The Treatment of the BMW 801D

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The Treatment of the BMW 801D-2Radial Aero Engine

Rescued From the Loiret River.ChristopherD. Adams

Conservation Scientist

Australian War

Memorial

Conductedat the

laboratoriesof

Groupe Valectra

(GDL).

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Electricitde France.

May June1992

ABSTRACT

An electrolytic stabilization treatment was conducted on a BMW 801D-2 radial aero engine,which had been rescued from the Loire River near Orleans, France in 1990, after having beensubmerged for some 45 years. Conducted at the laboratories of Groupe Valectra, Electricit deFrance, St. Denis, France, between May and June 1992, this project follows another successfultreatment conducted on a similar engine at the Conservation Laboratories of the Australian WarMemorial the previous year. The process involves the removal of superficial concretions and theextraction of chloride species from within the corroded metal using an electrolytic processdesigned so that large objects of technology may be treated as a whole. These items may betermed composite objects, consisting mainly of aluminium alloys, iron alloys and organicmaterial.

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INTRODUCTION It has been recognised for some time that the oceans, seas and waterways of the world are repositories for objects of our aviation andmaritime history that have become extinct on land. With the development of new detection and salvage methods, enthusiasts, collectors andmuseums are now recovering these valuable pieces of history. Thus, there is a demand to develop appropriate preservation techniques.

Unfortunately, these relics have little chance of long term survival if not treated correctly. The materials of construction have becomepolluted with salts and metal residues which promote extremely rapid corrosion on exposure to the atmosphere unless they are kept inenvironments of low relative humidity and oxygen level. In most cases this is quite impractical. Hence, almost all the rescued items arenow corroding faster than if they had been left in the water.

Preservation of these objects, requires treatments that remove the aggressive or initiating species from within the objec t, leaving it in apassive state. Surface coating or inhibitor treatments will not stop the long term co rrosion of the metals as all the necessary chemicalspecies that are involved in the corrosion reaction are still present under the coating. Inhibitors may to some extent, delay the corrosion buteventually it will again take hold.

In order to halt the corrosion of aluminium based objects which have been exposed to saline environments, it is necessary to extract the saltsthat have pervaded the construction materials. One method for doing this was developed by Dr Ian Macleod of the Western AustralianMaritime Museum .1 Although successful for its designed application, it has several drawbacks when dealing with objects of compositeconstruction such as aircraft engine. The time necessary for a treatment is often very long, sometimes taking years.

A commonly used process for the stabilization of ferrous objects utilises an electrolytic process. Here the contaminants ar e removed fromthe material using electrical poten tials. This method is arguably faster and more efficient than the previous immersion technique.

In 1990, scientists from Groupe Valectra of the Electricit de France adapted these techniques for the treatment of aluminium .2 The methodinvolves washing the object in several chemical complexing baths and applying an electrical potential while strictly regulating both potential

and pH. Thus the aggressive species are removed from the object. Having been found to be successful for the treatment of small objects, it was decided to scale up the treatment to include large technological items of composite construction.

1 MacLeod, LD 'Stabilization of Corroded Aluminium'. Studies in Conservation Vol 28 No. 1. Feb. 1983.2 Degrigny, C. 'Mise Au Point D'un Traitment Cathodic De Stabilisation De Pieces En Alliages D'Aluminium Degradees Par Corrosion En Miliue Aqueux'. Theses de Doctorat L'Universit Paris VI. 20 Nov. 1990.

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History of the PlaneBy June 1944 the German airforce was significantly weakened. When the Allied invasion of France began on 6th June 1944, the Germanfighter forces were in no way prepared.

The allies had mastery of the skies. It has been estimated that they could muster 4,190 fighters to meet a total 425 Luftwaffe fighters, of which only about 250 were serviceable on any one day. Thus at this time only few German planes could manage to get off the ground andattempt engagement with the Allied bombers and their escorts, which were now flying over France daily. It is during this period that aGerman plane crashed into the Loiret river, a tributary of the Loire, near a small settlement known as Port Arthur.

