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The Leading Edge Guide to Top Quality Anodizing using the Complete Spectrum Approach with a Universal Type I – II – III – (123) Mixed Electrolyte
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by Fred Charles Schaedel
Anodic Technical Services, Westminster , Calif.
ABSTRACT
ABSTRACT
This paper discusses a new advanced problem solving anodize technology for processing Types I, II and III anodize (A123) in a universal mixed electrolyte. This new chromium-free formulation will activate pore structure development, producing excellent Type IC and Type III hard anodize at lower voltages. The following technical areas will be discussed: (1) new anodize bath parameters in service, (2) simplified electronic power supply requirements and (3) slow pulse-step-ramp-dwell procedures. Data logger graphs presented will prove quality, efficiency and energy savings on critical alloy parts (2024, 2219, 7050, etc.) and actual leading edge configurations.
Background
Environmental considerations and other problem solving situations have necessitated ATS to develop a non-chromated polycarboxylic acid anodize electrolyte for Brush Type IC and II anodize which could also be used in larger Type II and III hard anodize process tanks on the line. This “greener” electrolyte has already been used successfully as a Type IC alternate at a time when nothing else was available (1991-92). In 1996, we also developed a modified oxalic acid anodize using tartaric acid, which was also used for hard anodizing on a Silicon Valley project. Due to red tape, their approvals were held-up in the aerospace arena because of structural concerns, even though the pore structure criteria did meet critical requirements. Some qualified technical people feel this has been ignored by the aerospace industry for far too long. Meanwhile, the present chromic acid anodize facilities have been so very busy meeting process deadlines, that hexavalent chromium pollution continues to escalate.
Because of RoHS requirements, European aircraft facilities have adopted a tartaric acid process as a Type IC alternate on commercial aircraft, which is similar to our Silicon Valley modified process. This indicates it has met their approval on structural aircraft assembles. In addition, Sikorsky Aircraft is pushing to use Type IC alternates exclusively for all of their helicopters.
At ATS, we have proceeded to integrate tartaric acid as the primary electrolyte in addition to our amino polycarboxylic formulation, along with further modifications for Type IC, Type II-23 and Type III hard anodize. Also, we have investigated polycarboxylic acid combinations for greater efficiency during the complete ramp cycle, which is most critical for Type I anodize, particularly on leading edge configurations.
It is important to remember that the problem-solving data which we are sharing has been a major part of our research and development work at ATS and PAC for well over 50 years. Our recent papers submitted since 2003 have led up to this paper, including the information we share, which serves to remind us how important the exchange of information is for both present and future production.
This innovative chemistry will become very important for all future anodizing in order to meet more demanding pore structure, coating weight and hardness requirements and should be controlled between minimum and maximum units as follows:
• Polycarboxylic acids controlled between 1-25% (or 25-1%), while we control…
• Sulfuric acid between 25-1 % (or 1-25%), to this we make calculated additions of…
• Amino-complex ion and inorganic/organic bonded groups from 2-10%
Specific control of this chemistry from a 1:25 ratio (or 25:1) has been key to our research and development on a practical basis. It is the ultimate determinant in attaining a complementary balance of pore structure, coating weight and wear resistance properties.
Introduction - moving forward
Our ultimate goal was to practice the best leading edge technology possible on all production part and leading edge configurations, using all organic based electrolytes. This meant getting rid of both the chromic and sulfuric acid. However, we also needed to present a powerful formulation, which could be added to all existing production sulfuric anodize tanks as a superior accelerator and modifier for Type II and Type III hard anodize, which would pass all applicable aerospace, military, medical, automotive and other commercial specifications.
The final chemistry presented here is not just leading edge from a basic technology standpoint, but also leading edge in expanding our complete spectrum package, which includes:
• Chemistry: Amino polycarboxylic complex ions
• Electronic power package: Slow pulse-step waveforms
• Procedures: Slow pulse-step-ramp-dwell (throughout the entire anodize cycle)
New, improved procedural developments have further enhanced leading edge pore structure development by pushing the envelope in eliminating burning on 2000 and 7000 series alloys.
