Surface preparation of metals

Originally, sol-gel TABLE 7.1 Methods of Treatment of Aluminum Substrates[1] “Light” Abrasion Wire wool or Scotchbrite “Heavy” Abrasion Grit-blast with alumina particles “Heavy” Abrasion

Surface Preparation of Metals

7.1 INTRODUCTION

The methods listed for surface preparation of metals in Chapter 6 are ally applicable, but the processes required for specific metals are differentfrom the general techniques The specific preparation and treatment (or pre-treatment) techniques described in this chapter have been reported to providestrong reproducible bonds and fit easily into the bonding operation It must

gener-be noted that the methods for preparing metal surfaces are generally mucholder than those of plastics because of the length of time that metals havebeen in use This is not to say that improvements have not been made inrecent decades, but the pace of upgrades has been slow in recent years.Improvements have often been driven by environmental rules promulgated

by governments to reduce emissions and toxic waste generated by the surfacepreparation methods of metals An important example is chromate-free etch-ing of aluminum

7.2 ALUMINUM

There are a number of methods to treat the aluminum surface prior to sion bonding The choice of the technique depends on the performancerequirements of the adhesive bond Table 7.1 lists the techniques availablefor the treatment of aluminum Figure 7.1 shows the durability of variousadhesive bonds

adhe-Chemical treatments have been traditionally most effective with num alloys, especially where long-term environmental exposure is required.The sulfuric acid
dichromate etch (FPL etch, named after Forest ProductsLaboratory, US Dept Agriculture) has been used successfully for many dec-ades The more recently developed techniques are often modifications of theFPL procedure Other important methods include chromate conversion coat-ing and anodizing Corrosion-resistant adhesive primer (CRAP), as well asanodic and chromate conversion coatings, help prevent corrosion failure ofadhesion[2]

alumi-139Surface Treatment of Materials for Adhesive Bonding DOI: http://dx.doi.org/10.1016/B978-0-323-26435-8.00007-1

© 2014 Elsevier Inc All rights reserved.

7.2.1 Sol-gel Process

Development of sol-gel process technology has received significant interestsince the 1980s The sol-gel reaction can produce homogeneous inorganicmaterials with desirable properties of hardness, optical transparency, chemi-cal durability, tailored porosity, and thermal resistance Originally, sol-gel

TABLE 7.1 Methods of Treatment of Aluminum Substrates[1]

“Light” Abrasion Wire wool or Scotchbrite

“Heavy” Abrasion Grit-blast with alumina particles

“Heavy” Abrasion Grit-blast with alumina particles plus silane treatment Chemical Chromic-sulfuric acid pickle

Electrochemical Chromic acid anodizing

Electrochemical Phosphoric acid anodizing

Electrochemical Sulfuric acid anodizing

Electrochemical Boric acid-sulfuric acid anodizing

Chemical Sol-gel procedures

Surface Bombardment Activated plasma

FIGURE 7.1 Durability of adhesive bonds to aluminum treated by various methods PAA 5 phosphoric acid anodizing, CAA 5 chromic acid anodizing, and Pickle 5 acid treatment [1]

technology was only aimed at the production of ceramic coatings, foams,fibers, and powders In 2001 it was discovered sol-gel processing could yieldboth inorganic and hybrid organic
inorganic materials, further widening theapplicability of this technology[3].

Chemistry is at the core of the sol-gel process technologies The sol-gelprocess, as the name implies, involves the evolution of inorganic nanoscalenetworks (Fig 7.2) through the formation of a colloidal suspension (sol) andthe gelation of the sol to form a network in a continuous liquid phase (gel).Different products are obtained by controlled hydrolysis and condensationreactions

The raw materials in sol-gel processing usually include silicon or metalalkoxide precursors (Fig 7.3) The most common metal alkoxides are thealkoxysilanes, such as tetramethoxysilane (TMOS) and tetraethoxysilane(TEOS) Water is required for the hydrolysis reactions during which alkoxide(OR) groups are substituted by hydroxyl groups (OH) The reaction isimproved by using alcohol as a solvent, because of the immiscibility of waterand alkoxides The alcohol also enhance the hydrolysis reaction by homoge-nizing the system The silanol (Si-OH) groups condense to form siloxanebonds (Si-O-Si) in condensation reactions Water and alcohol are by-products of the reaction The environmental aspects of the processes can byimproved by the selection of raw materials and by-products

