# Conclusions on Aluminum Adhesive Bonding Tests

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#### MalcolmW

##### Well-Known Member
Percy, BBerson & Gary;

Percy, thank you for your cautionary words about the 'slosh sealer.' I've never used it and seen references to its use in aircraft and automotive applications (sold by JCWhitney). It may have specific surface prep requirements, however, an all aluminum fuel cell / tank / wet wing sounds superior to me, also.

BB, I appreciate cautions and warnings. Yes, adhesives can be degraded by environmental exposures, however, Gary is doing environmental testing of his adhesives as part of the selection process which shows sound judgement.

I suggested chromate conversion (alodining) because it passivates or stabilizes the aluminum surface against corrosion, thus providing the adhesive with a substrate that doesn't change rapidly (everything changes over time, given enough time). In addition, since Gary has said that he will apply adhesive to 'lap' joints in his construction, this type of joint does reduce environmental exposure significantly. Also, his choice of acrylic (methacrylate) adhesives is a material which is slow to deteriorate under environmental exposure (many building caulks are made with acrylic polymers, which hold up well with the caulk being fully exposed to the elements).

Lastly, acrylic adhesives are used to assemble aluminum truck (delivery) bodies, which has a far more severe operating / environmental exposure than most general aviation aircraft ever see (no, not even in Ohio do we use deicing salt on aircraft). Yes, corrosion occurs, however, the trucks do not fall apart.

The key to good performance is following preparation procedures strictly (cleanliness!), consistency in adhesive mixing and application, and using a fixture or jig during assembly for precise alignment and bonding pressures. Not easy, especially at first, but once mastered, certainly faster and stronger than rivets.

Please forgive me for getting on my soap box, however, I believe Gary is doing a service to this forum with his openness about his adhesive bonding project, and I also believe that he is offering information that may lead to a more reliable method for experimental aircraft structures / frame construction.

All the best & fly safe,

MalcolmW

#### BBerson

##### Light Plane Philosopher
HBA Supporter
I suspect Richard Schreder has passed on. He must have had success with skin bonding but of course he also had some problems as well, that he mentioned in the forum. He was bonding PVC hard foam ribs to the skin.

As Malcolm said: "The key to good performance is following preparation procedures strictly (cleanliness!), consistency in adhesive mixing and application, and using a fixture or jig during assembly for precise alignment and bonding pressures. Not easy, especially at first, but once mastered, certainly faster and stronger than rivets."

Perhaps the industry procedure could be found and posted here.
BB

#### GESchwarz

##### Well-Known Member
The price for Partite 7350 is $9.50 for 50ml and$35 for 400ml. Mixing tubes for 50ml are 75 cents each, for 400ml \$1.25 each. It only makes since to use the mixing tubes when doing larger assemblies, otherwise if you're using them for every little job you're going to spend a fortune on the mixing tubes. If you're going to mix with a spatula you must be very thorough.

I don't have any data on usage, but if your joints are fairly precise (no big gaps) and you don't get sloppy with the stuff, it should go a long ways. After all, an ideal bond joint is only about .010" +-.002" thick.

#### GESchwarz

##### Well-Known Member
I visited the Sonerai forum and invited all who have knowledge of Monnett and his contact info, and aluminum bonding, to come and visit this forum to add what they know. Let's see what comes in.

#### GESchwarz

##### Well-Known Member
BB,

Here is a very good technical write up on Aluminum Bonding that I thought is appropriate to the subject of this post.

Bonding to Aluminum - Article

I've pasted it below, lest we loose the web link at some future date. It confirms much of what has been already discussed on this post. There are some surprising data as well. No mention is made of methacrylates in this article however.

The formatting got a little goofed up in the copy and paste process...

***

Bonding to Aluminum

SpecialChem - Jan 4, 2006

Edward M. Petrie, Member of SpecialChem Technical Expert Team
Introduction
The Basics of Aluminum
Surface Properties of Aluminum
Historical Aluminum Pretreating Processes
Trends in Aluminum Pretreating Processes
Selection of the Adhesive with Regard to Aluminum
Introduction

