Studies on anodization of magnesium alloy for thermal control applications

Studies on Anodization of Magnesium Alloy for Thermal Control Applications by A.K. Sharma, R. Uma Rani, and K. Giri, Thermal Systems Group, ISRO Satellite Centre, Bangalore, India

lectrochemical conversion coatings are often employed as an ideal means of improving one or more surface properties: chemical, mechanical, electrical, or optical. These are used to prevent atmospheric corrosion and/or reaction with propellant tanks; to provide better adhesion for paints, lubricants, and adhesives; to improve microhardness and reduce friction on sliding surfaces; to minimize contact resistance in electronic packages; to act as a solid film lubricant to prevent cold welding in space conditions; and to provide adequate optical surface for thermal control applications, etc. Chemical coatings for space application require a higher standard and better control than those used for ground application because on-orbit spacecraft are not approachable for repair, and space conditions are very severe. The chemical coatings designed for a space mission have to withstand the extreme temperatures of aerodynamic heating, acceleration, shock vibration, and acoustic noise of space flight, as well as the ultrahigh vacuum (10 -17 tort), extreme temperature cycling, bombardment of highenergy particles (electrons, protons), and ultraviolet (UV) radiation of the postlaunch environment. Magnesium and its alloys are increasingly used in aerospace and allied fields because of their exceptional properties, including ultralightness, good strength-to-weight ratio, and good, low-cost machinability and weldability. These alloys are also able to dampen shock waves and have excellent hot forming properties, good dimensional stability, and good heatsinking and pressure-tightness properties. The studies were carried out on magnesium alloy ZM-21. This alloy was selected due to its better mechanical and corrosion-resistance properties. The physical and metallurgical properties of the alloy are presented in Table I.

E

METAL FINISHING ° MARCH 1997

THERMAL CONTROL APPLICATIONS A spacecraft in orbit undergoes extreme temperature cycling due to direct sun load on one side and deep cold space on the other. This causes a large thermal gradient between the sunlit and shadowed sides of the vehicle. The various subsystems of the spacecraft can, however, work at their fullest efficiency within the specified temperature limits. In the absence of an atmosphere, heat exchanged in the spacecraft is limited to radiation. The equilibrium temperature of any subsystem is controlled by the ratio of solar absorptance to infrared (IR) emittance of its surfaces. The chemical coating applied to the spacecraft components plays an important role in thermal control by providing suitable optical properties. If we consider a spacecraft far away from the Earth's atmosphere and assume that the spacecraft does not have any internal power dissipation, the steady state temperature may be expressed by the following equation. 2~ [SAp ot/o-A El 1/4 where T is the absolute temperature of the spacecraft in °K S is the solar constant (mean value 1,353 W/m 2) ~r is the Stefan-Boltzmann constant (56.7 × 109 W/m2K 4) Table I. Chemical Composition and Metallurgical Properties of Magnesium Alloy ZM 21 Chemical composition(% weight) Zinc

Manganese Balance

1.9 0.95 Magnesium

MetallurgicalProperties

Density Tensilestrength Yield strength % Elongation Shear strength

1.77 g/ore3 220 MPa 102-108 MPa 20-30% 90 MPa

© Copyright Elsevier Science Inc.

Ap is the projected surface area of the spacecraft in m 2 A is the total surface area of the spacecraft o~is the absorptance of the surface of projected area is the emittance of the exposed surface to space Because S, Ap, A, and ~ are constants in this relationship, it clearly shows that the temperature of any given area is directly controlled by the e~:E ratio. Here, the term absorptance refers to all solar radiation (X-ray, UV visible, IR, radio frequency, etc.), whereas the term emittance is restricted to the IR range because thermal radiation occurs mainly in the IR region. EXPERIMENTAL