On the morning of 15th June 1944 at 0625 hrs, German fighters from the II and III/JG 26 and the III/JG 54 squadrons, took off fromGuyancourt, situated a few kilometres south-west of Versailles, on a mission of free chase. It occurred that the Commander of the JG 26squadron was Oberstleutnant Joseph Priller, aged 26, who was credited at that time with 99 kills (fig. 3.). Among them also was AlfredGunther of the II/JG 26, flying a Focke Wulf 190A-8.

Figure 3: Oberstleutnant Pips Priller, the Kommodore of the JG26.

Being helped from his aircraft duringthe battle for France in the summer of

1944.

His aircraft being serviced by ground crew.

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At 0650 hrs they were 100 kilometres south of Chartres. The German pilots noticed a formation of 70 to 80 allied bombers flying at analtitude of between 6,000 and 7,000 metres. They were identified as being B17 Flying Fortresses and B24 Liberators, heavily escorted by

American fighters. These formed part of the 1,361 bombers that the 8th airforce had in the air that day. A little before 0710 hrs, a B24Liberator became the 100th victim of the ace Pips Priller. Some minutes later the B24 was observed fall ing south-west of Chartres. Shortly after, the Luftwaffe fighters began their return, passing over Chartres at 0735 hrs, from where they set course for home, arriving atGuyancourt at 0840 hrs.

Oberfeldwebel Alfred Gunther, flying his Focke Wulf FW190-A8, failed to return and although German planes returned to the area thefollowing day, his fate remained a mystery until recently. Locals of the region always had stories of a German plane which crashed into theriver around that time, at a site known as Port Arthur. It was not until a team of divers from the Club Subaquatic Orleanaise discoveredand salvaged a BMW 801D-2 engine and other associated artefacts, that a positive identification could be made. The plane was certainly thatflown by Alfred Gunter. Most of these recovered items were eventually entrusted to Groupe Valectra for stabilization treatment.

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Project PerspectiveThe successful electrolytic treatment and stabilization, without dismantling, of a Pratt and Whitney Twin Wasp radial aero engine wascompleted at the conservation laboratories of the Australian War Memorial, Canberra, Australia in December 1991. 3, 4

This project, a world first, was made possible by the patronage of Electricit de France (EDF) and the Australian War Memorial (AWM).Further financial sponsorship was provided by TIGHAR (The International Group for Historic Aircraft Recovery) and HARS, (Historic

Aircraft Recovery Society). These studies enabled the refinement of techniques to a stage where further treatments of other relics of similarcomposite com position could be undertaken (fig. 1).

Figure 1: The Pratt and Whitney Twin Wasp Radial Aero Engine from the Hudson IA16-19. Treated at the Australian War Memorial.

Before treatment. After treatment.

With the recovery of a BMW 801D-2 radial aero engine and other associated relics from a Focke Wulf 190A-8 fighter from the Loiret River,France in late 1990, an opportunity was presented to continue the refinement of these treatment techniques at the laboratories of Groupe

Valectra, Groupe des Laboratoires, Electricit de France, situated at Saint Denis to the north of Paris (fig. 2.).

Figure 2: The BMW 801D-2 radial aero engine salvaged from the Loiret River.

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As rescued. Before treatment on display stand.

Sponsored once again by EDF and the AWM, this second project being a European first, was further facilitated by a French governmentscientific fellowship awarded to me for a period of three months.

AWM Conservation has for some time regarded that the development of techniques to treat and stabilize objects comprising modern metalalloys are high priority. A number of significant relics from the AWM collection are of this type and are in urgent need of attention. Withinthe Australian and South Pacific region a number of smaller collections have similar problems where many such objects have been salvagedfrom marine environments and deterioration has accelerated to crisis point. There also exists at present considerable interest in thesalvaging of other relics from the war against Japan a number of which will become part of our national collections. Thus, it is imperativethat adequate conservation and stabilization strategies are developed now.