Sol-gel technology was first implemented for several applications by theBoeing Company to treat titanium, followed by treatment of aluminum, tita-nium, and steel components to be repaired through bonding[5]

FIGURE 7.2 Nanostructured Boegel (Boeing’s original name for its sol-gel technology) Interface group [4]

NH2Si OH RO RO

+

NH2Si OH RO RO

NH2Si RO RO

O O

O

OH O

NH2Si RO

NH2Si RO

For example, the Boeing sol-gel system [4] (originally designatedBoegel) uses a dilute aqueous solution of tetra-n-propoxy zirconium (TPOZ)with a silane coupling agent (Fig 7.2) the actual silane is chosen to giveoptimum compatibility as well as having the ability to form strong bondsand to enhance the final surface durability When epoxy based adhesives are

to be used, the choice is often γ-glycidoxypropyl trimethoxy silane(Silquests A-187 offered by Momentive Company, www.momentive.com).Finally, acetic acid is added to the solution to control stability and rate ofreaction This system has been commercialized by Advanced Chemistry &Technology (acquired by 3M Company) as ACs-130 (ACs-130 is a trade-mark of 3M Company)[6]

With the sol-gel process, it is possible to achieve a reproducible surfacethat results in durable bonded interfaces using readily available materials.Using appropriate materials and conditions, sanding, used in conjunctionwith the sol-gel prebond treatment and a bond primer, can yield a robust,durable bond interface system for use at depot sites, repair facilities, oron-aircraft in the field.Figure 7.4shows a demonstration of this process on aB-52 fuselage component

7.2.2 Immersion Etch (Optimized FPL Process)

This method is specified in ASTM D2651-01[8]

1 Remove ink markings and stamped identification by wiping with mercial solvents such as acetone, methyl ethyl ketone, lacquer thinner,and naphtha

com-FIGURE 7.4 Demonstration of the sol-gel prebond process on a composite patch repair of fatigue cracks on a B-52 fuselage component [7]

2 Degrease by vapor degreasing with acetone or toluene, or by immersion

in a non-etching alkaline solution for 10 minutes at 70C
82C A

typi-cal solution is made by mixing 3.0 parts by weight sodium metasilicate,1.5 parts sodium hydroxide, and 0.5 parts sodium dodecylbenzene sulfo-nate such as Nacconols 90G8 (Nacconol 90G is available from theStepan Co.), to 133.0 parts water[9]

3 Immerse for 12
15 minutes at 66C
71C in the etching solution

(Table 7.2)

4 Rinse with water at 60C
65C for 30 minutes.

5 Air-dry in an oven, or use infrared lamps, not above 65.5C.

Figures 7.5 and 7.6show comparisons of smooth aluminum and treatedaluminum surfaces

7.2.3 FPL Paste Etch

FPL paste etch is used for secondary bonding of parts that contain previouslybonded areas, for repair of assemblies, or when the size of parts makesimmersion impractical The parts should be bonded in the temperature range

of 21C
32C A paste is prepared by mixing the sulfuric acid
sodiumdichromate solution described above with finely divided silica (availablefrom Stepan Chemical Co.) [12] or Fuller’s earth (composed mainly of

TABLE 7.2 Composition of FPL Etching Solution

Sulfuric acid (specific gravity 1.84) 10

Sodium dichromate (Na2Cr2O7  H2O) 1

FIGURE 7.5 Scanning electron microscopy (SEM) image of smooth aluminum surface [10]

alumina, silica, iron oxides, lime, magnesia, and water) and then applied tothe surface The paste is applied by brushing and should not be allowed todry after application Polypropylene (or similar) brushes should be usedbecause of their chemical resistance The paste should be allowed to remain

in place for 20
25 minutes Extra coats may be applied to prevent the pastefrom drying out or turning green Clean dry cheesecloth moistened withwater should be used to remove all traces of the paste at the end of the expo-sure period Water may be sprayed on if desired Drying should be carriedout at a maximum of 66C As might be expected, bond strengths obtained

by this technique are somewhat lower than those obtained by immersion[8]

7.2.4 Chromate-Free Etch Process

Acid chromate etching solution is not only toxic and hazardous during use,but also highly harmful if released into water supplies Equally effectivechromate-free etching solutions have been developed (Table 7.3) Russelland Garnis [13] found that an etching solution recommended for the pre-cleaning of aluminum prior to resistance-welding gave excellent results Thissolution consisted of nitric acid and sodium sulfate (N-S) In later modifica-tions, a “P” etch was developed containing sulfuric acid, sodium sulfate,nitric acid, and ferric sulfate