DETAILS

Modified acid fluoride anodizing 5 was investigated on magnesium alloy ZM 21. The experiments were conducted on samples sized 50 × 0 × 5 mm with a central tap 4 × 5 mm deep. The samples were processed according to the following sequences of operations: 1. Solvent degreasing Isopropyl alcohol for 5-10 min 2. Alkaline cleaning Sodium hydroxide (NaOH), 50 g/L Trisodium orthophosphate (Na3PO4:12H20), 10 g/L Temperature, 60 + 5°C Time, 5-10 rain Posttreatment, water rinse 3. Acid pickling Chromium trioxide (Cr03), 180 g/L Ferric nitrate [Fe(NO3)3-9H20],

40 g/L Potassium fluoride (KF), 3.75 g/L Time, 2 rain Posttreatment, water rinse 4. Anodizing Ammonium bifluoride [(NH4)HF2], 240 g/L Sodium dichromate (Na2Cr207), 100 g/L 43

Ortho phosphoric acid (H3PO4) 85%, sp gr 1.71, 90 ml/L Temperature, 60-90°C Agitation, intermittent by Teflon rod Current density, 10-50 A/ft 2 Voltage, 60-110 V (AC) Anodizing time, 30-45 min Posttreatment, water rinse 5. Sealing Sodium silicate, 50-55 g/L Temperature, 93-100°C Time, 15 min

RESULTS AND DISCUSSION Magnesium is categorized as a difficult metal for electrochemical deposition because of its high chemical affinity for aqueous solutions. It reacts severely with atmospheric oxygen and water, resulting in the formation of an oxide-carbonate film on the surface. This oxide film on magnesium is porous and is not self-healing; consequently, it does not offer protection. The presence of this oxide film prevents the formation of metal-to-coat bonding during electrochemical treatments, resulting in a nonadherent deposit. The highly reactive nature of magnesium is clearly indicated by its position in the electromotive series (standard oxidation potential, E ° = +2.37 V). The situation is still more complex for magnesium alloys. The alloy constituents, together with the magnesium matrix, form local cathodic and anodic sites introducing electrochemical heterogeneity. If the cathodic site has low hydrogen overvoltage, the hydrogen desorption is facilitated in these areas, and corrosion current is thus substantially increased. Alloy composition, quality and design of castings, mechanical surface finishing, etc., have to be carefully examined before proceeding to any chemical treatment. The presence of alloy impurity phases and different intermetallic phases, which differ in their chemical activity with different reagents, leads to patchy deposition. If the castings have surface porosity, flaws, or oxide and flux inclusions, the quality of the coating will be seriously affected. If there are deep recesses, narrow cavities, and sharp comers in the design, then uniformity of coating will be very difficult to achieve. Because magnesium alloys are soft, the possibility of inclusion of 44

particles of harder materials--such as silicon carbide, aluminum oxide, diamond, or heavy metals--
Mechanism of Film Formation The anodic coating is formed by a chemical reaction between the magnesium alloy surface and hexavalent chromium. Magnesium is oxidized by the hexavalent chromium, which is reduced to the trivalent state. 6'7 As the concentration of chromium and other electrolyte constituent ions reduces due to the chemical reaction at metalelectrolyte interface, an alternating current supply is required. Alternating current enables replenishment of the reactant concentrations at the metalelectrolyte interface on one electrode, while producing a coating on the other. The coating is composed of hexavalent and trivalent chromium and the substrate metal (chromate, phosphate, hydroxide, and bifluoride). In addition, some traces of sodium silicate due to sealing in silicate solution were also present. The energy dispersive X-ray analysis of the anodic film is shown in Figure 1. The figure clearly indicates the presence of sodium, magnesium, phosphorus, silicon, and chromium. Process Optimization Process optimization has been carried out by investigating the influence of various operating parameters, including variation in electrolyte temper-

,-__m

L

,=,-

D

C m

tin=

&==

~

~

Tm

w

Imy

Figure 1. Energy dispersive X-ray analysis of the anodic coating. ature, applied current density, and postdeposition sealing on the physicooptical properties of the anodic coating. The influence of the operating parameters on the physico-optical properties of the anodic film is presented in Table II.

