Magnesium Protection using Indestructible Paint's Organic
Coatings RangeIntroduction Indestructible Paint has been functioning in one form or another for many years and its first experience of magnesium protection was in the 1950's with the introduction of the Rockhard range of coatings. These are a range of stoving (baking) products which have been developed along the way.
They are still extremely successful, but since the late 1970's, Indestructible Paint has been making ranges of coatings for the Rolls Royce aero engine company which include a variety of alternative magnesium coatings. The reason why Rolls Royce have several alternative systems is that Rolls Royce is an association of older British aero engine companies such as Hawker Siddeley, Bristol Aerospace and others.
Along the way, systems have been developed in line with the thinking of the particular site and we often hear reference to Glasgow systems, Derby systems, Bristol systems and even Leavesdon systems. This is not only the situation with magnesium protection but also with other systems as well. As a result of this, Indestructible Paint manufacture a variety of systems which may appear to do the same job.
We obviously have our own ideas of which are perfect systems, but it is a fact that different systems from time to time do perform better than others. Also, different systems have been formulated for different temperature ranges, from an air drying touch up system right through to high temperature magnesium protection at in excess of 300°C.
We believe that the American AMS 3132 phenolic based system which appears to be their standard magnesium protection is the fore runner of our Rockhard range of products. This was the technology used for protecting magnesium in the 1940's in the UK, and has been superseded a long while ago. However, some of the thin film principles are still used, for example in the Rolls Royce system MSRR 9032.
We do not have any particular major stipulations here although in the UK, we have tended to stick with the chromate pretreatments. This is a thin film technology, and in America significantly more use has been made of heavy anodising, and such new technology as Tagnite and Magoxid. However, we believe all of them have one thing in common, that surface sealing using Rockhard improves their performance significantly. In fact we would go so far as to say, particularly on aircraft, that surface sealing is essential.
Our products are tested to a range of standards, and some are tested individually whereas some are tested as part of a system. Historically they were tested to United Kingdom Ministry of Defence , DTD, Def Stan, and AFS, but systems were also tested individually by such authorities as British Aerospace (Hawker Siddeley), Rolls Royce and Westland.
DTD 5562 - This test states that the material will pass the following:
A normal repair system for magnesium is to use a two pack epoxy chromate primer, in the UK this is to DTD 5555 or BSX 33, which is very similar to MILP23377 in the United States. However, Rolls Royce also have a system whereby they use a touch-up system to MSRR 9037, which incorporates a clear sealant air drying.
We have recently done some work for a UK manufacturer where we put this sealant on, although baked at 125ºC and over coated it with two pack chromate. The subsequent protection was significant and in fact gave excess of 1000 hours with 1 thou film thickness of combined coating on a AZ31 panels (ASTMB117 salt spray). We therefore believe that some work should be done with this and if there was a problem of magnesium touch-up this could perhaps give a very workable process.
We have also got a two pack chromate free replacement for these materials which have performed equally as well on magnesium in the salt spray over 1000 hours as the chromate product.This is currently on test at an American manufacturer. Some thought should perhaps be given to previously putting on two pack acid based etch primer, such as BSX 32 or DEF 80-15. This would give a completely comprehensive system.
Pretreatment being equal, we have recently completed an exercise with a variety of coatings which we used on magnesium at 125ºC. The first alternative was to try a simple two pack chromate primer to DTD 5555 or MILP23377. This was compared with a variety of our low temperature cure one part and two part coatings which were put on underneath the two pack chromate primer.
After 1000 hours on AZ31 the worst performance was straight two pack chromate, and the best was utilising our 985-111-800. However, running it a close second was two pack IP9064/Clear and IP9170 Clear. Interestingly, both of these products along with the two pack clear primer could be used cold, with we believe similar effect.
Please apply for detailed report and photographs showing testing after 1000 hours on AZ31 Magnesium utilising systems cured at 125ºC (255ºF).
