Liberators by Ford

By Chester S Ricker

Analyzing the fabrication and assembly methods employed at the much-discussed Willow Run plant. which is now held to be hitting production stride.

Willow Run is more than a plant for building Liberator bombers. It represents the impact of two great, but different, manufacturing philosophies: The very flexible and easily changed methods common to the aircraft industry, and the rigid and fixed methods used on high unit production where designs can be frozen for a year or more.

Willow Run is, as a result, like a child born under the constellation of Gemini, the "Twins" — trying to do two different things at the same time. Entirely aside from the initial personnel training and transportation problems which naturally delay starting new production, Ford engineers and production men have had to reorient themselves.

For 20 yr, the key men have thought in terms of tremendous unit production — millions of duplicate parts a year and from one to five cars a minute coming off the production line. The reconciliation of two widely divergent manufacturing viewpoints has been a tremendous problem for both Consolidated, who conceived this child, and Ford, who has fathered it. Under the stress of combat experience, the former has been forced, in womanlike fashion, to constantly change her mind about what was required and the latter, in fatherly fashion, has tried to satisfy every whim.

Getting production started under these circumstances has been the task of both Charles E Sorensen and Logan Miller, who have been the guiding hands in getting Willow Run launched.

The difference in what has been done here and at other automotive plants converted to aircraft production can be summed up very quickly. Ford introduced manufacturing methods which were conceived out of 30 yr experience in building auto chassis and bodies. Almost all other automotive air frame subcontractors took aircraft methods and applied to them as much automotive knowledge as they could. Now that Willow Run is producing, the entire manufacturing personnel has turned its attention to methods which will increase output. In one particular case, a complete rearrangement of the spar department and the use of new tools has increased production nearly 20 times with the same number of employees.

Willow Run is just getting out of swaddling clothes, but growing fast. It should reach full manhood before the year has passed.

In such a huge plant, where the assembly line distance from first wing parts assemblies to the airport doors is almost a mile, where the machine department is a good quarter mile long, and where the ship assembly bays are a clear 150 ft in width and at least 36 ft high, one would need to write an entire treatise on aircraft production methods in order to tell all the interesting details.

Therefore, only the principles followed in plant layout, the special fabrication processes, the unique Ford mixtures, and the method of coordination of these subassemblies on the final line will be given detailed consideration.

Willow Run may be divided into three distinct manufacturing units: One in which raw material is converted into finished parts suitable for subassemblies; a second where the fabricated units are put together in subassemblies; and last, the combination of subassemblies into the complete product. The latter is the line on which Ford-built bombers are constructed. The first two supply the parts for both Ford ships and "knock-down" parts for other plants assembling similar ships.

The plant area is roughly divided as follows:

Parts manufacturing area19.0%
Parts subassemblies18.7
Wing and fuselage assembly19.2
Ship assembly line22.9
Inspecting and shipping8.5
Extra departments (stores, tool room, etc.)11.7

Note that each of the major divisions represents roughly one-fifth of the total floor area.

In arrangement, the plant follows Ford manufacturing principles: All material comes in at one end and the finished product flows out at the other. This has been modified in only one respect: Government furnished equipment, such as complete engines, propellers, instruments, and armament, are received and kept in separate bonded areas and included in the extra departments mentioned above.

Raw material may be in the form of sheets, coils, wire, forgings, or castings, either finished or unfinished, delivered by truck or rail to a covered section of the plant where it can be unloaded by cranes where necessary. Receiving inspection is adjacent to the unloading dock; so, also, are the shearing department for sheet metal and the cold heading department for making rivets.

Aluminum wire stock for rivets is inspected as it enters the cold heading department, where millions of rivets are made each day; over 550 different sizes and types are used regularly.

All rivets, except some made from pure aluminum, must be heat treated after cold heading. Those formed from 17S aluminum alloy are degreased, Alroked (which turns them black), and stored until needed. When removed from storage, they are heat-treated and quenched, put in centrifugal dryers, then stored under deep-freeze refrigeration at -20" F. This type constitutes about 20 percent of the 700 lb. of rivets used in each ship. The frozen rivets are packed in cans or cellophane sacks and properly marked, then distributed to other deep-freeze boxes conveniently located along the assembly lines.

Remaining rivets are made of A17S aluminum alloy, and are handled differently. First they are degreased, then heat treated, and finally Alroked. They do not require refrigeration and so can be used at any time.

