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.
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 tank
cleaning 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 tank cleaning 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 tank cleaning 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 tank cleaning equipment will be attached, if it is
not already the case to the discharge manifold of the first
tank . The tank cleaning 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.