Aluminum is an almost ideal substrate for adhesives. It has a high surface energy and is very resistant to most environments. It is also a material with good formability and high strength-to-weight ratio that can benefit greatly from properties offered by adhesive joining. As a result, adhesive bonded aluminum joints are commonly used in the aircraft and automotive industries as well as in a multitude of other structural and non-structural applications.
Aluminum's surface oxide forming ability provides excellent corrosion resistance and a surface providing high initial adhesive strength. However, this oxide layer is relatively complex and can be formed in many ways depending on the conditions present during its formation. The oxide layer can also provide significant problems for the adhesive formulator and the end-user - especially when it comes to bond durability in warm and humid climates. As a result, a prebond surface treatment is generally required for aluminum to stabilize and specifically define the oxide structure.
The technical and commercial advantages of using aluminum have significantly encouraged its use as an adhesive substrate, and aluminum is perhaps the most scrutinized of all adhesive substrates. Aluminum joints are commonly used in adhesive research and for comparing different adhesive materials and processes. In their marketing literature, adhesive manufacturers usually describe the properties of their products measured on aluminum substrates.
This article will explore the important considerations necessary when using aluminum as an adhesive substrate. The surface properties of aluminum will be considered as well as the various processing methods used to modify the surface for effective adhesive bonding. The unique and interactive aspects of the adhesive, substrate surface, and service environment will be explained. Because of the plethora of technical data available on aluminum, bonded aluminum joints, and the adhesive-aluminum interface, this article cannot hope to cover all possible factors. It will, however, serve as a useful guide to bonding aluminum and an introduction into the more comprehensive aspects of the subject.

The Basics of Aluminum

The use of aluminum exceeds that of any other metal except iron. It is important in virtually all segments of the world economy. The strength and durability of aluminum varies widely depending on the components of the specific alloy, and also on the particular manufacturing process. Aluminum is generally characterized by its alloy content (a four digit number) and the temper or condition of the alloy (a letter designation). The quality of adhesive joints formed with aluminum greatly depend on these factors.
The corrosion resistance and the durability of joints made with adhesives are primarily dependent on the type of aluminum alloy that is used. Bonds made with relatively corrosion resistant 6061-T6 aluminum alloy will last about four times as long as equivalent joints made with 2024-T3 alloy when exposed to marine environments. The permanence of adhesive bonded aluminum joints is dependent on the alloy type because of their different corrosion degradation rates under extreme environmental conditions. With the proper surface treatment, both initial joint strength and the durability to aggressive environments can be greatly improved as shown in Table 1.
Tensile Shear Strength, psi
Alloy 2036
Alloy 6151
Alloy X5085
A
B
C
A
B
C
A
B
C
Initial, no aging
1930
1850
2200
2550
2690
2530
2250
2270
2110​
After 3 mos aging at 23°C, 85% RH
1390
1050
2400
1350
1550
2410
1860
1890
1910​
After 3 mos aging at 52°C, 100% RH
420
350
1110
920
1050
1100
1260
1520
1230​
After 3 weeks aging in 5% salt spray solution
0
0
1410
80
150
2090
690
530
1210​

Table 1: Effect of Surface Treatment on Aluminum Joints Bonded with a Heat Cured Epoxy Adhesive (EC-3443, 3M Company) 1​
A - Mill finish
B - Vapor degreased
C - Alodine 401-45​
The yield strength of the alloy and other bulk properties will also influence the joint strength to a certain degree. This effect will depend on the specific joint design and type of mechanical stress. Although the bulk material is important in the design and ultimate performance of adhesive joints, the main requirements for high bond strength and lasting durability reside at the adhesive - substrate interface.

Surface Properties of Aluminum

The oxide layer that forms on aluminum is more complex than with other metal substrates. Aluminum is a very reactive surface, and oxide forms almost instantaneously when a freshly machined aluminum surface is exposed to the atmosphere. Fortunately, the oxide is extremely stable, and it adheres to the base metal with strengths higher than could be provided by most adhesives. The oxide is also cohesively strong and electrically nonconductive. Once a very small (on the order of 0.001 mm) layer forms, it protects the base metal from atmospheric corrosion. These surface characteristics make aluminum a desirable metal for adhesive bonding, and they are the reasons why many adhesive comparisons and studies are done with aluminum substrates.
Various interactions between surface oxides and environmental factors (e.g., water, temperature, atmospheric chemicals) can create bonding complications. Under normal conditions the surface oxygen atoms will hydrate to form a layer of hydroxyl groups. These adsorb and retain molecular layers of bound water. The presence of bound water on the surface can be a positive factor since it encourages wetting by polar organic adhesives, such as epoxy. A hydrated metal oxide surface layer is illustrated in Figure 1.