Handling of sheet metal offers another good example of the Ford technique with materials. The planning and shearing department funnels all stock from the unloading dock to the shears and on to the press department. The work required in handling stock from the raw material to the finished product is reduced to a minimum. For example, practically all sheet stock now arrives on platforms or skids and is slip-sheeted with tissue paper. These skids have side covers fastened with quickly detachable metal bands to protect the stock while in transit. This method eliminates the unpacking of sheets, since they can be removed directly from the stack.

Sheet aluminum formerly came in plywood crates holding but a few sheets. This required hours for uncrating and lots of room, while presenting more chances for scratching, damaging, and wasted material, the salvage value of the crates being negligible. The new method also makes it easier for the producer to pack and ship them.

The sheet metal planning department has a carefully worked out schedule which gives the maximum number of pieces and minimum waste from each stock sheet of aluminum. Blanking dies are made so strips may be run through in either direction, and stops are arranged so re-run blanks will be taken from unused portions of the original sheet. In some cases, scrap loss is less than 10 percent due to this preplanning with proper shearing instead of squaring of the stock.

Die stocks are adjacent to the shearing and press departments, so each order with dies and stock are drawn by the planning department and routed to the presses. Coil stock is routed by this same department to the slitting machines in the draw-bench section. Here again, by careful preplanning, the big rolls can be cut with practically no loss of this critical material.

Castings, forgings, and other parts requiring machining move directly to the machine shop from receiving inspection. Throughout the fabricating division, there is no back-flow of parts.

As soon as parts begin to take finished form they are put on an overhead conveyor system almost a mile long, which picks up finished material from the press department and distributes it to the doors and hatches section, cowling, miscellaneous small parts, and nose assembly departments. This conveyor runs the entire length of the fabricating shop — up one side of all departments with stock, returning on the opposite to pick up completed parts, then twice through the paint shop and into subassembly.

The small-parts paint department has its own internal conveyor system, which reduces trucking and crane work and keeps every man on the job in his department. It also reduces to a minimum the amount of material in process.

Some of the fabricating department processes are revolutionary in aircraft building, but old in making auto bodies. In forming bulkheads and other intricate parts from ST material, for example, not a single drop hammer or hydropress with rubber draw dies is used. With one exception, all pieces — whether blanked or formed — are fashioned in big presses, single or double acting, most with air cushions. Steel is used on all blanking and piercing dies and on small forming dies. Large draw dies are made of a composition of cast iron to which some steel has been added. Hence, the parts are accurately formed at one stroke. Fitting or reworking parts on the assembly line is not tolerated, except where unexpected changes make it necessary until the dies can be corrected. The only exception is a very heavy spar plate that is too long to put in any available press. This is blanked out with an Onsrud router that runs on rails so as to reach every portion of this piece. A pair of templates are used so one blank can be set up while another is being shaped. The setups are on opposite sides of the track carrying the router.

Most press-formed parts are heat treated. Quantities are not so great that dies cannot be held in the press while the parts are heat treated. Then they are re-struck immediately after quenching. With heat treated flat stock the parts are passed through Farnham Rolls for leveling. This department. has a huge ice box into which parts are placed to retard air-hardening when they cannot be refabricated at once.

To facilitate handling the amount of material being heat treated, a continuous type furnace equipped with a conveyor system was developed. Suspended from the conveyor are carriers, made from metal lath which has a diamond shaped pattern-carriers which are like big rectangular boxes without ends. Three-eighths inch iron rods, each with one end bent at right angles to form a handle, are struck through these carriers. The rods can be put in vertically or horizontally to be used as shelving or vertical dividers. Big bulkheads, for example, are put in vertically; small ones are laid in horizontally. At regular intervals one carrier goes into the furnace at one end while another drops out at the other end into the quenching spray. To conserve space, the oven is overhead, each carrier being lifted before it enters the oven, and being dropped down into the quench when it leaves. Parts are inspected before they leave the press department and are then distributed via the continuous overhead conveyor.