Effect of Electrolyte Temperature In most of the chemical reactions, the rate of reaction is dependent on the electrolyte temperature. At higher temperatures, the deposition rate is generally higher due primarily to the increase in activation energy and conductivity of the electrolyte,s Figure 2 shows the variation in bath voltage with time at various electrolyte temperatures, 50, 60, 82, and 90°C, at a constant current density of 10 A/ft2. As expected, the terminating bath voltage is higher for higher electrolyte temperatures. Some anomalies, however, were noticed in initial bath voltage. Because the anodic film is an electrical insulator due to a sharp increase in nucleation sites, a heavy fluctuation in initial bath voltage was observed, making it difficult to adjust the constant current. At latter stages, however,

Table II. Influence of Operating Parameterson Physico-OpticalProperties Process Variable

Coating Thickness (pro)

Solar Absorptance

Infrared Emittance

15,40 35.40 40.20 41.20

0.80 0.83 0.84 0.84

0.79 0.86 0,87 0.88

22.80 32.10 48.00 63.35 89.00

0.81 0.84 0.84 0.85 0.86

0.80 0.86 0.88 0.92 0.92

Electrolyte Temperature (°C)a 50 60 82 90

Applied current density (A/fl2)b 10 15 20 25 50 "Current density of 10 Nft2, 45 rain, bElectrolytetemperatureof 82°C, 30 rnin.

METAL FINISHING

• M A R C H 1997

120 /

"

100

g I

110 l

,3 I

8 r

9O

7O

!

,

;, 1

; , , i, 2 ,30 40 ANODIZINGTIME (Hin)

~d 10 20 ANODIZING11ME(MIN.)

Figure 2. Variation in bath voltage with time at various operating temperatures: O, 50°C; +, 60°C; *, 82°C; and [], 90°C. (Current density, 10 A/fl2). when the entire surface of the specimen is covered with an anodic film, the increase in bath voltage becomes somewhat steady. In the experiment at 50°C, a very low voltage raise, from initial to terminating stage, was observed with only a partial covering of the specimen by the anodic film. The low voltage raise was due to the availability of conductive bare alloy surface during the entire course of electrolysis. These results clearly indicate that the optimum electrolyte temperature at a constant current density of 10 A/ft 2 is 60 to 90°C. Because the anodizing electrolyte produces pungent fumes, it is desirable to use the lower electrolyte temperatures with a suitable fume extraction facility. Figure 3 shows the influence of electrolyte temperature on the film growth at a constant current density of 50

I

a~ ~ICKNESS0~) 8 8 8 I [ I

_.-----o 4O

2O

I

50

60

r82

ELECTROLYTE TEMPERATURE

90

i

95

('C)

Figure 3. Inf uence of electrolyte temperature on film growth (current density, 10 Mft2; anodizing time, 45 min). METAL FINISHING

° M A R C H 1997

30

Figure 4. Variation in bath voltage with time at various applied current densities: O, 10; +, 15; *, 20; I~, 25; and X, 50 A/ft2. (Electrolyte temperature, 82°C). 10 A/ft 2 and anodizing time of 45 minutes. As is apparent from the figure, the rate of film formation increased with the electrolyte temperature. The relative increase in film growth, however, was rapid with initial increases in electrolyte temperature, but later fell and became almost constant after 82°C.