Figure 1: Metal surfaces are actually hydrated metal oxides2​
Note:1.The oxide layer is typically 40-80°A thick
2.The hydrated layer is tightly bound
3.A pure metal surface is rarely available for engineering use.​

The initial strength of an aluminum joint can be improved by surface pretreatment. This generally involves cleaning the surface to remove contaminants and / or converting the existing surface to a new surface of higher surface energy. In the case of the aluminum surface, chemical conversion can also protect the base metal from corrosion and enhance the durability of the bonded joint in various service environments. The most common surface preparations that have been used for bonding aluminum can be generally segregated into three groups:
• chemical etching, and
• primers and conversion coatings.
The parameters of the pretreating process and their control can be very important in assuring consistency from joint-to-joint. It has been reported that the temperatures used for rinsing and drying aluminum should not exceed 70°C since different oxides are formed above and below this temperature. The oxide (bayerite, ß-Al2O3.H2O) formed at lower temperatures supposedly provides better adhesion.3 It has also been recognized that the use of distilled or deionized water rather than tap water for cleaning and etching baths is preferred for greater bond strength and consistency.
Alloying elements influence the growth mechanism of the oxide layer and, hence, the corrosion behavior of the surface. In moist air the oxide can undergo corrosion, the products of which are deposited on the top of the oxide layer and form a weak boundary. The fracture of these joints always occurs within the oxide layer. To obtain a strong and stable bond between the metal and the adhesive, the naturally formed surface oxide has to be removed (usually by abrasion or chemically) and replaced with a new oxide layer that is continuous, solid, and corrosion resistant.
In selecting a pretreatment process, for aluminum or any other substrate, both the initial strength and the permanence in a specific operating environment must be considered. Mechanical abrasion is a useful pretreatment in that it removes the oxide and exposes bare aluminum. When this is done, however, many of the benefits of the protective oxide layer are lost. For example, if bare abraded aluminum is bonded, the reactive metal at the joint interface can potentially become hydrolyzed and oxidized, which will displace the adhesive. Hence, this bonded joint may initially be much stronger than one made with unabraided metal, but it will deteriorate rapidly when exposed to a harsh environment such as high heat and humidity.
This is why pretreatments that modify the oxide layer or create a new, stable oxide layer are especially desirable when permanence is a primary consideration. They improve bondability and maintain protection. To protect the aluminum joint from the affects of the environment, especially water and corrosion, an artificially thickened oxide layer is generally formed on the surface. Historically, chemical etching or anodizing surface treating processes have provided the surest way of obtaining durable adhesive bonds with aluminum.
Once the aluminum is treated by one of these processes (discussed below), the work of protecting the interface is still not over. The usual approach to good bonding practice is to prepare the aluminum surface as thoroughly as possible, then wet it with the adhesive as soon afterwards as practical. In any event, aluminum parts should ordinarily be bonded within 48 hours after surface preparation. However, in certain applications this may not be practical, and primers are used to protect the surface between the time of treatment and the time of bonding.
The use of primers for structural aluminum joints in the aerospace and automotive industries is especially common. Primers are applied as a low viscosity solution which enables them to chemically wet a metal surface more effectively than more viscous, higher solid content adhesives. Corrosion resistant primers are often used to protect the etched surface during assembly operations and exposure to environmental conditions when in service.4

Historical Aluminum Pretreating Processes

When bonding aluminum to itself or to other materials, the optimal surface preparation should be determined for the application based on the initial strength and durability required, and then the process should be rigidly followed. Over-specifying the strength requirements should be avoided since it could result in the selection of a surface preparation process that is time consuming, difficult to control, and expensive. Table 2 serves as a guideline for selecting which pretreatment to try first. Conventional pretreating processes for aluminum are described in ASTM D2651.
Surface Treatment
Type of Bond
Solvent wipe (MEK, MIBK, trichloroethylene)
Low to medium strength
Abrasion of surface (sandblasting, coarse sandpaper, etc.) plus solvent wipe
Medium to high strength
Hot vapor degrease (trichloroethylene)
Medium strength
Abrasion of surface plus vapor degrease
Medium to high strength
Alodine treatment
Low strength
Anodize
Medium strength
Caustic etch
High strength
Chromic acid etch (sodium dichromate - sulfuric acid)
Maximum strength​

