BRUSH AND FLOW SELECTIVE SULFURIC ACID ANODIZING by Joseph C. Norrls SIFCO Selactlve Platlng, Dlvlslon of SIFCO Industries g p 8 ~ 4 4 1 3 1
P 04213 27030 .pbF
- s
Abstract
The brush and flow selective sulfuric acid anodizing processes are reviewed. The areas covered include: the equipment, supplies, and procedures used; the effect of operating conditions on the process; and the properties of the coatings. Examptes of applications are given.
2.
Formationof A1203 at the aluminum suriace.
3.
2.
They do not necessarily require the disassembly of a unit.
3.
They reduce the amount of masking
4.
They permit anodWng of pans too large for existingtanks.
5.
They reduce downtimeand production delays.
Anodiiing is the formation of an oxide film on aluminum using reverse current (part is positive) and a suitable electrolyte. As the coating is formed, three processes occur simultaneously. They are: Consumptionof aluminum to form A$03
They are portable.
required.
Introduction
1.
I.
This paper will deal with brush and flow selective sulfuric acid anodiiing. Brush Anodlzlng Brush anodiied coatings are applied using a handheld tool. The tools are often identical to those used in brush electroplating. Table 1 Four Types of AnodWng
Dissolution of some of the A1203 by the anodiiingsolution.
Type of anodizinq
The anodiiing process, therefore, is more complex than the single build-up process that occurs in electroplating. There are four principaltypes of anodiiing: they are phosphoric, chromic, sulfuric, and hard coat. The four types of anodizing differ markedly in the typical thickness of the coatings and in the purposes of the coatings. See Table 1.
M o s t anodizing is done by immersing the part in a tank of solution. m e anodizing, however, may be done using brush or flow techniques similar to those used in selective plating operations. The brush and flow techniques have a number of advantages Over tank anodizing. Some of them are:
Typical thickness
Purpose of
coatinq
Phosphoric
0.25 (0.000010)
Improve strength of adhesive bonds
Chromic
25 (0.0001)
Corrosion pmtection
sulfuric
12.5 (0.0005)
Corrosion protectionwith some wear resistance
Hard Coat
50 (0.002)
Wear resistance and often corrosion protection
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The conversion to zinc phosphate plus dry film lubricant resulted in some added benefits. The processing time was reduced by elimination of the separate hydrogen emhrittlement relief step after the dry film lubricant is adequate for the hydrogen embrittlement relief of the coated steel. This result was confirmed using sustained load tests per the MIL-STD 1312/5. 4.1.1 Torque Method. The coating is field repairable using an air drying dry film lubricant. Also the galvanic harrier of epoxy primer is no longer necessary. SUMMARY:
.. ............................
:
Successfull replacment of cadmium plating on parts which were being plated in-house has heen acheived. Outside vendors have obtained some benefit for parts of these types which they make. The next step in the process will be to eliminate cadmium plating from parts which are made only by outside vendors.
ii
Figure 2: Dry film lubricant without zinc phosphate pretreatment. Neutral salt spray (ASTM B117) for 96 hours.
REFERENCES:
1. Cook A. R., Proceedings from the workshop "Alternatives for Cadmium Electroplating in Metal Finishing", 1977. 241-57. 2. Lucas, J. M. and Hague, J. M.. Proceedings h m the workshop "Alternatives for Cadmium Electroplating in Metal Finishing", 1977,465-78.
3. Federal Specification QQ-P-416F. "Plating, Cadmium (Electrodeposited)", October 1, 1991.
4. Lowenheim F. A,, "Electroplating", McGrawHill, Inc.. New York, 1977, 182-7.
5. Zaki, N. and Budman, E., Produns Finishing, 56 (1). 1991,46-51. 6. Lynn, J. C., Warke, W. R., and Gordon, P., Marerials Science and Engineering. Is, 1975, 5162. 7. Military Specification MIL-S-5002D. "Surface Treatments and Inorganic Coatings for Metal Surfaces of Weapons Systems", November 30, Figure 3: Dry film lubricant with zinc phosphate pretreatment. Neutral salt spray (ASTM B117) for 96 hours.