Effect of A p p l i e d C u r r e n t Density The next variable studied was the operating current density. The bath voltage-time curves for magnesium anodizing at constant temperature of 82°C at current densities of 10, 15, 20, 25, and 50 A/ft 2 are shown in Figure 4. As expected at higher current densities, the anodizing voltage tends to be higher. Some anomalies, however, were observed in the initial stage, which can be attributed to high fluctuation in bath voltage with initial settings of constant current. Experiments were also conducted with a current density of 5 A/ft 2 at a constant electrolyte temperature of 82°C. Instead of film deposition, a heavy dissolution of substrate in the electrolyte was observed. The influence of applied current density of 10, 15, 20, 25, and 50 A/ft 2 on the film growth at a constant electrolyte temperature of 82°C and anodizing time of 30 minutes is shown in Figure 5. As expected, the rate of film deposition increased with increase in electrolyte temperature. The relative increase in film growth was almost linear up to an applied current density of 20 A/ft 2. It dropped slightly after 20 A/ft 2 and became almost constant after

Figure 5. Influence of applied current density on the film growth (electrolyte temperature, 82°C; anodizing time, 30 min). 50 A/ft 2, where a limiting film thickness is approached after 30 minutes of electrolysis.

Effect of P o s t d e p o s i t i o n Sealing The anodic coatings obtained immediately after anodization are porous and may not be able to provide adequate corrosion protection to Iong4erm exposure. Sealing in sodium silicate solution was carried out to close the pores of the coating. In this process, the anodic film at the pore sites is hydrated. In the course of change, the coating swells and pores are closed. The coating becomes smoother after sealing; however, no change in its optical properties was noticed. Optical Properties In vacuum (space conditions), radiation is the only predominant mode of heat transfer. Consequently, the equilibrium temperature in space is attained soon after exposure to the thermal environment. A passive thermal control system, with known radiation characteristics of its surface, has the advantage of high reliability and no added equipment, moving parts, or external power, as are needed for active elements. The anodizing process can effectively be exploited in space application to minimize temperature gradients across the internal components of the spacecraft. The radiation properties, i.e., solar absorptance and IR emittance, have only little variation with process variables. The influence of anodic film thickness on IR emittance is shown in Figure 6. The IR emittance of the film was found to increase with an increase 45

1.0

0.95

0,5

f

0,9

0.0

~o.2

-O,5

0.85

,

I

~oo

i

I

2OO

,

I

300

~

4OO

i

=fro.6

"~MPE~A~.IRE ('13) 03

Figure 9. Differential scanning calorimetry curve of the anodic film. [

13.7

I

26.8

I

30

I

40

r

433

I

85

93

Figure 6, Influence of film thickness on the infrared emittance. in film thickness. Similar behavior was observed with solar absorptance, but the variation was of lesser magnitude. In a wide range of experimental conditions, the solar absorptance was found to vary only within 0.80 to 0.86, whereas IR emittance Was found to vary between 0.79 to 0.92. The increase in the IR emittance was due to an increase in surface roughness and spalling deposition at higher film thicknesses.

Morphological Studies The microstructure of the anodic coating was examined with a scanning electron microscope. Coatings of this type are usually considered soft and powdery. The scanning electron micrographs of anodic coating are presented in Figure 7. The coatings have a rough and granular texture. At higher coating thickness ( > 50 pm), due to higher terminating bath voltages, sparking was observed. This results into very rough, spalled, and fibrous deposits.

Thermoanalytical Studies Thermoanalytical studies thermogravimetry (TG), derivative thermogravimetry (DTG), and differential scanning calorimetry (DSC)--were investigated. The TG and DTG curves were recorded in the 25 to 800°C temperature range with a sample size of 22.671 mg and heating rate of 10°C/ min; however, due to a limited temperature range the DSC was 25 to 500°C, with a sample size of 30.322 mg and a heating rate of 5°C/min. Figure 8 shows TG and DTG curves, and Figure 9, DSC data for the anodic coatings. The thermograms re46