Testing of the following systems on magnesium means that the following minimum performance will be achieved:
System 1 :
Utilising our Aerolac varnishes to MSRR 9051, in either green tinted or aluminium versions, simply apply a very thin film to magnesium pretreatment. This should then be cured as per RPS 661/2. The metal protective varnish should be sprayed on according to RPS but normally with a minimum thickness of 0.040mm. However, this thickness can be varied or reduced to virtually nothing and on magnesium components with a no varnish requirement, one coat of varnish is sprayed and then wiped off.
This leaves a residual film to sill the normally chromate conversion coating. The varnish will then be stoved by heating the part to 150 ±5ºC (300ºF). Over it apply our baking materials IP9165 primer and IP9166 grey top coat. The primer, IP9165 will be sprayed with one coat and allowed to air dry 15 to 20 minutes, it will then be stoved by heating the part to 150 ±5ºC and maintaining the temperature for 30 minutes.
Then the top coat, IP9166, will be sprayed on with one coat and allowed to air dry for a minimum of 3 minutes. A further coat should be applied and allowed to air dry for a minimum of 15 minutes, the IP9166 will then be fully cured by heating the parts to 150°C for 30 minutes. If further coats are required these will be sprayed again at 150°C.
Finally the coating will be stoved by heating the parts to 190 ±5ºC (375ºF) for two hours. The IP9165 will have a minimum thickness of 0.008mm. Unless otherwise stated, the thickness of the complete paint film after stoving should be .09mm minimum. IP9165 and IP9166 is to MSRR 9032 and should be applied to RPS 661/3. This is a very tough system.
System 2 :
This is using a simple Rockhard system, utilising Rockhard chrate primer 700-155-003 over which is applied stoving grey top coat, 985-000-693.
Materials are released to MSRR 9226 and application is in accordance with RPS 661/21. The part will be sprayed with one coat of primer 700-155-003, which will be allowed to air dry from a minimum of 3 minutes. A further coat of primer can then be applied and allowed to air dry for a minimum period of 15 minutes, which will give a dry film thickness of approximately .016 to .025mm.
Parts that are only to be coated with the primer will then be coated and stoved at 190 ±5ºC maintaining the temperature for one hour. Where the application of enamel is to follow the product will be stoved at 150ºC maintaining the temperature for 30 minutes, then over coated with a coat of enamel and allowed to air dry for a minimum of 3 minutes.
A further coat of enamel will then be applied and allowed to air dry for a minimum of 15 minutes. This will give a dry film thickness of approximately .025 to .5mm. Where further coats are to follow the enamel should be stoved at 150ºC maintaining the temperature for 30 minutes and repeated if necessary.
The final big coat will be stoved for 190ºC for a period of one hour. Air drying time immediately before stoving is essential, where static ovens are used. If conveyor ovens are used however, the temperature increases gradual and air drying is not necessary. Please note that the primer has a tendency to darken when heated above 150ºC and particularly if under load from bolted joints. This is acceptable providing the primer remains physically undamaged.
On parts where attrition coatings are specified it is essential that both primer and enamel cured to 150ºC only if application of attrition coating is carried out after painting.The final cure at 190ºC will be achieved during subsequent stoving of the attrition coating. If touch up for the cured 700-155-003 is required ask for IP9064/5555. This is being colour matched to give a good colour alternative to the stoved Rockhard chromate primer.
System 3:
Utilising what was an original British Aerospace system, apply clear Rockhard 576-450-002, an intermediate coat of chromate Rockhard and then a decorative top coat of grey Rockhard such as 692-155-004.
Rockhard stoving primers and enamels were first produced in the early 1950's and have shown a steady record of technical development over the years to comply with the needs of the time.
Of the many challenging requirements of anti-corrosive coatings, it is still today difficult to envisage more exacting requirements than those quoted in the design of the Westland Wessex anti submarine helicopter during 1956 and 1957. In addition to the very considerable corrosion fatigue problems inherent in a helicopter, dust, sea water spray, snow, light objects are continuously recycled over the aircraft by the wash of the rotors when hovering near the ground in a wide variety of locations. In view of all these factors, the use of magnesium alloys appeared impracticable although very desirable to meet design needs due to its high strength to weight ratio. Much of the magnesium sheet used on this aircraft was thin gauge and since this was used as external skinning the maximum possible protection had to be developed.