1989.
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Figure 1 shows a drawing of a typical twl that can be used for either brush electroplating or sulfuric acid anodizing. Figure 2 shows the tool being used to process test panels. m e p z l f & w b i complete the DC circuit necessary for anodizing. As soon as the tool touches the part, the operator wiii move the tool on the pan or ‘brush’ the surface being anodized.
per minute) is employed in flow anodizing. m e exchange rate is the volume of solution that flows between the anode and the cathode per minute divided by the volume between the +lwxhwMm-. anodizing flow -rate i the anodeto-cathode gap volume is 6 cm3. the exchange rate is 15.OOO divided by 6. or 2,509 times per minute.
Flow Anodldna
In flow anodizing. the anodizing solution is pumped rapidly through a gap between the part and the cathode. Figure 3 shows a sketch of a simple fxture to do an inside diameter. Brush Versus Flow Anodldng
The same sulfuric acid anodiiing solution is used with the brush and flow methods. Both methods are equally effective. that is, the same operating conditions may be used, and the coatings are identical when using a certain set ofoperating conditions. Thii is provided that asufficient exchange rate (about 2500timas
Fig. 2 Brush anodizing.
handle
G r O P h t F electrode
tive Polority LSolution Inlet I
I
Fig. 3 Flow Anodizing Fixture for sulfuric acid anodiiing.
Fig.1 Drawingof brush anodizingtool. 2
30
There are a number of factors that enter into determining which method is the best for an application. Some of them are given below.
Alloy Being Anodized
Factor
are:
Type of application
Complexity ofpart andor accessibility of area requiring anodiiing
Necessity for a masking fOaure Number of
Parts
Each aluminum alloy responds differently
Comments
I. Certain aluminum alloys bum more easily than others; these require lower current densities. 2. me factor (coating efficiency) varies slightly depending on the alloy.
B N S ~anodiiing is usually more convenient for touch up of anodiied coatings since they are usually one of a kind and the areasis small and accessible.
Table 2 shows how the aluminum alloy affects the anodiiing process'
ENSh,anodibing generally is more convenient for a simple part and an area that can be easily masked around with tape. An example is a bore in a plate. Flow anodizing is generally more convenient when it is difficult to mask around the area, such as when there are complex surfam nearby, and when the surface to be anodiied is not very accessible.
Table 2 Effect of Alumlnum Alloy on Sulfuric Add Anodldng Aluminum Current Density' FactoP A/cm* (Mn.2) Alloy
Max
Avg
- - 2024-T3
For Max Conosion Protection
0.16
0.08
(1.0)
(03)
0.03 (0.2)
0.49 (0.008)
0.19 (25) (1.25)
0.03 (0.2)
0.43 (0.007)
A masking focture may not be required for brush anodizing. Flow anodizing requiresthat a chamber be built up around the area to be anodiied.
3003-HI 0.39
5052-H32 0.78 (5.0)
0.39 (25)
0.03
0.43
(0.2)
(0.007)
Flow anodizing is generally preferred for a large number of parts since masking can be rapidly performed using masking factures, and since flow plating Is less labor intensive.
6061-T6
0.47 (3.0)
0.23 (1.5)
0.03
0.43
(0.2)
(0.007)
0.39 (2.5)
0.19 (1.25)
0.03 (0.2)
0.43
0.16 (1.0)
0.03
0.49 (0.008)
7075T6
A-356T6 0.31 (20)
Important Sulfurlc Acid Anodldng Operatlng Condltlons
(0.2)
(0.007)
*Coatings will bum above the maximum current density. Use the average current density when anodizing for wear resistance or in dimensional restoration. Use a current density of 0.03 Ncmz (0.2 A/in.*) when anodizing for corrosion protection.
