Case Study: Aluminum Ceramic Coatings Proper Application and Testing

Introduction

The coating I am discussing today was brought about in the automotive and aerospace industry, as protection from high heat on car fires and turbine blades for jet engines. It was brought to the oil and gas industry, by engineers that had a use for it as an anode on fasteners and performed well on ASTM B-117, giving 5,000 salt spray hours to carbon steel. The only draw back was that the nuts needed to be over sized by 10/10000 in allowance to the 1.8 mil total coating thickness.

Getting the coating tested and specified from the automotive industry to subsea industry was no easy task and there were many standard test organizations that did not comply to one another and new testing was needed. The rules for application were poorly addressed, or I should say left open for interpretation by the applicator.

Methods / Materials

Aluminum Ceramic Coatings (or ACC as we will call it through out the paper), have been in oil and gas use since 1999. As a sacrificial coating for subsea systems.

ACC is a high performance coating that can be applied to metallic and non-metallic surfaces to provide resistance to corrosion, thermal oxidation, abrasion and erosion. ACC is a

water based slurry comprised of an acidic chromate/phosphate binder system containing dispersed aluminum particles. The material is applied by spraying with conventional. After being heat cured, ACC produces a ceramic/ metallic composite coating that can be effective in thickness as low as .75 of a mil.(0.00075 inches.)

ACC provides protection to metal surfaces against atmospheric and chemical corrosion, wear, erosion, thermal oxidation, pitting and other destructive forces. The coating can withstand salt spray exposure for thousands of hours and protect parts at temperatures up to1200°F. By protecting the surface from corrosion and oxidation, ACC can improve the fatigue strength characteristics. It can be made conductive and sacrificial to all steels and many other alloys. It will extend the life of critical parts and, therefore, cut costs through savings in parts replacement, repairs and lost production time. Often, lighter gauge or less expensive base metals can be used to take advantage of the unique protective properties of ACC.

ACC is used on all types of metal parts and surfaces. In the oil and gas industry, it can give protection to riser bolts, running tools, subsea BOPs, works well with respect to Thermal Spray Aluminum parts, valves. High heat applications include boilers, heat exchangers, furnaces. In the chemical process industry ACC provides protection from corrosion and oxidation of steam valves, pipes and other high-temperature applications.

Technical Data

Color. (When mixed). Gray-green

Total solids by weight. 47- 56%

Weight per gallon 12 - 13.3 lbs.

Specific gravity 1.44 - 1.6

The coating cures 30 minutes after the entire part has reached 650°F. It must reach the 650°F for two reasons; one is that the ceramic will return to slurry (mud) if not fully cured and two is that a chemical change occurs in the hexavalent chrome molecular structure and transforms into trivalent chrome at that temperature.

The coating can act as an anode, because the ceramic holds aluminum flake particles in suspension against the surface of the metal substrate. Forming an electrochemical bond in an electrolyte such as salt water and allowing the aluminum to sacrifice it's self in place of the metal substrate and forms aluminum oxide and acts as a seal.

The aluminum oxide then becomes an even better barrier layer and anode. This occurs at a coating thickness of at least (>.75 mils), no less. Even when working with fasteners you must have this thickness of the ceramic , in order to have the system work.

The issues we see with the coating system is in its documentation of application, quality control and

specification as something other than it was intended.

Improper Applications and Quality Checks

1. Abstract

The discussion of improper application and quality checkpoints of a ceramic coating has come up with applicators applying by academic direction only and very little hands on experience. A manufacture can only direct an application so much and an OEM can only write obvious quality check points. Even with a safe specific analysis of the application it is still left up to experience and interpretation of the product.

2. Instructions

The instructions for application of the ACC listed as they are can be preserved different ways.

Application instructions for Aluminum Ceramic Coatings

Surface preparation

Deposits of oils and organic material must be removed by thermal degreasing or alkaline cleaning prior to abrasion procedures. Parts should only be handled while wearing gloves subsequent to degreasing. Grit blast parts using new 60 -100 mesh alumina at 80 psi air pressure.

Application of coating

(ACC) is supplied ready for use at application viscosity and does not require thinning. Some name brands will give special instruction as to what viscosity Adjustment Solution to use if any.Condition the five gallon pail of coating on a five-gallon paint spinner to disperse any settled solids. Transfer the coating to a separate container by screening

through a 100-mesh screen or medium mesh paint filter cone. Screen the entire contents of the original container to assure that no solids remain on the bottom. Be sure to inspect the container after emptying.