This leading edge technology starts at 2 to 3 volts, with early pore structure development, which is completed at low final voltages and energy levels. The pore structure is improved at the very base nanostructure and then continues to build at a lower exothermic electrochemical energy level. This means less energy usage while producing higher quality anodize. This higher quality anodic pore structure allows for better sealability and promotes longer in-service quality performance.
The leading edge technology presented herein comes with priorities, some of which have to do with our commitment to the industry. There are specific commitments which the customer needs and/or wants, depending upon his particular facility. Regardless of the motivations, they should all be based around the following factors:
• Quality
• Efficiency (anodize time)
• Energy savings
Always bear in mind, the leading edge technology for the top quality anodize presented in this paper is integrated into our complete spectrum approach as a total package. This entire package must be considered as one, for optimum results. To better explain the technology, this paper has been divided into the following seven sections:
1. Chemistry: process tank electrolyte
2. Electronic power package requirements
3. Slow anodic pulse control discharge
4. Process procedure ramp and run requirements
5. Data logger graphs
6. Air agitation requirements
7. Conclusions
Chemistry: process tank electrolyte
The aim was to develop a single complete universal electrolyte for Types IC, II, IIB, 23 and III hard anodize coatings. The Type IC formulations were reviewed by people who know and understand aircraft structural and leading edge requirements. The choice of organic acid used as a base had to be compatible with and enhance or improve sulfuric acid anodizing to satisfy specification requirements and organic dye pore structure considerations. The ATS Group wanted the organic acid formulation to include at least two different polycarboxylic acids for better efficiency and compatibility with existing organic, sulfuric acid and phosphoric acid electrolytes. Finally, we also wanted this electrolyte, as a concentrate, to perform as a super additive or modifier when added to any existing sulfuric acid anodize tank and stand alone as a superior brush anodizing solution for all MIL-A-8625 types.
Since 1961 we have studied and reviewed more than 160 carboxylic acids as potential useful anodize electrolytes. Over 30 of these have been used as basic anodize electrolytes in laboratory and/or production tanks. We have integrated three of these acids into the concentrated amino polycarboxylic electrolyte/accelerator (ACEA) for one good scientific reason - they absorb heat at different energy levels which one organic acid such as tartaric will not do by itself. This heat absorption is very important to pore structure development as related to sealability, hardness and process time and is equally important to eliminate burning of difficult-to-anodize alloys.
The high concentration of amino, dicarboxylic and polycarboxylic groups (ACEA) are very beneficial in three different applications. First, when added to a basic preferred electrolyte (tartaric and/or oxalic), the resulting electrolyte has much greater efficiency as related to anodize time and potential structural bonding. Second, this concentrated mixed organic electrolyte activator (ACEA) will also increase the efficiency of any sulfuric acid anodize electrolyte, even when the aluminum is controlled in the 10-20 g/L range. In fact, when a sulfuric anodize tank is maintained with 5-10% of this mixed organic electrolyte, the free sulfuric acid can be lowered to 7-12 vol% with no burning on 2000 series alloys, producing a superior hard anodize which passes Taber abrasion tests at 0.8-1.8 wear index on 1000-3000 A production loads. Third, the mixed organic activator formulation also works as an excellent brush anodizing electrolyte. We have even touched up scratches on existing chromic acid anodize panels with an exact grey visible match to the existing anodize.
Through exhaustive testing, we have established that this highly concentrated universal electrolyte can be used by itself or with other basic acid electrolytes to meet applicable specifications. The basic organic and inorganic electrolytes which we have used in activation, anodize and brush anodize/electropolish applications are as follows:
Organic: | Tartaric acid | Inorganic: | Sulfuric acid |
Oxalic acid | Phosphoric acid | ||
Citric acid | Boric acid |
The amino carboxylic electrolyte/activator (ACEA) presented in the anodize tank operation ranges shown in Table 1 is a 75% active concentrate. Amine complex ion chelates are also included in the remaining 25% aqueous part of the formulation. All formulations are presented as they have been tested in laboratory and production tanks up to 1,000 A loads at this time.
