The Treatment
of the BMW 801D-2 Radial Aero Engine Rescued From the Loiret River.
TREATMENT OF THE ENGINE
1. Precleaning.
Due to the relatively good condition of the engine, the internal components
and engine block were almost entirely intact. For this reason, much river
debris and loose corrosion products still remained contained within. Although
physical access to this was extremely limited, as much as possible was removed
using vacuum extraction.
The externals of the object were mechanically cleaned of loose debris and
concretions using brushes, probes and scalpels to a point where physical
damage to metal surfaces would not occur.
A high pressure water spray was
used for a final wash prior to immersion in the pre-treatment bath.
2. Pretreatment.
a. Preparation of Solution.
The nominal internal dimensions of the treatment tank were 2.05 metres
by 1.8 metres and the required height of the solution required to cover the
engine on its treatment stand was 1.2 metres. This corresponded to a liquid
volume of 4,428 litres.
The buffered citrate solution was prepared in the ratio of 9.6g/1 anhydrous
citric acid and 4.8 g/1 sodium hydroxide. Thus the total weight of chemicals
required for the first pretreatment was 42.5 kg of anhydrous citric acid
and 21.25 kg of sodium hydroxide.
b. Polarization of the Tank.
As mentioned earlier, the uncoated inside surface of the treatment tank
was cathodically protected during the entire treatment process. An initial
trial of the technique was first conducted.
The tank was polarized for three days before engine immersion and the pH
was monitored, as was a visual observation of the colour of the solution.
Although a slightly green colouration was imparted to the solution, the pH
remained stable at approximately 5.2. This indicated that little dissolution
of ferrous ions had occurred, demonstrating the effectiveness of the technique.
The power supply used was an ACORE type RGT444 with an output rating of
25V and 100A (this power supply was used throughout the treatment). Large,
2 cm diameter power leads were used to connect both anode and tank (cathode)
to minimise voltage.
The tank was polarized to a value of -1. 2V/ SSE after an initial period
of stabilization. The power requirement was very low, being approximately
2V and 5A.
c. Engine immersion.
The pH of the solution was closely monitored:
DAY
TIME
pH
Prior to immersion
5.23
1
1800hrs
5.17
2
0800hrs
5.53
2
1900hrs
5.63
5
0900hrs
5.90
6
0900hrs
5.91
7
1000hrs
5.94
Figure 17 a and b, left and right.
Immersion of engine for pretreatment.
Although
buffered, the pH rises slowly. This may be attributed to the dissolution
of iron corrosion products. The consumption of the complexing ions
by ferrous species leads to a decrease in buffer capacity. As the
iron concentration levels out, so does the pH stabilize.
d. End of the Pretreatment.
After 7 days of immersion the engine was removed from solution and washed
down with high pressure water. It was noted that most of the superficial
corrosion products and concretions had been removed from the ferrous alloys,
which now had the typical black appearance of magnetite (fig. 18.). Riverbed
debris, such as sand and small stones were still wedged within the fins
of the cylinder heads and in other areas (fig. 19.). Magnesium alloy corrosion
products were now quite soft, and were easily removed by mechanical methods.
This process was conducted for approximately 1 week. As the front section
of the engine was cleaned, the ferrous alloy
“cam followers” were revealed. These were found to be
unattached and were thus removed and treated separately.
Figure 18. After pretreatment. The iron components of
the engine have the typical black appearance of magnetite. Note the
presence of concretions on the upper portion of the piston rod which
partially protruded from the solution during treatment.
Left, after pretreatment. Right, after
removal. Note the removal of the cam followers.
Figure 19. Magnesium corrosion products found at
the front of the engine could be easily removed by mechanical methods
after the pretreatment.
3. Removal of Iron Concretions Using Cathodic Polarization.
The tank was filled with a total of 4,428 litres of
0.04M sodium metasilicate pentahydrate solution (MW 122.06). A total weight
of 21.62kg was first made up as a concentrated solution in small tanks and
diluted to the correct concentration in the larger treatment tank prior to
the commencement of the treatment. During the process, a fine white precipitate
formed in the tanks. It was speculated that this was due to the high carbonate
content of the available tap water forming a precipitate with the high initial
concentration of metasilicate, although this theory was never tested.