For manual spray operations, transfer the coating to the spray gun cup and lightly shake the spray gun regularly during use to keep the solids suspended and to assure a homogeneous mixture. Suitable spray gun equipment is the DeVibiss EGA 502 (F tip/395 air cap), EGA 530HV series (F tip and 395HV air cap) or TGA-515 ( F tip and CV 39-90 air cap). The Blinks 115 or 26 spray guns with 76SS needle and 76 x D76S fluid and air cap are also suitable.

Other spray guns may be used provided fluid tip and needle orifice are sized at 0.040- 0.045 inches.

When fully automatic or semi-automatic spray equipment will be used, a separate coating reservoir equipped with a centrifugal pump and auxiliary prop stir agitator must be used. Additionally, the automatic spray guns must be fitted with a flow through closed loop fluid body ( I.e., Paasche A-JU type gun).

For optimal coating performance, the surfaces to be sprayed, dried and cured twice; however, many applications may be specified where one coat deposition will be sufficient. Typically, dry film thickness of 1.0 - 1.5 mils will be satisfactory for most applications. Drying and curing are explained in the following section.

Adjust the atomizing air pressure to 30-40 psi for manual conventional air atomized spray guns and 50-75 psi for manual HVLP spray guns. For automatic spray guns, follow the manufactures suggested air pressure recommendations for the style gun being used. Set the fluid adjustment nozzle

to deliver a strong fine mist. Hold or position the spray gun approximately eight (8) inches away from and directly perpendicular to the surface to be coated. Apply a wet even coating to the surface by gradually building up the coating in an overlay pattern using multiple passes as required. Porous surfaces such as powdered metal and cast iron tend to absorb coating and will require more passes to develop the proper film build. The wet coating should be green colored when applied, but should not create build ups or drips and runs or fail to dry or flash to gray. Increasing spraying distance, lessening air pressure or fluid flow or changing to a finer spray nozzle will prevent excess buildup of coating. The coating must not be applied to the part in such a way that the coating dries to gray immediately. Should this condition occur, then reducing the spray gun to part distance, or reducing the atomization air pressure, or increasing the fluid delivery rate or a combination of above should correct the problem. ACC can be washed off with warm water, dried and recoated prior to curing if required. This is possible both before and after oven drying. Cleanup is simple using warm water.

Note: Cleanup water will contain hexavalent chromium compounds and must be captured and disposed of properly as chromate containing hazardous waste. Do Not dispose of chromate containing cleanup water in municipal or sanitary sewage systems. Refer to MSDS for additional waste treatment information.

Drying and curing procedure

After allowing ACC to air dry for about ten minutes, during which time it changes from green to light grey, the parts are then ready to be dried at 150- 175°F for a minimum of 15 minutes. The air-dry procedure can be modified to

accommodate continuos process applications. ACC is cured in a 650°F -0°/ +25°F oven to achieve a minimum part dwell time of 30 minutes at 650°F. If feasible, parts may be self-cured by placing them right into their intended high heat application and allowing the process heat to do the curing. ACC cannot be over cured, and under cured coatings are detectable after burnishment by placing a wet cloth on the coated surface and observing if any binder leaches onto the fabric as exhibited by yellow color.

Electrical conductivity must be established to obtain maximum salt spray performance. Mechanical burnishing subsequent to curing will accomplish this. Recommended methods are: glass bead or grit burnishing @ 20-30 psi (suction type blasters). Another method by which ACC is made electrically conductive is to post cure at 1050°F (part temperature) for at least 30 minutes. Conductivity may be tested by placing resistance meter probes one inch apart. The reading should measure below 5 ohms.

(Sealers and other top coatsMany top coats are used to either seal the porous ceramic, or to allow for in service use. Fluropolymers, fusion bonded epoxies and even rubber has been used over the surface of the ceramic.)

3. Methodology

Case Study Investigation: The reason I broke the test up into four segments is to show what is possible when using a two coat system. In the study, I under cured the ceramic coating on plates 3 and 4 and under cured the top sealer coating on plates 2 and 3.In doing so I am able to make mistakes and problem solve issues that may have come about in production, but have yet to be covered.

4. Findings

What were the results of your investigation and what answers do they give you?

There is not always a consistency to every test and multiple tests are required. Certifications of Coating and quality check points would not only need to be signed off on but thermal coupling records, burnishment mil readings before and after water test and multiple ohm readings would all go a long way to securing a well processed application. ( Many written procedures are not direct enough as to when in process quality checks should be made. For instance the DW test should only be done after the burnishment of the ACC.)