Table 2: Effect of Substrate Treatment on Strength of Aluminum Joints Bonded with Epoxy Adhesives 5​
The simplest form of aluminum pretreatment consists of (1) solvent wiping, (2) vapor degreasing, or (3) either of these methods combined with mechanical abrading. In each instance, care must be taken to assure that the cleaning materials themselves do not become unknowingly contaminated, thus providing ineffectual cleaning or cross-contamination resulting in poor bond performance. For low to moderate strength aluminum joints, vapor degreasing and alkaline cleaning are often used. These processes will generally remove mill oil and debris from the surface of the substrate. The processes also provide a uniform surface for subsequent bonding.
Sandblasting is commonly used for treating aluminum surfaces prior to adhesive bonding because of its simplicity and economics. This process removes the existing aluminum oxide surface layer and allows a new one to develop. The new oxide layer will depend on the environmental conditions during which it forms.
Chemical pretreating processes, such as etching, produce higher reliability and longer service life in a bonded assembly. If aluminum adherends are first cleaned, then sandblasted, and finally chemically treated, the surface area is increased, the contaminants are removed, and a new oxide layer forms which is conducive to excellent initial and long-term strengths. However, this three step process is often not necessary when only low-to-moderate strengths (500-2000 psi) are required or if the finished adhesive joint will not be exposed to aggressive service environments (especially high moisture or humidity conditions). Useful bonds in these low-to-medium strength applications can be achieved simply with cleaning and / or abrasion.
While various acidic or caustic procedures can be employed with or without vapor degreasing, the most recognized etching pretreatment for bonding aluminum has been the sulfuric acid - dichromate solution used by the aircraft industry and described in ASTM D 2651. This process is sometimes known as the FPL etch, named after its developers, Forest Products Laboratories.6 The first step in this process is vapor degreasing followed by alkaline cleaning and then chemical immersion. The substrates are finally forced air-dried. There are several modifications of this treatment including a paste-like etching solution to allow for parts that cannot be immersed in the acid solution.
Other important methods of pretreating aluminum for adhesive bonding include anodizing and chromate conversion coating.7 In anodizing, the aluminum is immersed in various concentrations of acids (usually phosphoric or chromic) while an electrostatic charge is applied. The oxide reacts with the etchant to form a compound that protects the surface and is compatible with the adhesive. In this way the aluminum oxide is retained, but it is rendered more receptive to bonding. It has been shown that an anodized surface on aluminum alloy can constitute a very durable surface for epoxy adhesive bonding with excellent resistance to seacoast or other saltwater types of exposure. Examples of widely used anodizing processes are the Boeing phosphoric acid anodize (PAA) process8 and the chromic acid anodize (CAA) process . 9

The landmark U.S. Air Force Primary Adhesively Bonded Structure Technology (PABST) Program in the late 1970s demonstrated that properly designed and manufactured bonded fuselage panels made from the correct aluminum materials can actually operate safely at higher stress levels than comparable rivet joined aluminum structures.10,11 The results of this program show phosphoric acid anodizing as an optimal way to achieve durable aluminum bonds.
• PAA is the most durable pretreatment for aluminum that is processable within reasonable production tolerances.
• PAA with a corrosion resistant primer provides the best corrosion resistance.
• The adhesive should be selected on the basis of best durability as defined by slow cyclic testing in a hot / humid environment.
The chemical conversion coating method is also commonly used to treat aluminum substrates prior to bonding. Chemical conversion coating is an amorphous phosphatization process where the aluminum is treated with a solution containing phosphoric acid, chromic acid and fluorides. Chromate conversion coatings on aluminum constitute an effective way to enhance the surface bondability and also improve the corrosion resistance of the bond line.12 The resulting durability observed with mechanical abrasion and chemical conversion coatings are variable and depend on the particular processing conditions. Whereas, anodizing and etching processes produce consistent and generally durable aluminum joints.

Trends in Aluminum Pretreating Processes

The recognition that chrome is carcinogenic has forced alterations in surface treatment processes. Chromic acid anodizing and FPL etching are being phased out in many locations.
There have been several processes that are suggested as more environmentally friendly alternatives. One such process is a chromate-free etching process (designated PT2) for improved environmental and occupational health and safety perspectives.13
Work on sulfuric acid anodizing and sulfuric boric acid anodizing is now in progress.14 In the automotive industry, a pretreatment has been developed for aluminum coil that is nontoxic and compatible with weld bonding. This proprietary treatment is claimed to be as effective as chromium based pretreatment processes on exposure to salt spray.15
An environmentally benign sol-gel process has also been developed to provide aluminum with good bondability and corrosion resistance.16 In addition this process is claimed to be low cost and energy saving. The sol-gel consists of a ceramic inorganic coating to establish and aluminum oxide based coating. The ingredients are alumina and organosilane.