After the extensive mechanical cleaning described above,
the iron components of the engine were prepared for polarization by connecting
all major iron components using electrical wires and clamps. It must be remembered
that this treatment stage is designed to treat the iron components only while
inhibiting corrosion of the aluminium alloys (fig. 20).
Figure 20. Wiring of the iron components
ready for cathodic polarization.
Figure 21. Positioning of the circulation hose.
Although the aluminium components are not directly polarized at this stage
they are obviously in some sort of electrical contact to a greater or lesser
extent with the iron components and are thus indirectly subjected to some
degree of cathodic polarization.
To minimise the risk of cathodic corrosion then, circulation of solution
around the object was facilitated by the strategic placement of a hose with
outlet holes, attached to a circulation pump. This countered any possible
formation of hydrogen bubbles on the surface of the aluminium components
(fig. 21).
The PVC flooring material was placed into position as was
the stainless steel anode attached to its frame. The engine
was carefully lowered into position and potentials measured
on various representative parts prior to polarization:
Position
Potential V/SSE
front of engine (ferrous alloy)
-0.73
rear of engine (ferrous alloy)
-0.80
cylinder head (aluminium alloy)
-0.83
baffle cover (aluminium alloy)
-0.81
Table 2:
Recorded potentials prior to polarization.
With initial polarization, potentials were then set at:
Table
3: Potentials after initial polarization.
Position
Potential V/SSE
front of engine (ferrous alloy)
-1.04
rear of engine (ferrous alloy)
-1.40
cylinder head (aluminium alloy)
-1.30
baffle cover (aluminium alloy)
-1.22
tank surface
-1.55
Power supply settings were 4.5V and 40 Amps.
Position
Potential V/SSE
front of engine (ferrous alloy)
-1.08
rear of engine (ferrous alloy)
-1-38
cylinder head (aluminium alloy)
-1.31
baffle cover (aluminium alloy)
-1.26
tank surface
-1.65
The tank and engine were polarized using a single power supply for both. After
a short period of instability with fluctuating potentials, power could be decreased
to 3V and 20 Amps obtaining stable average potentials of E= -1.33V/SSE for
the iron components and a reading of E= -1.60V/SSE for the tank surface. The
system was left for four days undisturbed, after which the potentials were
checked and the following values obtained:
Table 4: Potentials obtained after four days.
After a further 24 hours of polarization, the power was disconnected and the
engine lifted for inspection. Although covered with the white precipitate formed
during the solution preparation, all metal surfaces appeared to be in good
condition with the precipitate being nonadherent and easily washed off. On
reimmersion at the same power settings the potentials obtained were as obtained
previously.
The polarization was continued for another 6 days, after which time the engine
was removed from the tank and thoroughly cleaned with a high pressure water
spray. The surfaces were then kept wet with a water spray. The general appearance
indicated that most of the iron concretions were now removed with a small amount
of what appeared to be carbonate species still present.
Because of its high pH, the sodium metasilicate solution had to be first neutralized
before disposal. This was accomplished using citric acid monohydrate in the
proportion of 5g per litre of metasilicate solution. This gave a final solution
pH averaging 6.1 which was suitable for drainage disposal. Tanks, hoses, pumps,
leads and anode had to be thoroughly cleaned of the tenacious film of metasilicate
which formed on all surfaces.
4. Dechlorination in Buffered Citrate.
Again a sodium citrate solution, buffered to a pH of 5.4, was prepared
in the same manner as for the pretreatment although this time deionized water
was used. The total volume of solution was prepared in small open tanks ready
for immediate transfer to the treatment tank once the engine was placed into
position. This avoided any practical problems which could arise due to delays
in the collection of such a large volume of deionized water from a conventional,
laboratory water treatment unit.
Since this stage of the treatment is a global one, all major metal components,
both aluminium and iron, were wired (fig. 22.), making sure that good electrical
contact was achieved. The pump and hose were positioned, as in the previous
stage, to facilitate good circulation of the solution.