DW Test- The deionized water test has been standard ceramic cure test for the past almost twenty years. The only issue with it is that it may take 24 hours for the hexavalent chrome if not cured, to show up on the outside of a non-burnish part. Usually a green or yellow tint on a wet rag, or tint in a pool of water left on the ceramic. Indicates that the part is not yet fully cured. Surface ceramic can cure on the surface practically, that is why it is important to water test the ceramic after burnishment.

The deionized water test which is standard with checking the

hexavalent chrome binder has transferred into trivalent chrome and cure of the ceramic, came out negative for hexavalent and positive for cure on all samples. The test was performed for 20 minutes and the substrate had cooled to room temperature before performing the test.( this was prior to the burnishment of the ceramic.) When the deionized water test was performed after burnishment the test responded quit well to the uncured ceramic hexavalent chrome still present.

Case Study: When applying the de-ionized water test, all the samples past while the plates were at room temperature. Even though two of the four were under cured.

5. Conclusions

What were the results of your investigation and what answers do they give you?

The conclusion was that the water test needed to be performed after the burnishment was completed in order to reveal the presents of uncured hexavalent in the ceramic. Too many variables are recorded on the outside of an unburnished ceramic. That is another reason to incorporate a thermal coupling device in order to read part temperature, rather than oven temperature.

Analysis

Sample 1. Ceramic Cure/ SealerCure. Bases to work from and match the other samples.Many people have questions on the look and feel of the product. Burnishment of the ACC will give off a shine when properly burnished

Sample 2. Ceramic Cure/ Sealer Uncured. Easily checked with MEK rub test and find at fault. If top coat fails rub test,

then deconstructive test to check ceramic level should be made.

Sample 3. Ceramic Uncured/ Sealer Cured. Findings depend on certification of in processes testing of coating records by applicator. Certification in processes records should include;

1. athermalcouplingtemperatureofthepartsreaching650°Fforanhourperinchof part thickness

2. a mil thickness reading after burnishment

3. adeionizedwatertestoftheceramiccoatingafterburnishment.*

4. Anothermilthicknessmeasurementafterthedeionizedwatertest,tocheckforloss of coating during the water test.If there were no records, or falsified records of the thermal coupling and the in process testing, you would have to then rely on destructive lab tests.

a) Hach CH-8 Hexavalent chromium test

b) Deionized Water test with removal of the sealer by burnishment, not burn-off.

c) ChromaVer3powderpillowsfromHachcompanyandWhatman#40filterpaper.

(Provide the three lab tests. Reference 3-6)

This is by far the most damaging occurrence and unfortunately the most common. If the ceramic is under cured at 450°F and allowed to pass, it will fail within days. If the ceramic is under cured at 550°F it will fail in a matter of months. If the ceramic coating is under cured at 650°F with out the part curing long enough. (Usually occurring due to part

metal thickness not being taken into consideration.) The part will fail in the field and return

back to its slurry application no matter what sealer is present. Coming in far short of its intended 5000 salt spray hours. Also being deployed into the field and failing while in service.

Sample 4. Ceramic Uncured/Sealer Uncured. Easily checked with MEK rub test and find at fault. A deconstructive water test and applicator's certification of coatings would determine the ceramic layer to be under cured as well. If the top coating is found to be under cured, a deconstructive water test should follow. Deconstructive testing would require the removal of the sealer and one of the three test listed above in sample 3, References 3, 4 and 5.

Final Note.

The results from the test Sample 3 again is the real reason for this study and is a concern for all the reasons stated. The coating does a good job and lives up to its title, but not all applications are equal, or should be treated so. The action taken was that several of the in process tests were made mandatory and thermal coupling was specified for both two coat systems. I would go one step further and ask for a submission of parts be coated and tested to ASTM B-117 for a total of 3,500-5,000 salt spray hours and a cut test made to check adhesion.

References:

1.ASTM B-117 salt spray test.

2.ASTM F1428 Standard Specification for Aluminum Particle-filled Base coat/ Organic or Inorganic topcoat, Corrosion Protective Coatings for Fasteners.

3.SLP 62-2 Cure Evaluation4.SLP 62-4 Cure Evaluation5.SLP 42-1 Coating Cure Evaluation

Thanks goes to Bruce McMordie with Coatings for Industry, Bill Fabiny with Dow Chemical, Lonnie Fuqua with NOV, Anand Samant with Praxair, Brian Harpold with Numerical Precision, Bolt Science and Array Coating Technology.