All stringers and formed shapes are made in the draw-bench department. The name "draw-bench" is really a misnomer here because the metal strip is compressed, instead of drawn out as when stretched through a die in real draw-bench forming. The material is usually reduced in section when drawn but is compressed when rolled. It is claimed that a shorter radius can be used in the corner of an angle when rolling and that thus a more rigid piece is obtained. Furthermore, one set of rolls can be used for several thicknesses of strip stock so long as the angle radius is correct for the heaviest stock rolled. This saves set-up time and number of rolls required. Without stopping the rolls, automatic shears that travel with the stock cut each piece off to the proper length as it is formed.

All rolled parts are made of SO stock and so have to be heat treated. The ovens have two chambers — the farthest one being the treating oven operating at about 925" F. After about one hour treatment, parts are withdrawn into the nearer chamber, where they are given a "shower bath" quench for four seconds.

After quenching, the stringers are often twisted, so special Ford-built machines which exert up to 35-tons pull stretch the parts about 3½ percent and thus straighten and set them.

Strippit punches used in Bath bending presses and Cincinnati brakes punch guide holes in stringers and other straight parts. These presses are set up in line and connected so that it is possible to punch accurately stringers that are more than 18 ft long (the length of the biggest press) since the adjacent press can be operated simultaneously to take care of the extra length.

In assembling bomb bay doors, hand squeezers are used to rivet the roller guide brackets to the ends of the door while the skin is electrically spotwelded to the accordion section of the door. Chemical cleaning of parts to be electrically welded is the procedure throughout the plant, but on these doors a special nickel-silver wire brush scours the surfaces where they are to be spot welded. A special machine, adaptable to flat or corrugated sheets, was designed for this job.

Taylor-Winfield automatic welders sew these pieces together at the rate of 45 spotwelds per minute, the spots being automatically spaced.

All aluminum tubing for hydraulic lines is bent in one department, with all kinds of bending devices in use due to the lack of supply of any one kind. Today, more and more of the tubing is being formed in fiber-faced radius blocks after it is filled with Cerro-bend. This is preferred to bending on a mandrel because the inside of the tube does not have to be cleaned of drawing lubricant after bending. The Cerro-bend leaves a clean tube when it is dipped in boiling water and the alloy has run out.

The new spar assembly line is typical of developments at Willow Run today. Rivet hole drilling follows standard practice, but the methods used in riveting are typical of Ford ingenuity, Roller conveyors are provided so that an unriveted spar starts at one end and is ready to assemble on a plane at the other end. The conveyors are adjustable for height and are so constructed that they slide sidewise very easily, making it possible for riveters to align a spar readily with the riveting heads in the machine. From nine to eleven rivets are upset at a time, effecting a very important saving in man-hours compared with the old one-rivet-at-a-time method.

The rollers are used as a work bench as well as conveyor line. In between the riveting stations, girls assemble the stiffeners, ream the rivet holes, and put in the rivets. The spar is never removed from the line until it is finished.

When the spar has been drilled and reamed, with side angles clipped to the spar webbing, it is ready to enter a specially-built double-headed automatic riveting machine. It carries two multiple riveting heads — one stationary, the other with a follow up control so that the head moves sidewise as the width of the spar increases. There is also an automatic feed which advances the spar one group of rivets at each squeeze. The operator need only take out the clamps as they approach the squeezer head. This machine is used for both right and left spars and will handle both the front and rear spars.

From the dual riveter, right and left spars flow down their own assembly lines. At the end of each, bracket forgings are riveted to the finished spar, when the assembly is put on a special fixture. At the mating end there is a milling machine setup, with the cutter spindle mounted on a table which traverses the end of the spar. Two cuts are taken, a rough and a finish. A locating point, about half the length of the spar, secures it in its proper relation lengthwise to the cutter. The track on which the mill travels determines the proper angle in the face of the brackets.

In the assembly section, which is about 5/8 mi long, there are three distinct parallel sections of about the same width, one of which is used for larger subassemblies. In a parallel corridor are built the center wing sections and, in the third, other major subassemblies. The first and last differ in that the first corridor feeds parts transversely into the major assembly lines at about the assembly station where the parts are required. In the second and third corridors the flow of all parts and subassemblies is continuously forward in the direction of final assembly.

In the corridor where parts feed into the assembly line from the side, such subassemblies as wing spars, center wing bulkheads, outer wing leading edges, fuselage parts, fuselage bottom panels, pilot's floors, center wing flaps, trailing edges, stabilizers, empennages, ailerons, and engines may be found. As will be noted, each of these is a more or less complete subassembly in itself. The method of handling these is conventional, except in the ease of the pilot's floor, for which there is a "merry- go-round" conveyor. Each fixture is carried on a car which moves intermittently from one station to another until this subassembly is completed.