The project began in May 1992, and once finished, the engine is destined to become part of the collection of the Muse Pour La PaixMemoria, situated in Caen, Normandy.

[3] Degrigny, C. 'Traitements Electrolytiques De Vestiges Subaquatiques De Grandes Dimensions A Base D'Alliages D'Aluminium Dans Le Laboratoire De Restauration De L'Australian War Memorial.' AWM Publication, Dec. 199 1. [4] Adams, C.D., Electrolytic Treatments of Large Composite Objects Based on Alluminium Alloys Rescued from Sea Water'. AWM Publication, Jul. 1992.

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History of the PlaneBy June 1944 the German airforce was significantly weakened. When the Allied invasion of France began on 6th June 1944, the Germanfighter forces were in no way prepared.

The allies had mastery of the skies. It has been estimated that they could muster 4,190 fighters to meet a total 425 Luftwaffe fighters, of which only about 250 were serviceable on any one day. Thus at this time only few German planes could manage to get off the ground andattempt engagement with the Allied bombers and their escorts, which were now flying over France daily. It is during this period that aGerman plane crashed into the Loiret river, a tributary of the Loire, near a small settlement known as Port Arthur.

On the morning of 15th June 1944 at 0625 hrs, German fighters from the II and III/JG 26 and the III/JG 54 squadrons, took off fromGuyancourt, situated a few kilometres south-west of Versailles, on a mission of free chase. It occurred that the Commander of the JG 26squadron was Oberstleutnant Joseph Priller, aged 26, who was credited at that time with 99 kills (fig. 3.). Among them also was AlfredGunther of the II/JG 26, flying a Focke Wulf 190A-8.

Figure 3: Oberstleutnant Pips Priller, the Kommodore of the JG26.

Being helped from his aircraft duringthe battle for France in the summer of

1944.

His aircraft being serviced by ground crew.

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At 0650 hrs they were 100 kilometres south of Chartres. The German pilots noticed a formation of 70 to 80 allied bombers flying at analtitude of between 6,000 and 7,000 metres. They were identified as being B17 Flying Fortresses and B24 Liberators, heavily escorted by

American fighters. These formed part of the 1,361 bombers that the 8th airforce had in the air that day. A little before 0710 hrs, a B24Liberator became the 100th victim of the ace Pips Priller. Some minutes later the B24 was observed falling south -west of Chartres. Shortly after, the Luftwaffe fighters began their return, passing over Chartres at 0735 hrs, from where they set course for home, arriving atGuyancourt at 0840 hrs.

Oberfeldwebel Alfred Gunther, flying his Focke Wulf FW190-A8, failed to return and although German planes returned to the area thefollowing day, his fate remained a mystery until recently. Locals of the region always had stories of a German plane which crashed into theriver around that time, at a site known as Port Arthur. It was not until a team of divers from the Club Subaquatic Orleanaise discoveredand salvaged a BMW 801D-2 engine and other associated artefacts, that a positive identification could be made. The plane was certainly thatflown by Alfred Gunter. Most of these recovered items were eventually entrusted to Groupe Valectra for stabilization treatment.

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THE FOCKE WULF FW190A-8The Focke Wulf FW190 is a legendary German aircraft of the second world war which proved to be a deadly foe to Soviet, British and

American airforces. When it was first introduced into operation in the autumn of 1941, this aircraft could out-run, out-climb, and out-divethe best fighter the Royal Airforce had the Spitfire V.

There were many versions of this aircraft, but its most successful role was that of a fighter. It was also used for ground attack however and infact, was the only reliable light attack bomber that the Germans had in numbers.