Selection of the Adhesive with Regard to Aluminum

Once the surface considerations are taken care of, there are many types of adhesives that can bond well to aluminum. The selection will depend on the strength needed, the type of stress involved (e.g., peel or shear; static or dynamic), and the operating environment. Reynolds Metals Company 17 offers some general rules of thumb in selecting an adhesive for aluminum bonding.
• Bonds to aluminum are generally stronger than bonds to steel
• A chromic - sulfuric acid etch gives the best resistance to weathering and salt water environments
• Room temperature curing epoxies offer the best salt water resistance
• Higher strengths are usually obtained with heat curing epoxies than with room temperature curing epoxies
• Modified phenolic films give the highest peel and shear strength combinations
• The most severe adhesive environment is a hot, humid climate (temperature 30-50?; humidity +90%)
• Structural adhesives are strong in shear; weakest in peel and cleavage
• Heat curing adhesives are less sensitive to surface preparation than room temperature curing adhesives
The commercial epoxy, acrylic, and polyurethane adhesives will all bond well to aluminum and to a wide variety of other materials. Sell 18 has also ranked aluminum adhesives in order of decreasing durability as follows: nitrile-phenolics, high temperature epoxies, elevated temperature curing epoxies, elevated temperature curing rubber modified epoxies, vinyl epoxies, two-part room temperature curing epoxy paste with amine cure, and two-part urethanes.
The starting formulations presented in Table 3 are designed for general purpose bonding of aluminum where other substrates may also be involved. Note that aluminum powder is a key ingredient in these formulations to provide for a closer match in coefficient of thermal expansion between the adhesive and the substrate.
Component
Parts by Weight
A
B
C
Part A
DGEBA epoxy resin (EPON 828, Resolution Performance Products)
100
100
50​
Ground tubular alumina (Alumina T-60/T-64, Alcoa)
230​

Aluminum powder

100
50​
Part B
Aromatic amine eutectic (60/40 blend of MDA and MPDA)
12​

Aliphatic amine (EPI-CURE 3234, Resolution Performance Products)
5​

Aliphatic amine (EPI-CURE 3245, Resolution Performance Products)

5​
Polyamide (EPI-CURE 3125)

54.5​

Properties:

Mix ratios, Part A : Part B
• By weight
• By volume

19.4:1.0
9.6:1.0​

100:27
2:2.1​

100:5
---​

Viscosity, cps at 25°C
Paste
Paste
Paste​
Working life at 25°C
1-2 hrs
60 mins
60 mins​
Cure schedule
Elevated Temperature
Room Temperature
Room Temperature​

Tensile shear strength, psi, measured at 25°C on aluminum when cured:
• 7 days at 25°C
• 10 mins at 204°C
• 100 mins at 121°C

----
2250
2300​

2050
1050​

Table 3: Starting Formulations for Epoxy Adhesive for Bonding Aluminum19​
The durability of epoxy bonded aluminum joints that were immersed in water are shown in Figure 2. The anodized and grit blasted surface treatments, although giving different initial joint strengths, showed no deterioration after two years' exposure. Both the vapor degreased and conversion coating treatments were significantly degraded by the moist environment. Exposure of similarly prepared specimens to a more aggressive soak-freeze-thaw cycle gave rise to even greater differences in performance with only the anodized treated aluminum joint showing a high percentage of joint strength after a two year period.20

Figure 2: Effect of surface treatment on the durability of epoxy / aluminum joints exposed to room temperature water immersion. (1) anodized, (2) girt blasted plus vapor degrease, (3) vapor degrease, (4) chromate conversion coating.21​
References