The presence of nitric acid resulted in the production of oxides of gen when aluminum was treated These oxides are toxic and must be vented

nitro-In an effort to eliminate the necessity for venting the toxic etching fumes, anew etchant composition called “P2” was developed (seeFigs 7.5 and 7.6),FIGURE 7.6 SEM micrograph of etched aluminum surface by FPL (sulfuric acid
dichromate etch) and P2 (a chromate-free acid solution) methods [11]

which does not emit any appreciable fumes and results in good bond strengthand improved durability[13
15].

Degreasing or solvent cleaning may be carried out prior to using the P2etch using the procedure described in Section 7.2.4 The composition of P2etching solution is given inTable 7.4

To prepare a liter solution, the acid is added to approximately half liter of water while constantly stirring Ferric sulfate is then added andmixing is continued Next, water is added to bring the volume to 1 liter Thesolution is heated to 60C
65C and the parts are immersed in this solution

one-for 12
15 minutes Follow by rinsing in agitated tap water for 2 minutes

A second rinse, also at room temperature, using deionized water is sprayed

on the part to rinse off the tap water[13,16] This sulfuric-acid-ferric-sulfateetch yields bonds at least equal to those made using the sulfuric acid
dichromate (FPL) etch When used as a deoxidizer prior to phosphoric acidanodizing (PAA) (see below), the results are essentially equal to those usingthe sulfuric-acid
dichromate etch In a variation of this process, the finalrinse lasts 1
3 minutes in demineralized water at an ambient air temperature

of up to 71C, followed by drying in ambient air up to 71C[16].

7.2.5 Anodization

The anodization process is sometimes used for bare (nonclad) aluminummachined or chem-milled parts that must be protected against corrosion

TABLE 7.3 A Comparison of Corrosion Protection Performance of

Chromate Free Iridite NCP with Chromate[7]

Corrosion Performance Aluminum alloy 5052 6022 3003 1100 6111 6061 A356 Salt spray hours

Iridite NCP 1000 1 1000 1 1000 1 1000 1 1000 1 1000 1 576 Chromate 1000 1 1000 1 168 432 432 648 1000 1

TABLE 7.4 Composition of P2 Etching Solution

Anodic coatings include chromic acid (CAA), sulfuric acid (SAA), ric acid, boric sulfuric acid (BSAA) anodization processes The anodizingprocess involves an electrolytic treatment of metals during whichstable films or coatings are formed on the surface of the metals Anodic coat-ings can be formed on aluminum alloys in a wide variety of electrolytes,using either alternating or direct current.

phospho-Anodizing was first applied on an industrial scale in 1923 to protect minum seaplanes parts from corrosion Early on, chromic acid anodization(CAA) was the process of choice, sometimes called the Bengough
Stuartprocess as documented in British defense specification DEF STAN 03-24/3.Oxalic acid anodizing was patented in Japan in 1923 and later widelyused in Germany, particularly for architectural applications Anodized alumi-num extrusion was a popular architectural material in the 1960s and 1970s,but has since been displaced by cheaper plastics and powder coating A vari-ety of phosphoric acid processes are among the recent new development inpretreatment of aluminum parts for adhesive bonding or painting A widevariety of rather complex variations of anodizing processes using phosphoricacid continue to be developed The trend in military and industrial standards

alu-is to classify the anodization processes by coating properties in addition tothe identification of process chemistry

7.2.5.1 Chromic Acid Anodization (CAA)

CAA was the first major commercial pretreatment method for aluminum andhas remained in use in spite of requirements for a complicated voltage cycleeven though it has been found to be unnecessary Further it is highly toxicbecause of chromium and generates hazardous waste There have been var-iants of this process over the years One involves use of sulfuric and chromicacid anodizing processes, patented by Gower and O’Brien[17]

The most widely used anodizing specification is MIL-A-8625 TheUnited States Military Specification (Mil Spec) A-8625 specifies the CAAprocess, which has been adopted by the American Anodizing Council (AAC)