Operating condiions that affect the anodizing process and the thickness, structure, and properties of the coating are: 1. Alloy being anodized. 2. Temperature. 3. Current Density.
**The mAh for 1 pm on 1 cm2 (Ah for 0.0001 in. on 1 in..) at 0.03 Ncm2 (0.2 Mn.2). 3
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Temperature
2.
Repairing a damaged anodized coating to restore corrosion protection
The proper temperature range for sulfuric acid anodizing is 18 to 30 OC (65 to 85 OF). In
3.
Selectlve
anodizing
an
area
for
bore that has been machined oversize and anodizing will be used to get it back to sue. In this type of application, the maximum thickness of coating that can be applied depends on the alloy. The thickest coatings that have been applied to date on various alloys is as follows:
75 OF) is preferred. In corrosion protection applications, 24 to 30 “C (75 to 85 OF) is preferred. Heat is developed during the anodizing process. The amount of heat developed is dependent upon the thickness of the anodized coating and the size of the area to be anodized. The higher the thickness and the larger the area, the more the solution will be heated. Excessive overheating of the solution is prevented by:
2024-T3- 94pm (0.0037in.) 6061-T6 - 67pm (0.0027in.)
7075-T6- 97pm (0.0038in.) 1.
2.
Starting out with an adequate amount of solution.
A356-T6- 94pm (0.0037in.)
Cooling the solution, such as by running it through cooling coils or using an immersion cooler.
Equipment and material requirements will vary depending upon the type of application. Equlpment and Materials
Current Density Power Packs Sulfuric acid anodizing is best controlled by current density. When corrosion protection is desired, a current density of 0.03 Ncmz (0.2 amp/in.Z) is recommended. Higher current densities are used in wear resiaance or dimensional restoration applications. There is a current density above which burning of the coating will result even with an excellent set-up. This is called the maximum current density. percent of the maximum current density is called the average current density: it is recommendedthat this be used as a working current density in wear resistance and See dimensional reaoration applications. Table 2 for more information.
A thilty volt dc output rectifier is adequate for applications 1 and 2 listed above. A voltage output of 45 volts, however, is required for application 3 listed above, since higher current densities are used and higher thicknesses of coating are applied. A special power pack particulariy suited for sulfuric acid anoduing has been developed. The power pack is intendedfor anodizing areas up to approximately 322 cW (So in.? in corrosion resistance applications and 39 cm* (6 in.-.) in wear resistance or dimensional applications. Specificationson it are as follows:
Fm
Appllcatlons for Selectlve Anodldng
Max Voltage Under Load Max Amperage Output Resolution of Meters Ammeter Ah Meter
The three primary applications for brush and/or flow sulfuric acid anodizing are: 1.
Selective anodizing, for corrosion and/or wear resistance purposes, an area on a part that has not been anodized before.
45 10
0.01 0.001
A special feature of the power pack is the high resolution of the meters. The resolutionof the ammeter allows current density to be 4
32
controlled adequateiy for areas down to 1.6 cmz (0.25in2).
are often modlfied to allow pumping solution through them, particularly when the hardest, most wear resistant coatings must be obtained.
hours. The number of Ah required for a job is precalculatedby multiptyingthree numbers: the factor for the alloy to be anodized, the area to be anodized in cnV (in.*) and the thickness of the coating in terms of pm (0.0001 in.). This number of Ah, as measured on the power pack, is then passed while anodiiing. The resolution of the Ah meter on an 1.6 cW (0.25 in.? area is sufficient to control thickness to within -ell%
When special tools are made, the cathode material may be graphite, stainless steel, or platinumclad niobium. Standard or special tools should cover all, or as much as possible. of the area to be anodized. This is particularly important when the hardest, most wear resistant coating must be applied. Also, the coatings are applied as fast as possible when the tool covers the entire area Less than full coverage increases the required coating time and the amount of solution degradation of the coating. For example, if the tool covers only 25% of the area, the required coating time and the amount of degradation of the coating increases by a factor of four.