Lengthwise, the other two corridors are divided into four sections. In the first quarter, the wings are built from parts into a complete structure. In the second quarter two parallel rows of center wing sections go down one corridor, picking up all the smaller parts that help to complete this backbone of the bomber. In the parallel corridor the outer wings, forward and aft fuselages, and tail cones are constructed.

The forward section of the fuselage is divided into six parts: Front and rear top decks, the two sides, bottom, and pilot's floor. The unique part of this job is painting and installing furnishings of the sides while they move along on a conveyor. Until fully equipped they are not joined together with the floor or the top and bottom structures. The intricate equipment that must be put in at this time goes in much faster when the whole side is open than after the fuselage has been completed and workmen have to crawl inside to put it in place.

At the halfway point in the plant there is a wide cross corridor called the transfer bay section. Overhead there is a 25-ft high platform or mezzanine gallery on which there are two turntables and a big slat conveyor down the center. This mezzanine extends across the ends of the two assembly bays. Onto it come all major subassemblies, except the center wing section. From this transfer gallery the fore and aft fuselage sections are routed to the center wing assemblies on the final lines. The four primary assembly lines start from this transfer point. The big center wing units are fed to the assembly lines as rapidly as they move forward, and here the Ford-built units get their start. Parts which go to outside assembly plants are gathered on the balcony and loaded into huge trailers developed for transport purposes. Two of them can haul all the parts of a complete bomber (less the engines) to any of the other bomber assembly plants.

Four parallel lines of center wings are moved progressively in the third quarter of the plant, where complete Ford-built Liberators are assembled. The first steps in the assembly at this section are made while the center wing is supported by two-wheel trolley plates attached to its ends. These wheels run on rails that carry the center wing at its normal height of about 10 ft. It is supported in level flight position, or at an angle of 3°. After the fore and aft sections of the fuselage and the landing gear have been added, the trolley plates are removed and the ship lowered onto its own landing gear.

Between each pair of primary assembly lines, and parallel to them, there is an extension of the mezzanine gallery with a conveyor on which engines and both fore and aft fuselage sections are fed to the assembly line. At each station where this is done a special bridge and traveling crane pick up the correct fuselage section at that point, transfer it right or left, and lower it onto a mating car. On these cars the new section is aligned with the center wing, to which it is to be attached, and then moved into mating position. The engine units are supported by special hoists while the four attaching bolts are slipped into place and nuts run down.

To permit adding the outer wings in the last quarter of the building, it is necessary to change from two parallel lines in each bay to one. Thus there are only two final assembly lines. Dual slat-type floor conveyors permit movement of completed center wing assemblies from either one of the primary assembly lines to the center of the final assembly bay.

Later the lines turn at right angles and enter the preflight area of the final assembly. In this "L", off the main assembly building, is the final inspection section, final paint, Army inspection and, just before leaving the building, a section separated by heavy fireproof doors in which the planes are fueled. Then the big hangar doors at the end of the line are opened and the ship is taken out onto the airport apron for flight tests.

At the turn in the assembly line, there are four doughnut type turntables, each big enough to take a complete ship, located at the point of intersection of each of the final assembly lines. By their use, the planes are given a quarter turn so as to line up properly in the preflight area on the plant. By choosing the turntable, the planes may be put on either one of these preflight assembly lines. Big doors opposite the ends of the two final lines can be opened to bypass ships directly onto the airport apron should it be necessary.

With the general plant arrangement in mind, we can turn to the methods used in assembling the various elements. The outer wing is assembled in exactly the same manner as the center wing. Being a smaller unit, the structure of the fixtures can more easily be seen and the methods of fabricating more readily understood.

The first step is to rivet the brackets to the stringers and assemble the latter. This is done on tables with suitable alignment or locating fixtures. When riveted together they are ready to apply in the first wing fixture.

There are two general types of fixtures used for assembling the wings — one for assembling the top skin and the stringers, the other for assembling the spars and bulkheads and then picking up the upper and lower skins. In each type the fixture is designed to locate the parts and hold them securely in correct position until the job is finished.