1. Adhesive Bonding of Aluminum Automotive Body Sheet Alloys, The Aluminum Association, Inc. 1975.
2. Schneberger, G.L., "Adhesives for Specific Substrates", Chapter 21 in Adhesives in Manufacturing, G.L. Schneberger, ed., Marcel Dekker, New York, 1983.
3. Murphy, J.F. and Page, H.A., Am. Chem. Soc. Div. Paint Plastics Printing Ink Chem. Papers, vol. 15, no. 1, 1955 , p. 27.
4. Petrie, E.M., "The Use of Primers with Adhesives and Sealants", SpecialChem4Adhesives.com, August 27, 2003.
5. Adhesive Bonding of Aluminum, Reynolds Metals Company, 1966.
6. Eichner, H. W., and Schowalter, W. E., Forest Products Laboratory Report 1813, 1950.
7. Minford, J. D., "Surface Preparations and Their Effect on Adhesive Bonding", Adhesives Age, July 1974.
8. Kabayashi, G. S., and Donnelly, Boeing Company Report DG-41517, February 1974.
9. Bijlmer, P. F. A., Journal of Adhesion, 6 (1973), p. 319.
10. Thrall, E., "Bonded Joints and Preparation for Bonding, AGARD Lecture Series 102 5.1, 1979.
11. Shannon, et. al, "Primary Adhesively Bonded Structure Technology (PABST) General Materials Property Data", Douglas Aircraft Co., McDonnell Douglas Corp., Air Force Flight Dynamics Laboratory, Technical Report AFFDL-TR-77-107, September 1977.
12. Kim, G., and Ajersch, F., "Surface Energy and Chemical Characteristics of Interfaces of Adhesively Bonded Aluminum Joints", Journal of Materials Science, Vol. 24, 1994, pp. 676-681.
13. Wilson, I., et. al., "Pretreatment for Bonded Aluminum Structures", Advanced Materials and Processes, August, 1997.
14. Browne, J., "Aerospace Adhesives in the 90s", 38th International SAMPE Symposium, May 10-13, 1993.
15. Wilson, I., et. al., "Pretreatment for Bonded Aluminum Structures", Advanced Materials and Processes, August, 1997.
16. Zheng, H., et. al., "An Environmentally Benign Aluminum Alloy Surface Pretreatment Process for Adhesive Bonding", 28th International SAMPE Technical Conf., November, 1996.
17. Adhesive Bonding Aluminum, Reynolds Metals Company, Richmond, VA, 1966.
18. Sell, W. D., "Some Analytical Techniques for Durability Testing of Structural Adhesives", Proceedings 19th National SAMPE Symposium and Exhibition, Vol. 19, New Industries and Applications for Advanced Materials Technology, April 1974.
19. Resolution Performance Products, Starting Formulations 4011 and 4012.
20. Hartshorn, S. J., "Durability of Adhesive Joints", in Structural Adhesives, S. R. Hartshorn, ed., Plenum, Press, New York, 1986. Also in Minford, J. D., Treatise on Adhesion and Adhesives, R. L. Patrick, ed., vol. 5., p. 45, Marcel Dekker, New York, 1981.
21. Hartshorn, S.J., "Durability of Adhesive Joints", in Structural Adhesives, S.R. Hartshorn, ed., Plenum Press, New York, 1986.

#### GESchwarz

##### Well-Known Member
The concerns raised about corrosion as being a potential life-limiting factor for adhesives, I think favors the use of 6061 as possibly being an equivalent substitute for 2024 because of 6061's superior corrosion resistance.

My equivalency testing as discussed earlier will shed some light on the feasibility of substitution.

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##### Well-Known Member
Or 5086... I've been looking at this as a replacement for 6061 where strength isn't critical (although if it's already 6061 as opposed to 2024, it can't be too critical), and there's a lot to like. Avoid high temps, though.

#### GESchwarz

##### Well-Known Member
I'm thinking 6061 for my skins, ribs and formers, and 2024 for all my spars and longerons. I bought more than I'll need of 1" x .75" 2024 T3 angle for about 25 cents on the dollar from a surplus metal supply. The cockpit cage will be 4130.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Garry,

That's a good article and it talks about my concerns regarding the oxide surface treatment.
"For example, if bare abraded aluminum is bonded, the reactive metal at the joint interface can potentially become hydrolyzed and oxidized, which will displace the adhesive. Hence, this bonded joint may initially be much stronger than one made with unabraided metal, but it will deteriorate rapidly when exposed to a harsh environment such as high heat and humidity. "

Thanks for posting.
BB

#### GESchwarz

##### Well-Known Member
That got my attention too. What also got my attention was that the chromate conversion coating yielded the lowest strength. But really what we are after is durability. I believe that I have been able to prove that Partite 7350 is perhaps twice as strong as rivets and its performance is repeatable. Anodizing is out of the question for me and I would imagine most people. The article said "...heat and humidity", he didn't say "or". So the next big question is, Just how vulnerable are these joints to moisture intrusion from the edges of these joints, particularly in a generally arid environment, with a short cold rainy season? A lot of us just don't ever experience that harsh environment. Those who do could prevent moisture intrusion by sealing the bond joints with standard paint materials.

Tell me if I'm wrong.

Subjects such as this bring the following quotation to mind by

Hyman George Rickover (January 27, 1900), was a four-star admiral in the United States Navy. Rickover was known as the "Father of the Nuclear Navy"...