A second feature of the power pack is the voltage and current regulation controls. This allows the power pack to automatically assure that a preselected current, (current density), and voltage are not exceeded. This eliminates the needto make repeatedvoltage adjustments during an anodizing operation.
The anodiiing tool covers should be polyester, that is, Dacron@ batting, Dacr n tubegauze, Dacron felt, or white Scotch-Brite%I Cotton materialsshould not be used.
ImmersionProbe Solution C‘doler
.
An immersion probe solution cooler has been developed to maintain the anodiiing solution in the proper temperature range in production applications or when doing large areas. The unit has a refrigeration capacity of 941 watts (3200 BTUMR) at 6 OC (42 OF). Therefore, it has ample cooling capability for any anodizing job that can be done with the previously mentionedspecial power pack.
Pumps Standard, commercially available plating solution pumps are suitable for use in sulfuric acid anodring provided that they are large enough for the size of the area to be anodiied. General recommendations are as follows:
The solution cooler consists of a refrigeration cabinet, a stainless steel immersion cooling probe, a stainless steel immeniontemperature sensor, and a PVC tank The to contain the anodiiing solution. refrigeration cabinet includes an analog temperature control calibrated in OC. Solution temperature can be maintained to within k0.5 OC of the deslred temperature.
Size of Area to be Anodized cW (in.2) Up to 6.5 (Up to 1) 6.5 to 65 (1 to IO) 65 to 196 (10 to 30)
Solution S U D DMethod ~ Dip for solution. Pump not necessary. Pump using small submersible pump. Pump using large submersiblepump.
Brush Anodizing Tools Solutions Standard, commercially available brush plating tools may be used for anodizing. The need for solution-fed tools to maintain proper temperature, however, is greater in anodiiing as compared to electroplating; this should be considered in selecting tools. Standard tools
Two solutions used in all applications are: 1. Electrocleaningsolution. 2. Sulfuric acid anodizing solution.
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33
--
The electrocleaning solution is used to electroclean aluminum surfaces prior to masking and then again just prior to anodizing. The operation is carried out at 10 to 15 volts, . -ne subsequent rinse water does not break on the surface.
The protective coating strip solution is used to remove a film that has formed on the aluminum while using the anodize strip solution. The film protects the aluminum from
e
m
P
=
P
e
”
.
It is removed by applying the protective coating
strip solution on the surface using a Dacron pad and no current. Some gassing occurs on the surface as the film is being removed. The operation is continued until the gassing stops and there is a change in color on the surface. These changes indicate the film has Men removed.
The same sulfuric acid anodizing solution is used for all types of anodizing applications and when using either the brush or flow method.
Some other solutions that may or may not be required are:
Two room temperature seal solutions have been developed for use after anodizing. The use of either one will allow passing the salt spray requirements (336 hr) of MIL-A-8625E. The first solution provides for a greater tolerance for errors in processing while anodiiing. rinsing, and sealing. However, it imparts a light yellow color to the coating which might be undesirable for appearance reasons. When the light yellow color Cannot be tolerated, the other solution should be used.
1. Anodize strip silution. 2 Protective coating strip solution. 3. Seal solution.
4. Dye solution.
The anodie strip solution is used to remove thin, partially damaged anodized coating immediately adjacent to the area where wear or physical damage has penetratedto the base material. If the thin coating is not removed, the thin area will not pass current, and it will remain unchanged while anodzing. The end result would be a full thickness of repair coating in the area where penetration occurred into the base material, and a thin original coating around it This type of repair is generally not attractive and is suspect as far as wear resistanceand corrosion protection.