All these fixtures are rigid, steel and cast iron units mounted on heavy concrete foundations. The first fixture holds the stringers in correct position. Over these the skin is securely held in place by steel straps while it is drilled, countersunk, and riveted to the stringers. Rivet holes are back drilled through prepunched holes in the stringers and reamed at the same time. On center wing and outer wing top skins, these holes are countersunk in the skin, flush rivets installed, and the skin riveted to the stringers. Because of the height of the fixture and wing it would be difficult to work without a ladder for the top or without stooping to reach the lower edge. These fixtures are raised high above the floor to allow the elevator platforms on each side to drop low enough so that the lower edge of the wing comes at a comfortable working height for the operator. Air and electric outlets are conveniently located all along the base of the fixture.

Outer wing assembly fixtures are provided with a movable top running on rails so it can be moved to one side when a wing is completed. One spar rests on the bed of the fixture, the other is loaded into the movable top member of the fixture while it is displaced. Each end of the fixture has a retractable end plate that accurately secures the ends of the spars and the splice flanges during assembly. End plates, controlled by hand wheels, slide in gibs and are located by index pins. The movable overhead beam is also secured in proper position, by index pins at both ends, while the wing is being assembled.

In the fixture, the bulkheads are assembled first to the spars. There are hinged drill guides attached to the top and bottom of this fixture, used to drill the edges of the spars where the skin is attached. Bulkhead stations are clearly numbered on the overhead beams to guard against assembly mistakes. The bottom skins are then clamped to the spar and bulkhead assembly and drilled. When they have been properly fitted they are removed and the upper skin and stringer assembly applied.

Reason for locating the lower surface skin and stringer assemblies first and drilling the rivet holes where the three sections overlap, is found in the method used at Willow Run in attaching these assemblies to the spars. Because skins and stringers are assembled before they are fastened to the spars, blind spots are formed where "hat type" stringers are used. Also, since the entire top skin and stringer assembly is bolted in place at one time it is necessary to fit the bottom skin sections before the top skin is fastened in place.

The upper skins extend across the chord of the wing, the lower ones run lengthwise of the wing. The transverse arrangement of the bottom skins permits assembling them from the underside of the wing after the upper skin is in place. When the final strip is put on the bottom of the wing, bucking is done through access holes.

When the wing is finished and removed from the fixture by crane it is laid on its side and attached to an overhead conveyor line, where all fittings, aileron, controls, wires, wing tip, and leading edge are attached. One feature of the line is the hump where the wing is lifted high enough to allow work on the bottom side, instead of necessitating turning of the wing as when this work is done on bucks.

In the center wing section, after the wing leaves the last vertical riveting fixture, it is set up in cleanup stands, where drill guide plates are bolted to each end of the unit. These are used for reaming the bolt holes in the splice angles. These plates are left on the ends of the center wing to protect the splice plates against damage during transfer in and out of the giant Ingersoll machine, which accurately finishes all mating surfaces on the center wing. It has twelve special milling heads that may be operated simultaneously or individually. It finishes all of the vital connecting points. The bed plate of the machine measures 78 x 18 ft.

While in the Ingersoll, the wing is supported by two pivoted brackets mounted on posts at the base of the machine. When the machining is finished, the wing is lifted out by overhead crane and set on a transfer car. The drill guide plates are removed from its ends and the trolley plates, returned from the primary assembly line, are attached in their place.

Most interesting feature of the primary assembly line is the method of mating fore and aft fuselage units with the center wing section. The nose or fore fuselage consists of six other subassemblies finally brought together in two mating fixtures. In the first, the two side panels are joined to the pilot's floor, then the two top decks, front and rear, are added in side fixtures that move on transverse rails. These move in to mate with the side panels and the floor sections already mounted in the first fixture. When completed this section is lowered onto the bottom panel, which has previously been located in the second mating fixture.

In making up the five outside subassemblies of the forward fuselage, each unit is provided with two heavy cast-steel "T"-shaped straps, located in the primary assembly fixture so that their relationship to the complete unit is extremely accurate. They are bolted through the skin and bulkheads on each unit. The straps are located by removable pins in the assembly fixture, and when the finished part leaves the assembly fixture, they go with it.

When the part reaches the fixture in which the top, bottom and sides are joined together, these straps are used to locate the sections so the mating at the cleavage lines will be accurate and easy to make. When the fuselage section is completed, these straps are all pinned together and completely encompass the finished job. They not only accurately fix the assembly of the component parts but carry on to the semifinal assembly line to assure the accurate mating of the fuselage with the center wing section.