Fatal Confidence: "Doubtless", meaning a person who is rock-solid certain he knows all he needs to know and believes that nothing can go wrong. Take nothing for granted. Inspect every battle damaged ship that arrives for repairs to see what held up and what broke up. Rickover didn't simply invite bad news; he demanded it. Ignorance breeds confidence particularly in fields in which people know just enough to be dangerous. Innorance more frequently begets confidence than does knowledge.

Therefore, if you sense a high level of confidence, you must stop and test the critical assumptions upon which your safety relies.

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#### BBerson

##### Light Plane Philosopher
HBA Supporter
When I attended the Enstrom helicopter maintenance class we toured the blade bonding room. The room is sealed off and only one man builds blades using numerous long tanks and jigs and heating equipment.

I decided this attention to detail is beyond my ability or desire. What works in a factory may or may not be easily done in a home shop.

BB

#### GESchwarz

##### Well-Known Member
Thanks, and you're right. As an ASQ Certified Quality Engineer with 30 years in aerospace I can tell you it's all about process control. If I can validate the repeatability of my process, and learn by experience what will cause a weak joint, and understand what the variables are, then I've qualified my process. If I can manage to do it the same way on an assembly scale that I did on coupons, then I will get the same results.

Bonding may not be for most builders.

#### GESchwarz

##### Well-Known Member
Here are a couple of other thoughts I'd like to share...

When you desire safety, you welcome all the bad news you can get. Thorough testing is a badge of confidence: confidence that the machine can take abuse, and if it breaks, the designers will have a solution. And it's good business, reassuring users that there's no place they can go that the manufacturer hasn't been already.

***

An experienced 4-wheel driver once told me that he enages his 4-wheel drive only sparingly; if he kept it on all the time he would eventually meet a quicksand mire that he could not get out of because he had already burned up all his safety reserve. "I use the 4-wheel to get out of trouble, not into it" Again and again, people in high-hazard jobs have told me how they never go into a chancy situation without planning beforehand how to get out. You always have an alternate plan. Escape routes or Plan B's are a part of preserving a little safety margin for the really bad day.

#### MalcolmW

##### Well-Known Member
Hello, Gary, BB;

yes, that was an interesting article, and I was somewhat surprised at the poor rating given for the alodine chromate conversion, which is different than what I understand. Well, if you want the maximum strength & complete passivation of aluminum, there is nothing better than a chromic acid etch.

From personal experience in bonding aluminum, I know the chromic acid treatment produces excellent results. But, be **** careful around hot chromic acid, for it is an incredibly powerful oxidizing agent (of organic matter) and will cause burns in an instant. With the proper protection and a cautious attitude, it isn't difficult to use.

That said, here's how you do a chromic acid etch on aluminum:

Aluminum Cleaning & Chromic acid etch:

Remove ink and markings from the surface of the aluminum with a light wash using MEK or acetone and white paper towels, followed by a degreasing (vapor phase degreasing preferred. However a rinse using brake parts cleaner works well, which comes in spray cans and is available from most auto supply stores). Perform all of the above operations in a well ventilated area (outside or fan blowing), if inside, wear a respirator.

Etch the surface with a chromic acid solution by immersion at 65-70 degrees Celsius for 5-10 minutes. Chromic acid solution must be used above 20 Celsius, however, at this lower temperature, it will require about an hour immersion.

Rinse the metal (by now will have a brownish tint) thoroughly with distilled water and dry well. Do not use compressed air, because there is oil in most air compressors, which will contaminate the clean surface.

All cleaned parts cannot be touched by hand; use fresh, white cotton (mickey mouse style) gloves.
For best results, parts should be coated or bonded immediately after metal has dried completely. If you do not plan to bond the parts together within twenty-four hours, wrap in fresh ‘butcher’ paper to keep clean.

Chromic acid cleaning solution:
• 10 parts/wt. Sodium Dichromate.
• 30 parts/wt. 96% Sulfuric Acid.
• 100 parts/wt. distilled water.
(Dissolve the dichromate in the water, then add sulfuric acid slowly, stirring carefully.)

Distilled water produces best results. However, rainwater caught from the roof is very low in mineral ions and is an acceptable substitute (discard the first five minutes of runoff – that is the roof rinse water & is dirty).

When working with chromic acid, wear nitrile rubber gloves (Harbor Freight – low cost). Also a long rubber or plastic apron and shop face shield – safety first and last. Be **** careful around chromic acid solution, particularly when hot!