me seal solutions can be used by one of three methods: by dipping the pan in the solution, by making a dam and then pouring the solution in the dam, or by swabbing. When swabbing, use polyester covers and keep the surface wet with solution. Sealing conditions are as follows: Seal Solution
The stripping is done after solvent cleaning the repair and adjoining areas, and after masking for anodizing. Care is exercised in masking to assure sharply defined edges will be maintained throughout the masking, stripping, and anodiing process
Temp.% (OF)
Time
-
1
15.5 to 18 (60tO 75)
2 min
2
30 to 32 (85 to 90)
10 min
Dyeing occasionally must be done for optical reflectivity reasons or to color match an existing dyed coating. Brushan black, red, blue, etc. organic dyes are available for this purpose. A black, inorganic, electrolytic dye has also been developed for applications such as aerospace hardware.
The solution is applied to the area to be stripped using a Dacron pad. Rubbingthe pad over the area is necessary for stripping to take place. No current is used in the operation. Stripping is continued until there is an obvious change in color that indicates stripping has been completed. 6
34
Testing of Coatlngs
-
material and room temperature combinationstested were:
A test program was set up by an aircraft if a brush sulfuric acid anodized coating would crack after a bearing was press fn into the bore. The part was cast A 3 5 6 and it was not a fatigue sensitive part. The bores were 2.29 cm (0.900 in.) diameters, 0.86 cm (0.337 in.) long. Details on brush sulfuric acid anodiiing the part are given in Table 3. lt should be noted how quickly the anodizing operationwas carried out.
1. 2 3. 4. 5.
Corrosion resistance tests were also run on simulated spot repairs. Four panels were tank sulfuric acid anodized and sodium dichromate sealed; two panels wen? 3003-Hl4 alloy and two were 7075-T6 alloy. An area was sanded into the base material, exposing approximately 1.3 cm2 (0.2 in.3 of aluminum on each panel. In another area on each panel, the tank anodizing was stripped exposing aluminum on a 4.5 cm2 (0.7 in..) area The panels were then masked exposing only the two simulated spot repair areas on each panel. The panels were then brush sulfuric acid anodized to a 12.7 pm (0.0005 in.) thickness at 0.03 Alcmi: (0.2 Alin?). The panels were then unmasked and sealed with one of two seals. The combnation of base material and the seals used were as follows:
Part Thickness of Coatina Anodbing Time (in.) Minutes pm
-
1 2 3 4
(0.001) (0.001) (0.002) (0.0018)
15 15 50 46
4.5 4.5 9
9
After anodizing, the parts were SUpr Penetrant inspected. No cracks were found in the coatings. Four bushings were then manufactured out of 155 stainless steel to represent bearings. The bushings were then pressed into the anodized holes. The intetferencefns varied from 43 to 53 pm (0.001 7 to 0.0021 in.). The bushings were removed immediately on two parts: the two parts passed a subsequent Super Penetrant inspection. A third part was cut with the bushing in for metallographic inspection. There was no evidence of cracking in the coating. The coating was intact and coating was present all around the internal diameter. The fourth part was salt spray tested and it passed the test. Based on these results, it was recommended that the process be approved as a method for salvaging undersize holes and other features on parts not sensitive to fatigue.
2024, Seal 1 2024. Seal 2 6061. Seal 1 7075, Seal 1 7075, Seal 2
There was no evidence of corrosion on any of the above panels.
Table 3 Data on Sulfurlc Acld Anodlrlng Test Parts
- -
seal
1. 2 3. 4.