Similarly all the transmission gear-box castings holding hot ester lubricants required an organic protective with good resistance to organic solvents. The combination of turbine engines and the service requirements of prolonged hovering over the sea for 70% of the aircraft's life made a new approach to the protection of magnesium essential. Rockhard 444/176, subsequently described as 576-450-002 (or 961 type system) was eventually chosen and went into service in 1959.
Rockhard high performance coating have developed from this time and have been used widely in the United Kingdom and abroad.
We do not know in detail all the end used of our products but a few are listed below:
The latest uses includes use on the ALF 502 and 507 engine series, Sikorsky AH 60 Blackhawk gearbox, McDonnell Douglas MD500, the Pratt & Whitney Canada PT100 and PT6 series and GE F110 gearbox.
There are many more potential applications to utilise this experience, Indestructible Paints was formed to find them.
Surface Preparation
Grease and corrosion free surfaces are essential for optimum performance. DTD 911 pretreatment is strongly recommended for magnesium alloy surfaces. Approved processes for other metals greatly improve corrosion and chemical resistance. E.g. Aluminium - anodise, chemical pretreat or emery dress. Steel - Abrasive blast, phosphate or acid etch. Titanium - wet abrasive blast.
Application
Shop conditions are important for good results, application at temperatures between 15º and 30º Celsius (60º-85ºF) and a relative humidity of between 35 and 75% are ideal. Do not attempt application at temperatures below 5º Celsius (40ºF) or relative humidities above 75%.
Stoving products are very versatile and spray, dip, centrifuge and curtain coat applications are commonly used. Pigmented products are normally sprayed, apart from a few specialist applications.
Stoving Schedules
Where given are approximate and refer to article temperatures. In multicoat systems, care should be taken to cure early coats sufficiently to avoid "lifting'' due to solvent attack by subsequent coats. At the same time delamination - (poor intercoat adhesion) - due to over curing of early coats should be avoided.
At final curing temperatures of 190º-200º Celsius (375º - 400º F), it is difficult to overstove. Films progressively darken, flexibility and resistance properties tend to progressively improve well beyond normally accepted times.
Thinning
The standard thinner for use with the Rockhard range is 665-550-025, this is suitable for over 90% of all applications. If a faster thinner is required then 666-552-023 Fast thinner is available.
Surface Defects
Assuming that the substrate is clean and dry, should 'pin holing' occur an addition of 0.5 - 2% of 660-450-007 Anti Cissing Agent to the material prior to spraying may cure the problem.
For maximum protection of magnesium, thought should first be given to the whole process.
1. Selection of Magnesium - Use the best magnesium available, do the maximum amount of machining, and finally remove corrosion, degrease and clean.
2. Magnesium Pre-treatment - We do not have any particular major stipulations here although in the UK, we have tended to stick with the chromate pretreatments. This is a thin film technology, and in America significantly more use has been made of heavy anodising, and such new technology as Tagnite and Magoxid.However, we believe all of them have one thing in common, that surface sealing using Rockhard improves their performance significantly.In fact we would go so far as to say, particularly on aircraft, that surface sealing is essential.
There are several options of pre-treatment. Here we list some of the alternatives.
a) Chromate - Various chromate processes are in use throughout the world. Protection is, however, low. Refer to DTD 911C. Magnesium Elektron prefer the chrome manganese bath if this system is used. Relatively cheap, these systems can be done in house.
b) Hard anodising - HAE and Dow 17. These offer a far harder and superior abrasive resistant base for further treatment. However, they are porous treatments and will not by themselves protect magnesium in corrosive environments. HAE is the harder of the two with a hardness factor of about 700 vickers.
c)Magoxid - There are new pretreatments being developed all of the time, particularly because of the environment issue. In Europe, a process called Magoxid has been developed. Another anodic coating, this is not as hard as HAE, but is claimed to be wear resistant. It is a proprietary product.
d) Tagnite - As chromating has already been banned in California it is reasonable to expect further development in this area. A new process from the US is Tagnite. Again, currently a proprietary anodic process, this depends on very high current and includes a silicate which fuses into the surface.
e) Non chromate - For example the new Sundstrand system.
f) DOW 19 - For touch up. Please contact our laboratory for source information.
g) Preparation 18 - Non chromate alternative to DOW 19.