The basic layout of the Focke Wulf 190 was entirely conventional, being a monoplane with a nosemounted engine driving a tractor airscrew.The mainplane was fully cantilever and was fitted with split flaps of metal construction. These were operated by means of three electric push

buttons situated in the cockpit. A low wing gave adequate housing for the retractable undercarriage which meant that the legs could be keptshort, optimising pilot vision. This was further enhanced with a large frameless bubble canopy, which when first introduced, was quite aninnovation.

The A-8 version (fig.4) was powered by the BMW 801D-2 engine, which was a very rugged air cooled radial (see below). There were fittingsunder the fuselage to enable it to carry bombs or a jettisonable fuel tank of 300 litres capacity.

Figure 4: Schematic diagram of the FW190A-8.

In the cockpit 80 levers, slide controls, dials and buttons were divided into two instrument panels and three consoles. Amongst the

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apparatus on board, was the RM 16B sighting mechanism which permitted precise weapons aiming to a range of 900 meters. It could also beused with a night filter (fig.5.). In the rear section of the fuselage was found the FuG 25a radio installation which also served to identify friend and foe.

Figure 5: FW190A-8 cockpit layout.

Click on the small drawing toopen a full-size version in a

new window.

No less than nine model designations were assigned to the Fw 190A-8, most of these being based on varying armament configurations.

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THE BMW 801 RADIAL ENGINE

The BMW 801 twin row radial engine formed the basis of the Focke Wulf fw190 design. This engine has the reputation as being among the better engine designs of WW2 regardless of limitations in German supercharger technology which lead to some failings at high altitude. Italso powered many other Luftwaffe aircraft, from the Arado Ar 232A to the Junkers Ju 390.

The Bayerische Motroen Werke (BMW) based in Munich, were manufacturing Pratt and Whitney radials under license in the 1930s andused this experience to develop its own twin row engine. Despite this, it can be considered an original design incorporating fuel injection andother German features.

A remarkably compact installation, adequate cylinder cooling was obtained using pressure baffling augmented by a magnesium alloy fangeared to turn at 1.72 times engine RPM (3 times propeller speed). An oil tank and cooler are positioned in the nose bowl and are armourplated. The engine mount ring is a sealed unit of square cross-section and also acts as a hydraulic fluid reservoir. Additional streamlining

was achieved by the introduction of drag-inducing cowl flaps.

The BMW 801D-2 (fig.6.) was fed by methanol-water injection. Most revolutionary however, was the Kommandogerat. This hydraulicelectricbrain unit w as operated by a single control which was the pilot's throttle lever. It automatically adjusted fuel flow, mixture strength,propeller pitch setting and ignition timing. It also cut in a second stage of the supercharger at the correct altitude. The pilot could, if required, manually set the propeller pitch without altering any of the other settings.

Figure 6: The BMW 801D-2 radial aero engine.

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The entire unit had a dry weight of 1,228 kg and an overall diameter of 1,270 mm. With a displacement of 41.8 litres and both a bore andstroke of 156 mm, this square engine could develop 1,730 hp at take off.

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PREPARATION FOR TREATMENT Engine Stand

An engine stand was constructed of box section mild steel. This was designed to hold the engine in the most appropriate position for placement in thetreatment tank, keeping dimensions and volumes to a minimum. The devised treatment method called for the engine to remain on the stand during theentire treatment. Large lifting eyes were included in the design so that at no time would the hoist cables be in contact with treatment solutions.

Treatment Tank

A tank of mild steel sheet and box section reinforcing was designed and purpose built for this treatment (fig. 7.). The outside of the tank wasprotected with a coating of zinc anti-corrosion paint over which was applied several layers of outdoorgloss enamel.

No surface treatment of the inside of the tank was undertaken other than descaling and degreasing.

The rationale for this was that the base of the tank is isolated from the engine stand using rigid 7 mmPVC sheeting and the light gauge stainless steel mesh (the anode) is isolated from the walls of the tank by means of a frame constructed of 40 and 50 mm PVC tubing (fig.7.).