Figure 7. Scanning electron micrographs of the anodic coating (A) 8,000×, (B) 1,000 ×. veal three major changes that occur when particles of coating were heated: (1) dehydration, (2) dehydration along with decomposition of fluoride and hydroxide, and (3) decomposition of phosphate. Dehydration of the coating starts a 27°C, slows down at about 170°C, but continues until 240°C. The corresponding DTG peaks for dehydration are obtained around 55 and 260°C. The first-stage decomposition represents the dehydration, i.e., the loss of water of hydration from the coating. The second-stage decomposition represents the decomposition of chromium bifluoride and hydroxide, along with the loss of remaining hydration water. The total weight loss in the first two-stage decomposition is 3.4%. The expected 102

0.03

100

O.O2

i"

0,01 i

96

0,00 o

940

,

I

200

i

I

400

i

I 600

I

800

.o.01

"V~.MPERATU~('C)

Figure 8. Thermogravimetry and derivative thermogravimetry curves of the anodic coating.

endothermic behavior during dehydration/decomposition of chromium bifluoride and hydroxide was observed from the DSC curve. The decomposition of chromium/ magnesium phosphate starts at about 542°C and continues beyond 785°C. The total weight loss up to 785°C was 3.4 plus 4.2, for a total of 7.6%. Due to the limited operating temperature range, no DSC traces corresponding to this deposition step were obtained.

TESTING AND EVALUATION For process optimization, the coating properties were evaluated by visual inspection, adhesion test, thickness measurement, morphological studies, and measurement of optical properties. To evaluate the performance of the anodic film in prelaunch and postlaunch environments, test coupons anodized at optimum conditions were subjected to humidity, baking, thermal cycling, and thermovacuum performance tests. Humidity and corrosion resistance tests were conducted to examine the resistance of the anodic film to corrosive prelaunch and postlaunch conditions. The thermal cycling, thermovacuum performance, and baking tests were designed to evaluate the effect of on-orbit cycling temperature of the spacecraft on the physico-optical properties of anodic film.

Visual Inspection A l l the anodized specimens were visually examined for any defects. The

coatings were perfectly uniform, and no defects such as discontinuity, patches, or powdery deposition were observed. After visual inspection, the anodized specimens were subjected to evaluation tests and measurements. METAL FINISHING • MARCH 1997

Adhesion Test Adhesion was evaluated by a Scotch tape peel-off test. Masking tape of 25-mm width type 3M500 (pressure 200 g/cm:) was applied over the film by passing a 2-kg rubber roller over the tape twice. The tape was then quickly removed in a direction normal to the surface, and the test specimens were examined visually for any coating removal. No detachment of the film from the substrate was observed. This test was also conducted after humidity, corrosion resistance, thermal cycling, and thermovacuum performance tests. The test results showed excellent adhesion of the coating to the substrate.

recommended for thermal control application. Though the higher coating thickness ( > 50 pm) provides better IR emittance ( > 0.88 pm), these are not recommended because of large dimensional change and risk of job damage. At high coating thickness, due to high electrical resistance, a very high bath voltage was drawn. This has resulted in sparking at the coating surface. Consequently, there was a temperature raise at the coating-electrolyte interface, resulting in a higher rate of coating dissolution in the electrolyte. The overall impact of high coating thickness was deposition of rough, spalled, and powdery deposits.

Thickness Measurement The thickness of the coating was measured using an instrument that works on the eddy current principle and is used to measure the thickness of nonconductive coatings on conductive substrates within + 1 gm. An optimum coating thickness of 45 +- 5 pm is

Humidity Test This test is designed to check the effect of humidity and high temperature, which in turn shows the resistance of the coatings to corrosive atmosphere. The test was conducted in a thermostatically controlled humidity chamber. The relative humidity in the

chamber was maintained at 95 + 0.5% at 50 + I°C. After the test, specimens were visually examined, and their optical properties, such as [R emittance and solar absorptance, were measured. No changes in optical properties and no degradation (discoloration, corrosion spots, patches, or peeling) were observed. Baking Test This test is designed to evaluate the effect of continuous high temperature on. the appearance and optical properties of the coating. Anodic coating on magnesium alloy ZM 21 is proposed to be used for high-energy-dissipation components of spacecraft where temperature may exceed 200°C for an extended period. The test was conducted in an oven with the facility of circulating clean hot air at a temperature uniformity of + 2°C. The test was conducted at 300°C for 48 hours. No degradation in physical appearance and no change in optical properties of

AbsoluteAccuracy: You Can Put Your Hands On The Elcometer 355 is a small but powerful choice for the modern metal professional.The memory allows you to store batch calibrations and statistics for easy output to PC or printer. The 355 is also designed to accommodate a wide array of optional probes to measure coating thickness on various ferrous and non-ferrous metals.