Figure 22. Wiring of all metal components
ready for dechlorination in buffered citrate solution.
After transfer of the solution, the engine was positioned in the tank, power
connection strategically placed and polarization of both tank and engine
immediately commenced.
At the commencement of this treatment stage, as with that previously, one
power supply was used for both tank and engine. The initial values were:
Position
Potential V/SSE
tank
-1.50
aluminium parts (average)
-1.48
iron parts (average)
-1.11
Table 5: Initial average potentials for
the dechlorination treatment.
These values required a power setting of 5.5V and 48 Amps, but these values
tended to fluctuate somewhat and the current required seemed rather high.
Thus after two days, the circuit configuration was altered to a configuration
where two power supplies were used. In this way, the tank was cathodically
polarized separately from the engine. The ACORE power supply was continued
to be used for the tank and a SODILEC 12V, 30A unit was used for the engine.
Potentials were then set at the following values:
Position
Potential V/SSE
tank
-1.48 to -1.50
aluminium cast alloys (ave.)
-1.43
aluminium wrought alloys (ave.)
-1.27
iron alloys (ave.)
-1.13 to -1.19
Table 6: Stabilized potential settings
for dechlorination treatment.
The potentials remained stable and were maintained throughout the treatment.
During the entire treatment, iron components must be kept
at a potential of at least I.lV/SSE or below to avoid corrosion. At the same
time the aluminium components should not have a potential of less than -1.6V/SSE,
thus avoiding cathodic corrosion. Aluminiumcopper alloys are very sensitive
to this type of corrosion below this value, and other alloys to a lesser
extent.
Throughout the treatment, samples of the solution were
taken at regular intervals to closely monitor pH and chloride ion concentrations,
which were determined using a selective ion electrode:
Time
pH
Chloride conc. (mg/l)
0
5.58
8
19
5.58
230
25
5.39
277
27
5.43
309
40
5.55
309
48
5.86
319
105
5.92
355
117
5.94
230
129
5.95
570
138
5.98
674
153
6.10
319
162
6.10
230
177
6.15
355
These results can be represented graphically
(fig.23).
Figure 23. Increase in chloride ion concentration
with time in buffered citrate solution.
Unfortunately due to the limitations of immediate access to adequate
analytical facilities, these analyses were only conducted some considerable
time after the samples were taken. It was speculated therefore that
the erratic variation of Cl- concentrations was due to the presence
of chloride-consuming bacterial species.
After approximately 7 days the treatment was discontinued.
Table 7. PH and chloride concentrations for dechlorination
treatment.
5. Removal of Citrate Ions.
The engine was removed from the treatment solution and immediately washed
with high pressure water spray. The tank and associated equipment were washed
and prepared for a final immersion of the engine in tap water under cathodic
protection to remove contaminating citrate species.
This was conducted for a period of 24 hours, after which time the engine
was removed, washed with water and dried in preparation for the finishing
treatments.
6. Finishing Treatments.
The exterior of the object was dried as quickly as possible using a hot air
gun and the interior was pumped out and dried by the same method. A certain
amount of superficial mechanical cleaning was conducted prior to the application
of a protective surface coating. This was Dinol 4010 which was applied by spray
application (fig. 24.).
Figure 24. After the final treatment and
the application of Dinol 4010.
Finally replacement on its display stand indicated the last phase in the
treatment process (fig. 25.)
Figure 25. Comparisons. Engine mounted
on display stand.
Before treatment.
After treatment.
Treatment of Associated Objects
Some 57 items associated with the engine and plane were also put into
the care of the Valectra laboratories for treatment and storage/display preparation.
A detailed discussion of the treatment of these objects is the subject of
an EDF publication by Ms C. McLennan which is at present in production. However,
figures 26 to 31 present an indication of the diversity of the objects treated.
Although most of these followed the general plan as described, each treatment
was “tailored” to suit the specific needs of the object, taking
into account such parameters as construction materials, size, fragility and
condition.
Figure 26. Oxygen bottle.
Figure 27. Oil pump.
Figure 28. Undercarriage mechanism.
Figure 29. Clutch component of starter mechanism.
Figure 30. Electrical component of starter mechanism.
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