3. ROCKHARD SURFACE SEAL (ROCKHARD 576-450-002 AND 985-111-800) - These are clear versions of surface sealants, but each have slightly different properties.
'961' Type Resin Based
The 576-450-002clear,and colour variants on the same resin system have good high temperature capability. 100 Hours at 220ºC is a normal test. It is also significantly better for intercoat adhesion for subsequent primer or decorative coats. However, it is less colour stable at temperature, the clear coat going a dark chocolate brown if cured correctly. White versions go a very dark cream/brown when used at elevated temperatures.
576-450-002,961-450-002,614-150-002,690-450-005 are the only examples of this type of Rockhard stoving enamels.
'985' Type Resin Based
The 985-111-800 clear, and colour variants are inherently less stable products with shorter shelf life. It does however have some advantages. Although it has poor intercoat adhesion after full curing, it is possible to build up quite thick coats by air dry flashing off between coats with a final cure. This material then retains its colour better and can be used as a decorative coating, in for example aircraft grey for engines, undercarriages etc. 985 systems are understood to have slightly better resistance to chemicals used in anodising baths, but are more prone to problems with 'pinholing'.
All other Rockhard stoving products are 985 based.Including700-155-003, 690-152-004,615-155-001, 690-152-004, etc
It is quite acceptable to mix the systems, clear 576-450-002 with 985 system grey as the top coat.
Both systems are extremely chemical resistant (apply for resistance charts if required), very hard and erosion resistant. 576 systems are still used for coating turbine blades.
4. Overpainting - Either use multicoat stoving Rockhard as referred to in '3' or use our air drying 2 part epoxy chromate primer IP9064/6362 and IP9064/CAT, plus either 2 part epoxy top coat (IP9064 range) or 2 part polyurethane for maximum protection and aesthetic effect.
We do also have stoving zinc and strontium chromate primers (700-155-003) and (588/0066/2) for particularly arduous conditions.
5. Stripping Rockhard - Rockhard is a difficult product to strip. Effective methods are not environmentally friendly particularly from magnesium and aluminium and include (except b)).
a) Ardrox 26HT hot is reputed to strip it.
b) Plastic media bead has been successful.
c) 1 Hour in 100gm/litre chromic acid plus 5 minutes in orthosil.
Other methods are possibly available.
6. Touching up damaged coatings
a) Maximum clean and degrease the area.
b) Using our Dow 19, treat the damaged area.
c) Thoroughly wash off surplus and completely dry - preferably with heat.
d) 2 Part epoxy chromate air dry primer and 2 part epoxy top coat to match colour.
a) Components pretreated as specified, dry and free from oil and grease.
b) Operatives to handle with linen gloves and use jigs and low air pressures on intricately shaped articles.
2.
a)Spray or dip ROCKHARD Clear lacquer, 576-450-002 thinned to 18-20 seconds B4 cup.
b) Flash off 10 minutes.
c) Stove each double coat 30 minutes at 170ºC. (340°F).Note:Dipping may be practicable on some components but great care should be taken to de-tear before stoving. This is done first to seal the component.
3.
a) Two thin wet coats of ROCKHARD Chromate primer 700-155-003 thinned to 18-20 seconds B4 cup with thinner 665-550-025.
b) Flash off 10 minutes.
c) Stove 30 minutes at 170ºC.(340ºF).
4.Repeat process 3 sufficient times to achieve necessary film thickness. Allow 18-25 microns for stage 5 below.