The aft section of the fuselage is similarly handled during subassembly and in mating with the center wing section. The special hoists which lift these fuselage sections off the overhead slat conveyor and lower them into position for mating with the center wing use the straps as a safe means of handling these big units.

To mate the fuselage sections with the center wing section and accurately align the two assemblies at the cleavage line presented a problem that has been solved in a very ingenious manner. The fuselage section is lowered into a carrier car that moves on rails parallel to the axis of the ship. The fuselage is located on this car by the same straps which were used in its subassembly.

Location of the center wing section in relation to the fuselage is the next step. There are eight locating points on the center wing which have to be in line for this purpose. Four temporary locating plates are fastened to the bottom face of each center section spar just outside the fuselage line. These plates have spherical sockets which drop over ball-ended locating pins, which support the center of the wing. Mounted on huge iron pillars, they accurately fix the center wing in relation to the car carrying the fuselage section which is to be attached. These hold the wing in flying position or at about 3° from the horizontal. The outer ends of the center wing rest on two locating points in each of the end towers.

In order to support the center wing section on the locating pins at a mating station the wing must be lowered about six inches. This is done by having elevator sections in the track at each of these mating stations for this purpose. The truck which supports the fuselage is provided with a gear drive to roll the fuselage into mating position. The center wing mating pads have prepunched rivet holes which are used as guide holes when drilling the mating skin of the fuselage. After the sections are mated, the wing is lifted again to the level of the line.

To prevent the center wing from tip- ping after the forward fuselage has been attached, a special car running on rails supports the front end until the front landing gear can be attached at a station farther down the assembly line.

The aft section of the fuselage is added in exactly the same manner as the fore section.

Next hydraulic jacks in the end towers raise the whole assembly and the main landing gear is lowered, the nose wheel installed, and the oleos pumped up so the entire assembly is supported on its own wheels. At this point, the center wing conveyor line ends and the floor conveyor chain starts. The trolley conveyor plates are removed from the ends of the center wing and returned to the start of the center wing horizontal conveyor in the center wing department. This is where trolley conveyor plates replace the drill guide plates used on the Ingersoll mill.

Engines, trailing edges, and empennage are installed after the center wing is on its own landing wheels. The engines with their cowling, oil tanks, oil coolers, and intercoolers are a complete unit. They are kept in stock on the overhead gallery and lowered in place by their own special crane at the engine mounting station.

At the end of semifinal assembly, the planes run out onto two transverse slat conveyors in the floor. With the landing gear wheels on, a plane can be transferred to the center of the final assembly line from either primary line.

Platforms throughout the assembly line have folding floors and hand guard rails to allow the plane to be moved past as the assembly advances. The conveyor system in use is intermittent in action, not continuous as in automotive practice. Planes are moved from station to station by power on the final assembly line. Platforms, most of which are permanent, are constructed at each station to facilitate assembly work.

Elaborate spray booths are located on each of the final assembly lines beyond the turntables. Water sprays are used to catch the "over-spray". This is handled by down-draft ventilation through gratings in the floor.

After painting, the bombers enter the preflight area of five stations. Here propellers are added, seats, radios, and instruments installed. Fuel is added at the last station in the special fireproof room, at the end of the assembly line.

Here, the flight department takes over the craft and wheels it onto the flying field apron. This department has a huge service hangar in which operations can be carried on regardless of the weather. When accepted by the Army Air Forces, the Liberators are turned over to the Ferry Command for fly-away delivery from Willow Run Airport, which is an integral part of this tremendous project.

This article was originally published in the July, 1943, issue of Aviation magazine, vol 42, no 7, pp 152-158, 161, 362, 365-366, 369-370, 373-374.
The original article includes 14 photos of the Willow Run assembly line and 8 detail drawings of production fixtures.
Photos and drawings are not credited.

Footnote: Alrok was a passivation process for aluminum and many of its alloys. It involved boiling the part in a mixture of Na2CO3 and and an alkali chromate — sources differ, listing Na2CrO4 or K2Cr2O7. The coating was believed to be aluminum oxide (Al2O3) but later analysis showed the coating to be primarily a mixture of carbonates and chromates.