For details or a FREEbrochure,call:

1-0635

eloometec@ 1893 RochesterIndustrial Dr. RochesterHills,M148309

Circle 027 on readerinformationcard 48

Circle 016 on readerinformationcard METAL FINISHING • MARCH 1997

the coating were observed after the test. Corrosion

Resistance

Anodized magnesium alloy ZM 21 coupons were immersed in 5% sodium chloride solution, whose pH was adjusted to 7.0. The time taken for discoloration or formation of corrosion products on the coupons was carefully observed. No discoloration of coating or formation of significant corrosion spots were observed when the coatings were examined after 48 hours of immersion. This indicates adequate corrosion resistance for general and space applications. Thermal

Cycling Test

This test is designed to evaluate the effect of cycling temperatures, which are likely to be encountered throughout the life span of spacecraft, on the physico-optical properties of the coatings. The test was conducted in a thermostatically controlled chamber. A to-

tal of 1,000 cycles was applied. A cycle consists of lowering the temperature to -50°C, a dwell of 5 minutes, and raising the temperature to 200°C with a dwell of 5 minutes. The first six cycles were of 1 hour's duration each (thermal soak test), instead of 5 minutes. After thermal cycling, the test specimens were inspected visually, and their optical properties were measured. No degradation was observed. Thermovacuum Test

Performance

This test is designed to examine the effect of cycling temperature o n the coating in the space environment (i.e., in vacuum). The test was conducted in as thermostatically controlled vacuum chamber. The test consists of lowering the temperature to - 5 0 ° C with a dwell of 2 hours, and increasing the temperature to 200°C with a dwell of 2 hours. Ten cycles of hot and cold soak were applied, and a vacuum level below 10 .5 torr was maintained inside the

chamber during the test. No sign of any degradation on the coating was noticed after the test. Outgassing

Evaluation

Some materials and coatings outgas in the space environment. The outgassing matter escapes, forming a cloud of charged molecules in the vicinity of spacecraft due to the impact of particulate radiation. Some of the outgassed molecules may recondense on the surface at low temperatures. Although the weight loss could render a material unsuitable for flightworthy applications, it is the condensable material that is of much concern. In orbit, contamination of the spacecraft due to condensation of outgassing species may result in a change in optical and thermal properties of the surface or in an electrical opening between control relays. The ionizable molecules can cause corona and arcing phenomena; hence, it is imperative to use only those materials and coatings that have

COSIr.ElqmCIIVE

FAST

Maxi-Blast Plastic Media & Blast Systems • For Surface Cleaning • For Paint & Coating Removal • Non-abrasive, Non-damaging. • No toxic chemicals-safer to environment and employees • Free of metallic contamination • Pretreated with anti-stat • ISO-9002 certified • MiI-P-85891 approved

We have over 100 media types and sizes to fit your needs. Free parts testing available to determine what media and system will work best for you. Maxi-Blast will analyze your process and parts and recommend a solution. Maxi-Blast Plastic Blast Systems are made to customer requirements. Call today.

Maxi.Biast

;.c.

8 0 0 - 5 3 5 - 3 8 7 4 • 2 1 9 - 2 3 3 - 1 1 6 1 • Fax: 2 1 9 - 2 3 4 - 0 7 9 2 6 3 0 E. Bronson Street • South Bend, IN 4 6 6 0 1 http-J/www.maxiblast.com

l Ox Maxi Blast Multi Blaste M e d i a

Circle 097 on reader information card

METAL FINISHING

•

MARCH 1997

49

'lil l l l l lil l l IIIIII]1

1215 Karl

!

