to the deck by a nylon cord for easy removal. Upon completion of all operations, these test pieces will be removed to a convenient location for testing (e.g. ship's office). The following tests will be performed:

a) Measurement of passivity using electronic passivation meter.

b) Palladium chloride test for passivity.

The results of these tests will determine the passivity of the tanks. If necessary, further checks can be done in the tanks themselves.

11. Completion of documentary formalities and demobilisation.

After successful completion of all the above, the necessary work completion documentation will be signed by all parties, indicating acceptance of the work done according to the agreed scope. The MTT(S)PL personnel and equipment will then be repatriated to Singapore.

Further information on Stainless Steel AISI 316L - commonly used as cargo containment material in chemical transporting tankers and containers

The following encompasses a description of the steel, an explanation of how typical corrosion problems happen on stainless steel surfaces, how they can be avoided and remedied. A short description of the meaning of the so-called passivity measurements is given later.

Stainless steel type German material number 1.4435

Equivalent to AISI 316l

Chosen for this application due to rich Molybdenum and low carbon content.

The low carbon ensures minimum chromium carbide precipitation and as a consequence improved resistance to intergranular corrosion.

This alloy is widely used in the marine environment due to its pitting resistance in low temperature seawater. It is, however, susceptible to crevice attack.

To illustrate an alloy's resistance to pitting attack, use is made of the alloy's PREN.

PREN = Pitting Resistance Equivalent Number

It can be defined as : % Cr + 3,3 x % Mo + 16 x % N

It is an indicator of that particular alloy's corrosion resistance in corrosive environments. In cold, flowing seawater a PREN of 26 is only marginally acceptable a guide against corrosion resistance. Raising the temperature requires raising the PREN. An uninterrupted stainless steel surface of material 1.4435 in cold, flowing seawater has similar qualities as the same surface exposed to air. However if the surface is interrupted by crevices or fouling, then the PREN in the crevice or under the fouling can reduce to 25% of its air value. See below for consequences.

Using the example of stainless steel 316L recently analyzed on a typical chemical tanker, we can figure out a PREN value. (Sample was taken from a "clad" tank")

Average % Cr = 17,55

Average % Mo = 2,61

No N was measured, but in 1.4435 average N = 0,1%

Our PREN is therefore 27,8.

If anything occurs on the surface of the stainless steel to lower the PREN, then the likelihood of pitting will rise. This is especially likely when so-called crevice corrosion is stimulated by circumstances. It has been experimentally shown that during the course of crevice corrosion the following takes place:

- An area of the steel surface incorporating a crevice is surrounded by an electrolyte. We can use seawater as the electrolyte here and as a crevice, for example, the tiny gap between a nut and a flange, or, more seriously, the small gap or hole sometimes seen in a shoddily repaired welding seam joining the tank top to, for example, a bulkhead.

- Initially, the composition of the electrolyte inside and outside of the crevice is the same. The steel will begin with an equal corrosion resistance inside and outside of the crevice.

- Under these circumstances, however, the conditions exist to create what is known as the basic wet corrosion cell. It has 4 components :

a) The anode. In this area electrons are removed from the neutral metal atoms and the charged atoms enter the electrolyte as ions. This happens inside the crevice.

b) The cathode. Here the reaction depends upon the pH of the electrolyte.

i) The electrolyte is acidic (pH is less than 7 ). The electrons travelling from the anode will combine with the hydrogen ions in the acidic electrolyte producing hydrogen gas.

ii) The electrolyte is alkaline ( pH is greater than 7 ). The electrons travelling from the anode will combine with water and oxygen present in the electrolyte to produce hydroxyl ions. In both cases consumption of electrons produced at the anode will provide a stimulus to the anode to produce even more. The cathode is the steel surface outside the crevice.

c) The electrolyte. In our case this is seawater. It can be seawater mixed with fresh water. In both cases the liquid can conduct electricity (by ionic means).

d) An electrical connection between the anode and the cathode. In our case, this is the fact that the anode and the cathode are part of the same tank. No external connection, such as a wire, is necessary.