Dispose used chromic acid solution by neutralizing with bicarbonate of soda (lots of fizzing) and mixing liquid with dry redimix concrete. Use concrete to set posts, make weights, fix holes in driveway, etc.

All the best & fly safe,
MalcolmW

#### Norman

##### Well-Known Member
HBA Supporter
Anodizing and Fatigue Life

I found this article about anodizing and fatigue life posted on another forum this morning and was reminded that figure 2 of the article in post #45 shows that anodizing produces the strongest bond.

#### GESchwarz

##### Well-Known Member
Good call Norman and thanks for bringing this article to our attention. I do recall reading somewhere recently that anodizing does cause embrittlement.

#### MalcolmW

##### Well-Known Member
Hello, Norman, Gary;

very interesting article (http://www.experimentalhelo.com/Anodizing&Fatigue.pdf) and brings to mind (from long in the past) that anodizing was never used in adhesive applications. I had always thought that it was due to cost & complexity, however this article clearly indicates a fatigue factor.

That being said, I do know that adhesives are used extensively in aluminum assembly (aircraft, delivery trucks, etc.) without fatigue problems. In fact, LAP adhesive joints reduce or eliminate the stress risers associated with mechanical fasteners.

Gary, your choice of adhesives (elastomeric acrylics) with high elongation factors should have good fatigue resistance characteristics. As for the aluminum substrate, I believe the article commented: "Phosphoric and unsealed chromic acid anodic layers do not significantly affect fatigue life." This leads me to believe that 'anodizing' (the electrochemical process) does produce micro-cracking on the surface.

However, this is article does provide names of authorities in this field (of which I am not) who may have other articles or may even respond to a query as to the affect of fatigue on passivated aluminum in adhesive bonded structures.

All the best & fly safe,
MalcolmW

#### GESchwarz

##### Well-Known Member
The goal isn't in achieving peak strength. What is important is conforming to the tried and true surface preparation process steps, adhesive application and mating. These things should be done the same way every time that is known to produce good results. Every bonding practicioner should first work with the coupons and get familiar with what process variations will result in what failure modes, thereby gaining a knowledge and respect for that which is unacceptable workmanship.

I have not yet experienced failure due to corrosion. From what I've learned, If you do a good phosphoric etch and a good distilled water rinse and dry that you are good to go for a long lasting bond. I do not know to what degree, if any, these joints are vulnerable to corrosion here in the Pacific South West. I am assuming that methacrylate is as good as any good paint in sealing aluminum from all moisture intrusion, and if that is true, that corrosion cannot possibly occur. I have done some boil/soak/freeze cycles on the latest coupons in an effort to degrade the bonds to see how they do. I want to compare these methacrylate joints to some proseal joints but the 10 to 1 proseal mix ratio I did was off and the whole batch of coupons had to be scrapped. I knew I wasn't being careful when I did it, and I paid the price. So there is a delay there. Stay tuned, but don't hold your breath.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
I would not assume that corrosion cannot occur using modern materials. As an airplane paint shop owner I have seen a lot of corrosion. In fact, modern polyurethane paint seems to cause corrosion if applied without the proper primer. It absorbs moisture through the paint and creates some sort of acid to rapidly corrode. It can be worse than leaving the aluminum bare.
I had a piece of aluminum that I painted with Imron, It corroded after about five years.

Plastics and paints are not completely moisture proof. In fact it seems to me that moisture gets in and cannot get out, so the paint bubbles up in the sun. The older oil based paints do not hold the moisture in.
Just my experience.
BB

#### JohnG

##### Member
I have experience of re-building 3 Zeniths 701s that had been crashed. As per usual, where there was severe damage, the joints between .016 skins and ribs failed by ripping the rivets out of the ribs. But one builder had used Sikaflex polyurethane adhesive as well as the rivets in several joints. Quite notably, none of those joints failed, despite extreme distortion and rivets torn out nearby. As a matter of fact, the Sikaflex made disassembly extremely difficult. The only way to release the joints involved slicing with a thin blade.....

I've worked with Sikaflex polyurethane in autobody and experimental aircaft for many years, and it would be my much preferred adhesive for any application where there could be vibration and flexing. It remains flexible, and adhesion is absolutely tenacious. As distinct from brittle adhesives, a fracture does not travel, and 'peel' strength remains very high, right to the end. It does need a gap thickness to allow curing by atmospheric contact - ie not suitable for really tight fit joints. There are other brands that are probably equal, but Sikaflex is a well-proven leader in the field. Try it, you'll find lots of uses for it.

JG