[18] Table 7.5 shows the types of coatings obtained from anodizationprocesses

Table 7.6 provides data on different types of chromic acid anodization.Type 1 is chromic acid anodization Other anodizing specifications includeMIL-A-63576, AMS 2469, AMS 2470, AMS 2471, AMS 2472, AMS 2482,ASTM B580, ASTM D3933, ISO 10074, and BS 5599 AMS 2468 is obso-lete None of these specifications defines a detailed process or chemistry, butrather a set of tests and quality assurance measures which the anodized prod-uct must meet BS 1615 provides guidance in the selection of alloys foranodizing For British defense work, detailed chromic and sulfuric anodizingprocesses are described by DEF STAN 03-24/3 and DEF STAN 03-25/3respectively

Color will vary from clear to dark gray depending on alloy Copper ing alloys only yield gray colors.

bear-The anodized aluminum layer is grown by passing a direct currentthrough an electrolytic solution, with the aluminum part serving as the anode(the positive electrode) The current releases hydrogen at the cathode (the

TABLE 7.5 Classification of Coatings Obtained from Anodization

Processes According to Mil Spec 8625 F

Conventional coatings produced from chromic acid bath 0.5 µ
7.6 µ (microns)

Low voltage chromic acid anodizing (20 volts)

Used for 7xxx series alloys

Type I Chromic acid anodized coating This process is used principally for the

treatment of aircraft parts An example is the Bengough-Stewart process where a 30
50 g/I chromic acid bath is maintained at 100  F and the voltage is gradually raised to 50 V Adjustments are made for high copper, zinc, and silicon alloys Coating weights must be greater than 200 mg/ft 2 Criteria for corrosion resistance, paint adhesion, and paint adhesion testing must be specified.

Type IB Low voltage (20 V) chromic acid anodized coating Typically associated

with higher temperature, more concentrated chromic acid electrolytes Coating weights must be greater than 200 mg/ft2 Criteria for corrosion resistance, paint adhesion, and paint adhesion testing must be specified Type IC Anodized coating produced in a non-chromic acid electrolyte As with

other Type I coating processes, the treatment is designed to impart

corrosion resistance, paint adhesion, and/or fatigue resistance to an

aluminum part Coating weights must fall between 200 and 700 mg/ft 2 Criteria for corrosion resistance, paint adhesion, and paint adhesion testing must be specified.

negative electrode) and oxygen at the surface of the aluminum anode, ing a build-up of aluminum oxide Alternating current and pulsed currentcould be used but rarely are used The voltage required by various solutionsranges from 1 to 300 V DC although the common range is 15
21 V.

creat-Aluminum anodizing is usually performed[19]in an acid solution whichslowly dissolves the aluminum oxide The acid impact is balanced with theoxidation rate to form a coating with nanopores, 10
150 nm in diameter.These pores are what allow the electrolyte solution and current to reach thealuminum substrate and continue growing the coating to greater thicknessbeyond what is produced by autopassivation However, these same poreswill later permit air or water to reach the substrate and initiate corrosion ifnot sealed They are often filled with colored dyes and/or corrosion inhibitorsbefore sealing Because the dye is only superficial, the underlying oxide maycontinue to provide corrosion protection even if minor wear and scratchesmay break through the dyed layer

Conditions such as electrolyte concentration, acidity, solution ture, and current must be controlled to allow the formation of a consistentoxide layer (Table 7.7) Harder, thicker films tend to be produced by moredilute solutions at lower temperatures with higher voltages and currents Thefilm thickness can range from under 0.5 micrometers for bright decorativework up to 150 micrometers for architectural applications

tempera-7.2.5.2 Sulfuric Acid Anodization (SAA)

Mil Spec 8625 specifies Type II, which is sulfuric acid anodization (SAA)and Type III, which is a sulfuric acid hardcoat anodization (Table 7.5).Sulfuric acid is the most widely used solution to produce anodized coating(Table 7.7) Coatings of moderate thickness 1.8µm
25 µm are known asType II in North America, as named by MIL-A-8625, while coatings thickerthan 25µm (0.001") are known as Type III, hardcoat, hard anodizing, orengineered anodizing