- Corrosion will begin in the crevice if there is a difference in the free energies between the anode and the cathode. This means that an electrical potential difference exists between the anode and the cathode, and that the electrons produced at the cathode can move through the steel to the cathode, and from there into the surrounding electrolyte. The greater this potential difference (measured in mV ) , the greater the potential for corrosion.

The question here, therefore, is how does the reaction start, which initiates corrosion by setting up the potential difference between the anode and the cathode? The answer, in the case of stainless steel, is found by analyzing the influence of oxygen.

The pH of seawater is slightly alkaline i.e. greater than 7. This gives rise to the reaction described in b.ii) above. The electrolyte outside the crevice dissolves more oxygen from the air with which it has contact as long as the oxygen already dissolved in the electrolyte is used up to make hydroxyl ions. Inside the crevice this cannot happen ( lack of oxygen ) and the generation of hydroxyl ions will cease. We now have an imbalanced situation. A distinct anode and cathode emerge. More positive ions inside the crevice will build up. These positive ions are metal ions from the steel. Negative ions in the electrolyte outside the crevice will be attracted by the existence of the positive ions inside the crevice and move into the crevice. This would not be so bad if they were only hydroxyl ions, but our electrolyte is seawater. The most influential negative ion in seawater when discussing stainless steel is the chloride ion. The effect of this diffusion of negative ions into the crevice is to raise the pH of electrolyte inside the crevice. Hydrogen ions are produced. Together with the presence of the chloride ions there is a hydrochloric acid solution produced in the crevice.

To make matters worse, it has been shown experimentally that it is the dissolution of chromium atoms which leads to the most significant drop in pH inside the crevice.

In this way, returning to our PREN guideline, it is the leaching out of chromium inside a crevice which gives rise to a lowering of pitting resistance.

We can avoid crevice corrosion by removing any one of the 4 elements necessary to produce it.

a) The anode or the cathode. (2 elements inextricably intertwined). To do this we have to remove the crevice.

b) The electrolyte. To do this we have to remove the seawater.

c) The electrical connection. We cannot do this without scrapping the vessel.

As a consequence of the above, we can conclude that crevice corrosion is preventable in stainless steel German material number 1.4435 or AISI 316L by removing crevices, seawater or preferably both.

Some crevices are unavoidable. That's life. Crevices in-built by design such as those between gaskets and nuts, bolts and flanges are a part of the construction of the cargo system. They are concentrated inside tanks in the cargo pumps. Consequently manufacturers are aware of the corrosion potential and build in safety features. For example an inferior alloy of massive proportions can be incorporated into the pump construction just above the pump housing. This can act as a sacrificial anode protecting the pump in an electrolyte, and is easily replaced.

PITTING

The mechanism of pitting is essentially the same as crevice corrosion. The difference is that whereas in crevice corrosion the phenomenon is derived from the fact that a crevice already exists, pitting needs to be initiated - take for example cases of pitting in the tank tops in the centre of a steel plate, nowhere near a weld seam or a crevice.

Pitting is avoidable. Like crevice corrosion, only one of the factors leading to it has to be removed to stop the reaction.

When a clean tank is inspected and pitting is observed, then it is a case of locking the stable door after the horse has bolted. We are seeing the result of a previous corrosion cell. When we measure the passivity of the steel, even inside a corrosion pit, it almost invariably shows a passive area. Passivity is the unlikliehood of a particular area to corrode. Inside a pit under inspection conditions the electrolyte has been removed, as far as is observable. Inside the pitting the steel has access to oxygen and the passive chromium oxide layer has been restored. This is no reason, however, to ignore the pitting. The chromium concentration on the surface of the pit will be less than that of the surface surrounding it. The pit can trap seawater again in the future and reactivate a corrosion cell.

The pitting has to be removed mechanically and filled by the correct metal. The resulting repair must be devoid of crevices. In other words, the repair must have the same characteristics as the surrounding steel. This is achieved by careful welding followed by the minimum removal of new material by mechanical means to achieve a smooth finish. This is followed by firstly activating the steel in the area by pickling to remove unwanted oxides and contamination, followed by passivation to restore the passivating chromium oxide layer. Weld spatter must be similarly removed from the surrounding area and the area similarly pickled and passivated.