During the electrochemical process the tank is independently cathodically protected using an applied voltage. This was intended to prevent the dissolution of ferrous ions from the surface of the tank which would interfere with the chemical reactions. Further, the tank surface itself is protected fromcorrosion.

X-Radiography

The structure of the engine was examined using sophisticated industrial X-radiographic equipmentknown as MINAC. This miniaturised system allows penetration of approximately 1 metre of concrete and can generate 4 MeV of power.

As several cylinders of the engine had become detached due to the impact of the crash, it was decided that a comprehensive structuralexamination was required. Cracks and breaks held together by only corrosion products, if present, could cause complications during thetreatment process. The examination revealed however, that all major structural components were intact.

Internal Examination

Figure 7: Treatment tank showing thestainless steel anode and the PVC frame

and flooring.

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An endoscopic examination of the inside of the engine was conducted to ascertain as to whether it was visually apparent that the corrosionproducts present inside the engine were of similar nature to those present on the outside. It should be noted that the engine was recoveredfrom deep mud in which it had been embedded for some forty five years. Thus, it was possible that certain chemicals such as fuel, oil andcombustion products sealed within the engine, may have initiated different chemical reaction paths. The examination was videotaped forsubsequent detailed inspection. Results indicated that no unexpected reactions had taken place. Some signs of original lubricants werepresent, although none was sampled.

Analysis

Any treatment of a composite object must be based on the results obtained from the detailed analysis of the various alloys, corrosionproducts and other materials which comprise the object. Analytical investigations were undertaken in which metal samples were examined

by x-ray fluorescence spectroscopy and corrosion products were examined by x-ray diffraction analysis. Electron microscopy with Energy Dispersive Xray Analysis was also used to examine some samples. Due to the limitations of t ime and access to analytical facilities however,more comprehensive investigations could have been completed.

Of most interest to note was that no Al-Cu alloy appeared to be present (unlike the Pratt and Whitney engine). The composition of thecylinder head air-flow baffles was quite unusual, being an Al-Si alloy which was in very good condition. Inlet pipes are of almost purealuminium, being comparable to the modern 2000 series designation. Significant amounts of magnesium corrosion products were also

present, together with a small amount of residual magnesium alloy metal.It was also apparent that a number of aluminium components manifested various applied surface treatments. Some wrought sheet appearsto be covered with a chromate conversion coating and other intricate components appear to be anodised. These all appeared to be inremarkably good condition. Preliminary analytical investigations ( Appendix 1 ) indeed indicated the presence of chromium but moresophisticated analytical techniques are required to distinguish the true nature of these coatings.

Examination of Organic Material.

Figure 8: Some of thecomponents which appear to be

anodised.

Materials present on the engine include a semi-rigid plastic wrapping for

electrical cables (fig. 8), reinforced rubber oil hose (fig. 9) and O ring gasket

Figure 9: Semi rigid plastic wrappingmaterial.

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seals all in varying degrees of degradation. Although an infrared spectrum of the plastic material was obtained ( Appendix 2 ) using a Perkin Elmer 1750 FTIP,at the time of publication, no further analysis has been undertaken tocharacterise these.

Test Treatments

Two parts of the engine, which have become separated from the bulk, a piston and an air-cleaner, were selected as specimens to be used totest the effectiveness of ultrasonic agitation during the treatment (fig. 10). A 12 litre Sonoclean ultrasonic bath was used during a

pretreatment in a citric acid solution buffered to pH 5.4.

PH was monitored, as was the nature and appearance of the concretions andcorrosion products. For example, after 4 days of treatment of the aircleaner most

encrustations became quite soft, changing from solid to mud-like consistency, which could be easily removed. Interestingly, the pH of the solution rose from pH5.41 to 8.61 probably due to the dissolution of ferrous ions. At this point thetreatment was discontinued and the object thoroughly washed and dried.

These results proved most satisfactory. As with the Australian project, theusefulness of the technique was demonstrated. The high cost of an ultrasonic unitof sufficient size and power to be effective in the treatment of the engine as a wholeproved to be inhibitive.