Table 1 - Operating conditions for the ACEA universal mixed electrolyte.
Type IC / IIB / 23 | Type II / 23 / III | |
---|---|---|
Basic organic electrolyte (tartaric / oxalic / citric) | 2.0 / 5.0 vol% | 2.0 / 4.0 / 6.0 vol% |
Basic sulfuric acid | 0.0 / 5.0 / 7.5 vol% | 5.0 / 10.0 / 12.0 vol% |
ACEA (75% conc.) | 2.5 / 5.0 / 10.0 vol% | 5.0 / 10.0 / 20.0 vol% |
Voltage | 5.0 / 24.0 V | 10 / 25 / 50 V |
Current density | 2.0 / 25 A/ft2 | 10 / 25 / 50 A/ft2 |
Temperature | 85 / 105°F | 35 / 60-80 / 105°F |
Air agitation | 1.0 / 3.0 ft3/min | 3.0 / 5.0 / 7.5 ft3/min |
The Type IC and Type 23 anodic coatings have been excellent in the following three areas which have traditionally presented quality issues/problems:
- Excellent pore structure development (for adhesive bonding)
- Good bright microfinish maintained (for structural applications)
- Superior wear resistance on Type III hard anodize (all 2000 series including 2219)
Electronic power package requirements
Power supplies
The standard power supply (rectifier) is the full wave type with constant voltage (CV) and constant current (CC) controls for all Type IC, II, II B and III hard anodizing with one universal electrolyte. From here on, the electrolyte will be referred to and labeled “Type 123.” Secondary SCRs (silicon-controlled rectifiers) are preferred in a center-tap full-wave wiring configuration to perform both rectification and control. The voltage requirements for Type 123 anodizing are typically as follows:
Type IC - 123 0-20V
Type II - 123 10-18V
Type IIB - 123 0-10V
Type II - 123 Hard 0-24-30V
Type III - 123 Hard 0-50-75V
Type II - 123 10-18V
Type IIB - 123 0-10V
Type II - 123 Hard 0-24-30V
Type III - 123 Hard 0-50-75V
Most notable here are the lower voltage ranges needed when using the Type 123 universal electrolyte.
Special half wave rectifiers have also been used successfully for all Type 123 high-speed anodizing. However, filtering is preferred during the ramp cycle for Type IC until further testing on production parts is completed. The panels submitted in this paper have all been processed with both full-wave and half-wave rectification in order to substantiate the completeness of this technology. The current density requirements are also considerably lower when anodizing with the universal Type 123 electrolyte. As we have explained, this is due to the combination of dicarboxylic, polycarboxylic and amino acids which promote higher conductivity at different energy levels during the ramp cycle
Type IC - 123 2-5 - 10 A/ft2
Type II - 123 10-20 A/ft2
Type IIB - 123 5-10 A/ft2
Type II - 123 Hard 15-25 A/ft2
Type III - 123 Hard 15-50 A/ft2 +
Type II - 123 10-20 A/ft2
Type IIB - 123 5-10 A/ft2
Type II - 123 Hard 15-25 A/ft2
Type III - 123 Hard 15-50 A/ft2 +
Extremely low current density ranges are now attainable for Type IC and II B anodize. In addition, finer microfinishes are maintained throughout the complete anodize cycle for all types.
Slow pulse ramp
Extensive research and production dating back to a 1968 U.S. patent (#3,418,222) has proven that slow pulse always gives the best results. The pulse cycle ranges can vary from 1.0 cycles/sec to 3.0 cycles/min, depending on the alloy and specification requirements. Pulse parameters are also centered around slow pulse turn-on and turn-off times (depending on the load, 1,000-3,000 A), so that one can take advantage of both relaxation and recovery times during the pulse ramp cycle. In addition, we have added anodic pulse control capacitance discharge (APCCD) which has proven to be very important by allowing higher current densities at lower voltages when anodizing with the universal organic/inorganic Type 123 electrolyte. APCCD (and/or APCD) will be presented in detail in the next section. Basic slow pulse specifications are listed in Figure 1.