3003, Seal 1 3003, Seal 2 7075, Seal 1 7075, Seal 2
The panels were then salt spray tested per ASTM 8117 for 336 hr. No corrosion was noted on any of the panels. Selectlve Anodlzlng Applicatlons Selective Anodizing a Part Figure 4 shows an aluminum piston head that goes into a marine, gas fueled, combustion engine. A sulfuric acid anodized coating 7.5 pm (O.OOO3 in.) thick is desired on the wrist pin bore for improved wear resistance. The surface is very selective, that is, no coating is desired on other areas and none is permitted in the piston ring areas. The area is also relatively
Corrosion ResistanceTesting Corrosion resistance tests per ASTM B117 for 336 hr have been run on 7.6 cm (3 in.) by 7.6 cm (3in.) flat areas after brush sulfuric acid anodizing 12.5 pm (0.0005 in.) thick. The base
7
35
Fig. 4 Aluminum Piston Head Fig. 5 Brush Anodizing a Main Gear Box
Support Fitting
inaccessible. These factors make this a good selective anodizing application. An investigation is currently under way to determine whether the brush or flow method is the best way to proceed. The production rate will be approximately 300 pieces per hour.
Dimensional Restoration
Sulfuric Acid Anodized Coating Repair
Figure 6 shows an A 3 5 6 alloy generator frame with numerous, close tolerance inside diameters. A 5.199 to 5.202 cm (2.047 to 2048 in.) bore that is the second bore from the back,
Figure 5 shows a Main Gear Box Support Fining that is present on a CH-124 Sea King Helicopter used by the Maritime Command of the Canadian Forces. A bore in the part is being brush suhric acid anodiied during a training session. This bore must be frequently repaired on the aircra when damage occurs while installing or removing barrel nuts. Since the aircraft spends 90% of its flight time over salt water, it operates in a corrosive environment and the coating must be immediately repaired. When the area being repaired is less than 25% of the'total area, the preferred repair method is brush anodiiing. When the repair area is more than 25% of the total area, the entire bore is stripped and reanodiied. In this case, the flow method is more practical since access to the bore is very limited. It takes approximately 4 hours to clean, mask, anodiie. and unmask. The attemath is to replace the part, which costs approximately $5,000, and requires a replacement time of 150 hours.
Fig. 6 Generator Frame
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36
I
II I
I
I I
is periodically mismachined. Selective anodizing has proven to be a very successful method of repairing the bore. The bore is anodized to the middl1 subsequent machining or grinding operation is required. The complexity of the part made R unlikely that it could be successfully masked wRh tape and paint. The area was also relatively inaccessible. For these reasons the flow approach was selected for anodizing. Figure 7 shows the flow fxtures used. The rubber stopper shown at the bottom sealed off an adjacent smaller ID. A PVC cup wRh a rubber seal cemented to it (shown at the top) seals off an end face on the other side of the ID. The part Is oriented vertically and anodiiing solution enters the chamber from the bottom and exits through holes in the OD of the graphte electrode.
Fig. 8 Anodizing Generator Frame Figure 9 shows a 7075T6 helicopter control link, that is not fatigue sensitive, requiring a build up on an inside diameter. Also shown are the masking fature and the cathode used. The brush method was used in this application since the area was very accessible.
Figure 8 shows the anodizing in progress. Details on the application are as follows:
-
Area Build up Required-
38.7 cm* (6 in.2) 5opm (0.002 in.) on
Current Density Current Ah CoatingTime
diameter. 0.08 Ncmz (0.5 Nin.2) 3A 0.79 16 minutes
-
Fig. 7 Flow fmure for generator frame
Fig. 9 Helicopter control link and tooling for job
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37
. .
. .
.
.
Figure 10 shows the anodizing in progress. Auxiliary equipment is being used to rotate the cathode and to pump solution into the work area Details on the application are as follows: Area Build up Required ~
Current Density Current Ah CoatingTime
-
-
10.1 cmz(1.57 in.*) 5 Q m (0.002 in.) on
diameter 0.19 Ncm2(1.25 Nin.3 1.96A 0.22 6.7 minutes
Fig. 10 Brush anodizing helicopter control link
Summary Brush and flow sulfuric acid.anodiing are effective, reliable methods of anodilng a select area on a part or repairing a damaged tank anodized coating. The coatings applied by these methods meet the q u a l ! and performance requirementsof MIL-AS625E and AMS 24710 and 24726. There are important applications for these processes in aerospace and other industries.
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