Fig. 10: Items which had become detached from the engine, used to test the effectiveness of ultrasonic agitation --- a piston and

an air cleaner.

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TREATMENT PLAN & DISCUSSION A detailed theoretical explanation of the treatment of composite objects made up of aluminium and iron alloy have beendiscussed in detail elsewhere .1 However it is necessary to outline some basic theory behind the treatment plan.

Necessity of the treatment.

Most submarine artefacts made of aluminium alloys are also what is termed composites, that is they often contain largepercentages of more noble metals such as iron and copper and non-conductive material such as leather, wood, insulating fabrics,etc. During the period of immersion, intensive contact corrosion occurs between the metals at the expense of the aluminiumalloys. These corrosion reactions are irreversible.

During immersion, the rate of corrosion is slow but this increases with exposure to the atmosphere. The study of corrosion forms,particul arly in the corrosion layer covering the degraded alloys surface, has demonstrated that chloride species are present at the

very tips of the corrosion pits (formed during the immersion) and are responsible for continued degradation. Further, the very presence of corrosion products may accelerate this process.

The immersion of an object in an aqueous media (whether having the ability to complex the metallic species contained in thecorrosion products or not) designed to remove chlorides is a dangerous procedure. It is possible that the dissolved chlorideconcentration will increase to a point where they will re-attack the metal surface. This phenomenon is due to an increase in thecorrosion potential of the alloys during immersion (fig. 11.). After a certain time the area in which the alloy is sensitive to pittingcorrosion is entered (fig. 12.)

Figure 11. Evolution of Ecorr as a function of time foraluminium-magnesium-silicon (6082) alloy in sodium

citrate solution, pH=5.4 [NaCl]=10 -3 M, T=10C.

Figure 12. Determination of the field of probability of pittingcorrosion of 6082 alloy (Al-Mg-Si), determined from a series

of smaples subjected to anodic polarization, pH=5.4, [NaCl]=10 -3 M, T=10C.

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Treatment conditions.

A cathodic polarization avoids this drawback. However, it is important to be aware that in the pH range of between 4 and 9, where the surface isnormally protected, aluminium alloys are subject to cathodic corrosion due to a decrease in the isolating properties of the superficial oxide film.Depending on the conditions, this can take the appearance of pitting or a more generalised corrosion.

To avoid the risk of this corrosion due to localised alkalisation of the solution close to the metallic surface, the polari zation is conducted in aslightly acid and buffered solution of sodium citrate (pH = 5.4). The treatment is undertaken using potentiometric conditions which manifest intwo stages as the polarization preceeds (fig. 13).

Figure 13. Cathodic polarization at -1.4V(SSE) for various aluminium alloys in asodium citrate solution, pH=5.4, without agitation and deaeration. [NaCl]=10 -3 M

Figure 14. Mechanism for cathodic corrosion of aluminium alloys in a buffered solution.

The polarization currents begin at a low value, particularly for noncorroded alloys. This corresponds to the hydration of the initial oxide filmafter the incorporation of H+ ions from the media. This is more prevalent near inclusions containing more noble elements such as copper andiron which can be present in the aluminium alloy. The localised modification of the film now allows corrosion close to these inclusions. Thesurrounding matrix is dissolved and eventually the inclusions are removed (fig. 14).

The extent to which this process occurs is controlled by such factors as the type of alloy, the aeration of the media and the cathodic potential.Thus the increase in current densities is of importance.

After several days of polarization, pits appear on the surface either in groups or in isolation. Corrosion is particularly prevalent when hydrogen bubbles, which form on the surface of the metal, are not removed, since localised alkalisation can occur under them. Efficient stirring of thesolution minimises this problem.

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Figure 15. Experimental pH-potential diagram showing the sensitivity of 6082

alloy to cathodic corrosion.