1

~ ~

~11~i~Li ~i LI~L~[uu~1~1~1~q~q~j~LL~LLtt[HH~11111~111~1~1q~l~~i ~i ~i ~i L~LL~LHtH[Ht~m~11u~111~1111111111111~1

Court Unit203 Wau¢onda, IIIi~is

CUSTOM

VENTILATION

& AIR POLLUTION

26-6278/Fax CONTROL

SYSTEMS

ScrubAir Chrome Separators m e e t o r surpass ,015 mg/dscm - GUARANTEED!

Circle 075 on readerinformationcard

Custom automated system for surface treatment

EhrsamTM Blast Systems offers automated equipment for cleaning, etching, deburring, peening and

Custom designed other surface preparation. Custom designed to automated suit customer requirements, these systems range from simple semi-automatic units to sophisticated fully automatic

blasting systems robotic systems. Units are available for suction / TM or pressure operations and use a full range of by Ehl " s a blasting media. Call, write or fax for a fast response to your surface treatment needs or for original parts and service. ABB Air Preheater, Inc. Ehrsam T M Blast Systems P.O. Box 339, 2101 N. Buckeye, Abilene, KS 67410 USA Toll Free: 800-255-7910 • Fax: 913-263-3932

LIL lIP I | P IIPIIP @1998ABBAirPreheater,lnc,

1352-8618

low outgassing. For a material to be considered for flight usage, the maximum total mass loss (TML) and collected volatile condensable material (CVCM) percentage a r e <1 and <0.1%, respectively. A standard outgassing test as per ASTM E $95-83 was performed. The test specimen and the boat (container) were conditioned at 23°C and 50% relative humidity for 24 hours. The boat and test specimens were then weighed and placed in the specimen compartment over a copper heating bar. A preweighed collector plate was also placed inside the chamber and located directly opposite the specimen compartment. The test chamber was then closed and evacuated to a vacuum of 10 -5 tory or better. The temperature was raised to 125°C while the collector plate was maintained at 23°C. This caused the specimen to outgas. After 24 hours, the test chamber was allowed to cool and attain normal pressure. The specimen and the collector plate were then weighed and CVCM values were obtained. As the anodic film is inorganic, its outgassing value is very low. Hydrated water and some negligible trace of gasses and other volatile molecules ar the only materials that can outgas. The TML and CVCM values obtained of anodized specimens were only 0.593 and 0.001%, respectively. These coatings are, thus, suitable for use even in critical areas of the spacecraft.

Measurement of Optical Properties The optical properties, namely, solar reflectance and Ill emittance of the coating, were measured using a solar reflectometer emissometer. Both of these instruments provide an average value of solar absorptance and Ill emittance digitally over the entire solar or IR region: Because the anodization of magnesium alloy ZM 21 was investigated for thermal control application, the optical properties, such as solar absorptance and 1R emittance, of all the anodized specimens were carefully measured. To evaluate the environmental stability of the anodic coating, the measurements of optical propeities were carried out before and after each of the environment tests including humidity, corrosion, resistance, baking, thermal

Circle 001 on readerinformationcard 50

METAL FINISHING

• M A R C H 1997

Reliability, innovation and basic American ingenuity have made Kocour Thickness Testers the choice o f progressive platers f o r over 50 years. So, doesn't it make sense to rely on either a Model 6000 with micro-processor based circuitry providing access to over 300 coating/ substrate applications or the compact Series 10 Handheld unit available in both basic and enhanced models.

cycling, and thermovacuum. No change in o p t i c a l properties was observed. This clearly indicates excellent environment stability of the coating to the space environment.