Pitting is often associated with areas of uniform corrosion. For example, an area of steel about 1m2 is rougher than the area surrounding it. Inside this area are examples of even worse corrosion in the form of pitting, either shallow, open pitting or deep pinpoint pitting.

It is necessary to look at the causes of pitting to avoid it in the future.

Pitting is caused by starving a surface of the steel of oxygen relative to the surrounding surfaces, followed by exposing both to an electrolyte.

To starve a steel surface of oxygen you need to contaminate it.

Some examples of contamination are :

- Free iron. If iron from tools or boots imbeds itself into the stainless steel surface, then the surface will rust and the rust will form a porous layer on the stainless steel surface. The steel surface is deprived of oxygen but an electrolyte can penetrate through to the steel surface. We have created an 'artificial crevice'.

- Organic deposits

- Mud or silt

- Remains of impure cargoes such as wet phosphoric acid. This is usually the cause of the uniform deterioration of the surface texture of the steel, i.e. it becomes rough or abrasive.

Following the discharging of wet phosphoric acid, a thorough tank cleaning is required. This is needed to remove the tightly adhering sediments present in the acid. If this is not done, the large areas of the tank surface are deprived of oxygen. Subsequent immersion in an electrolyte will set up a gigantic corrosion cell between the 'clean' areas and the 'dirty' areas. These sediments, as far as corrosion is concerned, have the same properties as rust - they are adherent and porous to electrolytes.

The result is, that although through repeated use of the tank, eventually it is clean, a uniform corrosion has occurred during the time it was dirty and exposed to electrolytes. The dirty areas having sacrificed themselves as anodes to the clean cathodes.

Similarly, in depressions in the tank top, where sediments and electrolytes can collect undisturbed, limited uniform and associated pitting can and does occur.

Depending upon the nature of the cargoes to be carried in the future ( electrolyte or non-electrolyte ; i.e. aqueous or organic (solvents etc.)) then it is advisable during a period such as dry-docking to restore the surfaces of the affected tanks to a uniform quality by mechanical means such as pitting repair or even polishing of rough areas, followed by complete picking and passivation. In that way, the surface quality of the tank will once again be as intended when new.

Intergranular corrosion ( IGC)

IGC is a phenomenon whereby if the steel is heated or cooled through the temperature range 425-900oC, then the chromium can combine with carbon to form chromium carbide at the grain, or crystal boundaries, or edges. The adjacent or surrounding areas are thereby depleted of chromium and are susceptible to IGC. They are called sensitized areas. By rapid quenching of the steel with water after welding, the steel can be desensitized and IGC avoided.

IGC avoidance is a combination of choosing the correct alloy and performing the correct quenching after welding.

German material number 1.4435 is a correct choice of alloy for this application. The carbon level is low enough to allow welding repairs without quenching, so long as the welded surface is clean and free of oils. Quenching is something you can only observe during construction. However, observation of the areas in and around welding seams during inspection indicates correct compliance with procedures or not, i.e. that the steel surface at these areas was or was not different to the general surface condition, except where specific instances of crevice or pitting corrosion had occurred.

Passivation meter for stainless steel surfaces.

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 tip of the reference electrode is then placed on top of the soaked paper thereby making a steel / electrolyte / electrode connection. The electrode is connected to a mV meter and another contact extending from the meter to the steel completes the corrosion cell.

The mV meter contains a microchip, which performs the function of reducing the reference electrode's absolute potential to zero. The mV reading seen on the LCD gives an indication of the tendency that the steel will readily corrode. The manufacturer has set the limit that any reading above 60 mV indicates a passive surface, or in other words, an intact chromium oxide layer on the steel surface.

A reading of less than 30mV shows that the surface is still in an active state.

A reading of 30mV<reading<60mV shows that the surface is not fully passive, or that the protective oxide "film" is incomplete.

A reading of more than 60mV and stable or rising indicates a passive surface.

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