Thus anexperimentalPotential pH diagramcan now beconstructed,modified tothe requiredconditions. A protectionarea now appears in theregion whichis suitable fordechlorinationtreatments.(fig. 15).

It wasanticipatedthat thetreatmentprocedure

wouldgenerally follow that

which hasalready beenadopted in

Australia forthe treatmentof the Prattand Whitney Twin Waspengine.

The BMW

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engine, having been rescuedfrom afreshwaterenvironment,had theappearance of

being in a farless corrodedcondition

whencompared tothe Pratt and

Whitney, which wasrecoveredfrom sea-

water and had been leftexposed to theatmosphereuntreated forsome 20

years. Obvioussigns of corrosion dueto chloridecontamination

were absenton the BMW engine itself,althoughsome of thesmaller parts

which weresalvaged atthe same timemanifested

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typically blue/greencolouration of

Al/Cu alloy corrosionproducts. A thin layer of corrosionproducts andconcretions

were apparenton both ironandaluminiumcomponents(fig. 16.).

Figure 16. A thin layer of corrosion products and concretions cover both iron and aluminium components.

Notwithstanding, a decision was made to conduct the treatment according to the following global plan:

1. Pre-cleaning.

The employment of various mechanical methods to remove dirt, loose concretions and corrosion products before treatment begins.

2. Pre-treatment.

Immersion of the engine in a solution of citric acid buffered to a pH of 5.4 with sodium hydroxide. This is designed to remove concretions and

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7. Finishing Treatments.

Final mechanical work and the application of protective coating to meet conservation requirements standards are now undertaken to prepare theobject for museum display and/or storage.

[1] Degrigny, C.'Mise Au Point D'un Traitment Cathodic De Stabilisation De Pieces En Alliages D'Aluminium Degradees Par Corrosion En Miliue Aqueux'. These de Doctorat L'Universit Paris VI. 20 Nov. 1990. [2] Unpublished results obtained from experimental work conducted at AWM and Valectra.

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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, muchriver debris and loose corrosion products still remained contained within. Although physical access to this was extremely limited, as much aspossible 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 wherephysical 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 coverthe 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 initialtrial 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 thatlittle 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 thetreatment). 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.

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c. Engine immersion.The pH of the solution was closely monitored:

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 complexingions by ferrous species leads to a decrease in buffer capacity. As the iron concentration levels out, so does the pH stabilize.

DAY TIME pHPrior to immersion 5.23

1 1800hrs 5.17

2 0800hrs 5.532 1900hrs 5.63

5 0900hrs 5.90

6 0900hrs 5.91

7 1000hrs 5.94

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 thesuperficial 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 forapproximately 1 week. As the front section of the engine was cleaned, the ferrous alloy cam followers were revealed. These were found to beunattached and were thus removed and treated separately.

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Figure 18. After pretreatment. The iron components of the engine havethe 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 theremoval 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 wasfirst made up as a concentrated solution in small tanks and diluted to the correct concentration in the larger treatment tank prior to thecommencement of the treatment. During the process, a fine white precipitate formed in the tanks. It was speculated that this was due to the highcarbonate content of the available tap water forming a precipitate with the high initial concentration of metasilicate, although this theory wasnever tested.

After the extensive mechanical cleaning described above, the iron components of the engine were prepared for polarization by connecting allmajor iron components using electrical wires and clamps. It must be remembered that this t reatment stage is designed to treat the iron

components only while inhibiting corrosion of the aluminium alloys (fig. 20).

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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 ironcomponents and are thus indirectly subjected to some degreeof cathodic polarization.

To minimise the risk of cathodic corrosion then, circulation of solution around the object was facilitated by the strategicplacement of a hose with outlet holes, attached to a circulationpump. 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 thestainless steel anode attached to its frame. The engine was carefully lowered into position andpotentials measured on various representative parts prior to polarization:

Position PotentialV/SSE

front of engine (ferrous alloy) -0.73rear of engine (ferrous alloy) -0.80

cylinder head (aluminiumalloy) -0.83

baffle cover (aluminiumalloy) -0.81

Table 2: Recorded potentials prior to polarization.