The Model6000 features:• Versatility- measures anythingfrom a #2 screwto the largestcoil. • Multi-layercoatingsand alloylayers• WireCapability• Digitalcalibration,98% Accuracy• Easyto Read Direct Readout • STEP• Print Capability• RS232C Interface to PC • Proven Kocour Quality, Reliabilityand Value

CONCLUSIONS 1. The anodic coating on the indigenous magnesium alloy Z M 21 was obtained under the following optimum electrolyte composition and operating conditions: A m m o n i u m bifluoride, 240 g/L Sodium dichromate, 100 g/L Ortho phosphoric acid (85%, sp gr 1.71), 90 ml/L Temperature, 60-82°C Current density 10 A/ft 2 Time, 45 -+ 5 rain Coating thickness, 45 -+ 5 pm 2. The process yielded a satisfactory coating over a wide range of electrolyte temperatures (60-90°C) and applied current densities (10-50 A/ft2). 3. The bath voltage was found to increase with increase in applied current density for a constant value of electrolyte temperature, and to decrease with decreasing electrolyte temperature for a constant value of applied current density. 4. The rate of film deposition was found to increase with increase in electrolyte temperature and applied current density. The relative increase in film growth, however, was rapid at initial state and dropped slowly, approaching an almost constant value at electrolyte temperatures above 82°C and applied current densities above 50 A/ft 2. 5. The optical properties were found to be dependent on anodic film thickness. For the same film thickness, no significant difference in optical properties was observed. 6. The anodic coatings described herein are highly stable. The humidity, baking at high temperature, thermal cycling, and thermovacuum tests have no effect on the physico-optical properties of the coatings. This show high stability in adverse conditions. 7. As the anodic coating provides high solar absorptance (0.84) and IR emittance (0.88), low total mass loss and collected volatile condensable material, and can withstand high temperature for an extended period, it is suitMETAL FINISHING

° M A R C H 1997

Model 6000

Kocour's Thickness Testers

MEASUREUP Series 10 has 12 models.Choosethe one just right for your application.Theyare all lightweight,

durable and feature easy-to-readLCDdisp[ays.And,all ar and manufacturedwith Kocour'stotal commitmentto qua Basic modelsmeasure non-magneticcoatingson steel,or non-conductivecoatingson non-ferrous metals. Anythingfrom platingto paint, or anodizing,galvanizing powder coatings.Enhancedmodelsfeaturethese additio[ capabilities:• Printer Capability/ RS232 Interface • MultipleCalibrationMemoriesand Batch Statistics• Instantaneously DisplaysFullMeasurementStatistics

4800 South St. Louis Avenue, Chicago, Illinois 60632 U.S.A. Telephone: (773) 847-1111 FAX: (773) 847-3399 http://www, kocournet.com

Circle 041 on reader information

able for thermal control applications in a space environment.

References 1. Sharnaa, A.A., Metal Finishing, 91:57; 1993 2. Sharma, A.K. et al., Transactions of

the Society of Automotive Engineers, 3. 4. • 5.

Paper no. 932121; 1993 Sharma, A.K. et al., Journal of Spacecraft Technology, 1(2):35; 1992 Agarwal, B.N., Design of Geosynchlvnous Space Craft, Prentice Hall, N.J.; 1986, p. 281 Durney, L.J., ed., Electroplating Engi-

card

neering Hand Book, Van Nostrand Reinhold, New York; 1984, p. 410 6. Sharma, A.K., Thin Solid Films, 208: 48; 1992 7. Sharma, A.K. et al., Journal of Applied Electrochemistry, 23:500; 1993 8. Sharma, A.K., Metal Finishing, 87:73; 1989 9. Frankel, H.E., "Effect of Vacuum on Materials," Technical Note ESRO, TN-77; January 1969 10. Danphin, J. and A. Zwall, "The Outgassing of Space Materials and Its Measurement," Technical Note ESRO, TN-124 (ESTEC); February 1975 M F 51