Very thin coatings similar to those produced by chromic anodizing areknown as Type IIB Thick coatings require more process control, and areproduced in a refrigerated tank near the freezing point of water with highervoltages than the thinner coatings Hard anodized coatings can be obtained

in the thickness range of 25
150 microns Anodizing thickness increaseswear resistance, corrosion resistance, ability to retain lubricants and PTFEcoatings, and electrical and thermal insulation Standards for thin (Soft/Standard) sulfuric anodizing are given by MIL-A-8625 (Types II and IIB),AMS 2471 (undyed), and AMS 2472 (dyed), BS EN ISO 12373/1 (decora-tive), BS EN 3987 (Architectural) Standards for thick sulfuric anodizing aregiven by MIL-A-8625 (Type III), AMS 2469, BS 5599, BS EN 2536 and theobsolete AMS 2468 and DEF STAN 03-26/1

TABLE 7.7 Processing Steps for Different Aluminum Anodization[20]

CAAa PPA Bell Helicopter Fokker-DIN BSAA

Cleaning Deoxidation

Anodization

dry, T , 65  C

5
15 min, ,43  C air dry, T , 71  C

20
25  C Seal

75
125 ppm CrO 3

82
85  C 7
9 min, air dry

Raise to 50 V Hold for 10 min

5 min, 20
25  C air dry, T , 60  C

3
15 min, ,35  C air dry,

7.2.5.3 Phosphoric Acid

Phosphoric acid anodizing (PAA) is a widely used method, as described inASTM D3933-98 (2004)[21] In short, the process involves immersing thepart in a 9%
12% solution of phosphoric acid at 19C
25C at anywhere

from 9 to 16 volts under direct current (DC) for 20
25 minutes Rinsing,drying, inspecting, and priming (Table 7.8) all follow this step Durabilitydata obtained by this method is slightly better than that obtained by using anetch[16]

Phosphoric acid anodizing (PAA) was studied by Boeing Aircraft in theearly 1960s and commercially introduced in 1974 This process is less criti-cally dependent than etching on processing variables such as the timebetween treatment and rinsing It is also possible to use polarized light as aquality control test for the anodizing pretreatment The oxide layer formed

by this process is much thicker and the “whiskers” are longer than with mic acid etching, although the thickness of the anodic oxide is dependent onthe nature of the aluminum alloy being treated Phosphoric acid anodizingproduces surfaces that are more resistant to hydration than those producedwith other anodizing methods, including chromic acid[22] Phosphoric acidanodizing is also known to give more consistent results in durability studiesthan chromic acid etching

chro-7.2.5.4 Boric Sulfuric Acids Anodization (BSAA)

Chromium (hexavalent) has serious health and safety problems and is closelyregulated, hence a number of chromate-free processes have been developed

An important process is boric
sulfuric acid anodization (BSAA) [5]which,while not as popular as PAA or CAA, is used in some aerospace applications

by the Boeing Corporation The BSAA oxide exhibits a porous morphologyintermediate to those of PAA and CAA (Fig 7.7) although its total oxidethickness is closer to that of CAA With regard to its chemistry, a smallamount of sulfur is incorporated in the oxide surface, but nodetectable boron Durability results are similar to those for PAA and CAA

Table 7.7shows a typical set of processing conditions for BSAA

TABLE 7.8 Phosphoric Acid Anodization (PAA) Conditions

FIGURE 7.7 Scanning electron micrographs of BSAA oxide at different magnifications [20]

7.2.6 Brush Plated Etch (Stylus Method)

This method of electrochemical surface preparation is not broadly known,but has been effectively applied for small batch runs where tanks and othercapital-intensive methods might have been used Almost all of the solutions,

“brushes” (or styli), and electrical power supplies are proprietary When thismethod is used, the entire “family” of proprietary items should be used(mixed usage of items between vendors should not occur) Additional detailscan be found in ASTM D2651-01

7.2.7 Ciba Laser Pretreatment (CLP)

The CLP process for the treatment of aluminum is comprised of a mere twosteps (Fig 7.8):

1 Primer application (incl evaporation of solvent)

2 Laser treatment

Laser instruments, from small mobile lasers to high-performance ary laser units, accommodate a broad range of CLP-uses, including repair tohigh volume production Speed of the treatment operation depends on thepower of the laser apparatus For example, a CLP equipped with a high-performance laser is easily able to pretreat a width of 4 cm at a speed of

station-8 m/min; this is not the upper limit The choice of the lasers allows CLPwith short cycle times The primer applicator and the laser can be adapted toindustrial robots and in this way CLP can be integrated as an in-line pretreat-ment into the overall production process[24]