5.
a) Spray with matt black ROCKHARD 615-155-001 thinned to 16-18 seconds B4 cup with thinner 665-550-025, wet coats using low air pressure and ensure wet coats.
b) Flash off 10 minutes.
c) Stove 1 hour at 200ºC (390º/400ºF) metal temperature.
Total film thickness 100 ± 5 microns.
Note:
1. Rockhard matt black may be substituted by Gloss Black or Grey.
2. Clear lacquer built up coats can be reduced or increased at Stage 4 to accord with performance / weight / cost requirements.
Materials:
Casting Sealer 576-450-002 and Thinner 665-550-025
Rockhard lacquer has been effectively used as a casting sealer for high performance uses for over 3 decades.
Typical Process
Castings to be processed must be clean and free from oil and grease or pretreated to an approved process.
1. Preheat casting to 105ºCelcius (220ºF), and allow to cool to 38ºCelcius (100ºF). At this stage casting must be dry following water tests.
2. Immerse completely in Sealer 576-450-002 and leave until all bubbling ceases.
3. Remove from sealer, drain and clean up, removing all surplus sealer. Ensure any interior pockets, drain holes, passages etc are clear.
4. Following a minimum of 20 minutes flash off period, stove for not less than 30 minutes at 200ºC (390º/400ºF) metal temperature.
This single application has proved sufficient in may operations, especially in the case of thick wall castings, however, when necessary a second application is permitted to specification.
When castings are being processed as a 2 coat job the initial stoving time can be reduced by 50% but the final stoving must be maintained.
Alternative Process
Dependent upon the degree of porosity and wall thickness some successful sealings have been achieved without preheating. Castings must of course be dry and free from oil and grease.
Some attempts at spot surface application have been made but with only a very limited success rate obtained. Only trial and error would show the feasibility of this method, no process can be laid down.
When castings are known to have a high ratio of micro-porosity it can be very advantageous to treat castings before machining, as this reduces greatly the care, and time taken to clean up castings before stoving.
Finally it must be emphasised that this is a sealer formicro-porosity and should not be confused with a metal stopper.
Whilst viscosity control with thinner adjustment is routine practice in most paint shops, effect of resin polymerisation is frequently misunderstood. It is hoped that in such cases, the following notes may be of assistance.
It is generally good practice to:
1. Use minimum tank size for articles to 'turn over' tank contents in minimum practical time.
2. The exposed surface area should be as small as possible to reduce evaporation. Tanks should be covered when not in use.
3. Avoid contamination due to carry over of pretreatment solutions, water and other extraneous matter. Lacquer tanks which have been repeatedly thinned tend to have a lower non-volatile (or solids) content for a given viscosity. particularly on upper surfaces and drained edges.
Recommended Test Method
Materials: Rockhard756-450-002 Thinner665-550-025
Rockhard 576-450-002 is supplied at 29% solids.This should not be allowed to vary by more than for ± 2%.
1. Take an approximately 25 gram sample from dip tank into a clean, dry tared weighing bottle.
a) Weigh by difference a 2 gm ± 25% sample onto a Petri dish or similar small flat receptacle.
b) Bake in oven at 140 ± 10ºC for approximately 2 hours,weigh and return to oven, weigh again after 15 minutes. If reading is within 2% offirst, take this figure as 'b' otherwise return to oven and repeat.
Non Volatile =(b/a)100 =%
2.Prepare standard. Thin a sample of fresh lacquer to tank viscosity and repeat above procedure.
3.Compare tank N.V. with standard.
Extreme example Tank N.V/Standard N.V.= 10%/20% = 50%
Resultant film thickness per coat would be approximately halved.
System 1:
Our first endeavours to make coatings to this temperature were using chromated high temperature primer IP9310 over coated with high temperature lead free stoving aluminium IP9029, with a top coat of PL163 black spraying. This was our first attempt and is satisfactory on surfaces giving a black coloured surface. However, we are currently having tested a new system which is only a two coat system.
System 2:
This material is using clear high temperature primer LR?? over coated with high temperature coloured anti corrosive top coat LR??.
Please contact our technical team and Research and Development people for more information.