With initial polarization, potentials were thenset at:

Table 3: Potentials after initial polarization.

Position PotentialV/SSE

front of engine (ferrous alloy) -1.04

rear of engine (ferrous alloy) -1.40

cylinder head (aluminiumalloy) -1.30

baffle cover (aluminiumalloy) -1.22

tank surface -1.55

Power supply settings were 4.5V and 40 Amps.

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 obtainingstable 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 thepotentials were checked and the following values obtained:

Table 4: Potentials obtained after four days.

Figure 20. Wiring of the iron componentsready for cathodic polarization.

Figure 21. Positioning of the circulationhose.

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

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After a further 24 hours of polarization, the power was disconnected and the engine lifted for inspection. Although covered with the whiteprecipitate formed during the solution preparation, all metal surfaces appeared to be in good condition with the precipitate being nonadherentand 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 highpressure 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 acidmonohydrate in the proportion of 5g per litre of metasilicate solution. This gave a final solution pH averaging 6.1 which was suitable for drainagedisposal. 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 theengine 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 thatgood electrical contact was achieved. The pump and hose were positioned, as in the previousstage, to facilitate good circulation of the solution.

After transfer of the solution, the engine was positioned in the tank, power connectionstrategically placed and polarization of both tank and engine immediately commenced.

At the commencement of this treatment stage, as with that previously, one power supply wasused for both tank and engine. The initial values were:

Position Potential V/SSE

tank -1.50aluminium parts (average) -1.48

iron parts (average) -1.11

Table 5: Initial average potentials for the dechlorinationtreatment.

These values required a power setting of 5.5V and 48 Amps, but these values tended to

Figure 22. Wiring of all metal components ready fordechlorination in buffered citrate solution.

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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 theengine. Potentials were then set at the following values:

The potentials remained stable and were maintained throughout the treatment.

During the entire treatment, iron components must be kept at a potential of at leastI.lV/SSE or below to avoid corrosion. At the same time the aluminium componentsshould 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 toclosely monitor pH and chloride ion concentrations, which were determined using a selective ion electrode:

Time pH Chlorideconc.

(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 wereonly conducted some considerable time after the samples were taken. It was speculated therefore that theerratic variation of Cl- concentrations was due to the presence of chloride-consuming bacterial species.

After approximately 7 days the treatment was discontinued.

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 dechlorinationtreatment.

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Table 7. PH and chlorideconcentrations 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 associatedequipment were washed and prepared for a final immersion of the engine in tap water under cathodic protection to remove contaminating citratespecies.

This was conducted for a period of 24 hours, after which time the engine was removed, washed with water and dried in preparation for thefinishing 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.

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Before treatment. After treatment.

TREATMENT OF ASSOCIATED OBJECTSSome 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 asdescribed, each treatment was tailored to suit the specific needs of the object, taking into account such parameters as con struction materials,size, fragility and condition.

Figure 26. Oxygen bottle. Figure 27. Oil pump. Figure 28. Undercarriage mechanism.

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Figure 29. Clutch component of startermechanism.

Figure 30. Electrical component of starter mechanism.

Figure 31. Electrical motor.

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CONCLUSIONS AND OUTLOOK Building upon the experience gained from the first treatment conducted in Australia, further refinement of the electrolytic stabilizationprocess has been achieved. Further, as a practical conservation exercise, the object is now considered stable and appropriately presented.

The treatment process can be considered effective and eff icient. Notwithstanding, development and refinement should continue. Future work should address the following:

Comprehensive analysis of all metal components should precede treatment. Significant time delays for chloride analyses should be avoided. The effect of treatment solutions on organic components (rubbers, plastics etc.) should be studied. The effect of treatment procedures and solutions on surface treatments (such as anodizing) should be studie