CLP was developed for typical substrates used in the automotive industrybut as it is equally efficacious in pretreating aluminum, titanium, and stain-less steel, it could work for the aerospace parts The process is a two-stepone where the initial stage is to prime and then dry/stove the substrate to be

FIGURE 7.8 Schematic diagram of the Ciba laser pretreatment [25]

bonded Although the literature does not give examples of the primer used, it

is fairly clear that a suitable surface protection or corrosion protection primercould be utilized; by inference, this could be extended to a silane-basedprimer The area to be bonded, and only the area to be bonded, is thenexposed to the beam of a suitable laser; laser type, power, and speed of treat-ment have to be optimized for each type of substrate[26]

A comparison of a wet pretreatment and a CLP line is presented in

Figure 7.9 The most important advantage of CLP is that it is tally friendly Wet treatment methods generate toxic by-products which must

environmen-be disposed in the face of increasingly stricter regulations around the world

7.3 BERYLLIUM

Beryllium and its alloys must be heated with care Handling and processingproduce dust, chips, scale, slivers, mists, or fumes Air-borne particles of ber-yllium and beryllium oxide are extremely toxic with serious latent effects.Abrasives and chemicals used with beryllium must be properly disposed of

[2] One procedure is to degrease with trichloroethylene, followed by sion in the etching solution (Table 7.9) for 5
10 minutes at 20C[27].

immer-Rinse in distilled water after washing in tap water and oven-dry for 10minutes at 121C
177C Caution should be exercised because beryllium

reacts quickly with methyl alcohol, fluorocarbons, perchloroethylene, andmethyl ethyl ketone/Freonsand can be pitted by long-term exposure to tapwater containing chlorides or sulfates[27]

A proprietary coating used to provide a corrosion-resistant barrier isBerylcoat “D”, currently available from Materion Corporation, Inc.,Mayfield Heights, OH 44124 (http://materion.com)

FIGURE 7.9 Comparison of wet treatment and CLP lines [25]

7.4 BRASS

Brass is an alloy of copper and zinc Sandblasting or other mechanical means

of surface preparation may be used The following procedure combinesmechanical and chemical treatment[28]:

1 Abrasive blast, using either dry or wet methods Particle size is not cially critical

espe-2 Rinse with deionized water

3 Treat with a 5% solution of sodium dichromate in deionized water

4 Rinse in deionized water

5 Dry

Another method is the following[27,29]:

1 Degrease in trichloroethylene

2 Immerse for 5 min at 20C in etching solution (Table 7.10).

3 Rinse in water below 65C.

4 Re-etch in the acid solution for 5 minutes at 49C.

5 Rinse in distilled water after washing

6 Dry in air (temperatures of washing and drying must not exceed 65C).

7.5 BRONZE

Bronze is an alloy of copper and tin The surface treatment involving zincoxide, sulfuric acid, and nitric acid given above for brass is satisfactory forbronze

TABLE 7.9 Composition of Etching Solution for Beryllium

TABLE 7.10 Composition of Brass Etching Solution

Nitric acid, 67% (specific gravity 1.41) 360

7.6 CADMIUM

Cadmium is ordinarily used as a coating on steel It can best be made bondable

by electroplating with silver or nickel Another procedure is the following[30]:

1 Degrease or solvent clean with trichloroethylene

2 Scour with a commercial, nonchlorinated abrasive cleaner [31] (such asAjaxsfrom Phoenix Brands)

3 Rinse with distilled water

4 Dry with clean, filtered air at room temperature

It may be desirable to use a primer or sealant Adhesive choice is larly important with cadmium coatings[30]

particu-7.7 COPPER AND COPPER ALLOYS

Copper is used in three basic forms: pure, alloyed with zinc (brass), andalloyed with tin (bronze) Copper has a tendency to form brittle amine com-pounds with curing agents from some adhesive systems The most successfulsurface treatments are black oxide (see below), and chromate conversioncoatings, which are especially recommended when the adhesive is slightlycorrosive to copper Other treatment methods have also been listed

7.7.1 Nitric Acid
Sodium Chlorite (Black Oxide)

This method is intended for relatively pure copper alloys containing over95% copper [8] It tends to leave a stable surface It is not recommended foruse with adhesives that contain chlorides or for hot bonding to polyethylene.The procedure follows:

1 Degrease

2 Immerse in the etching solution for 30 seconds at room temperature Thesolution is made by mixing 30 parts of nitric acid (70% technical) and 90parts of water, all by volume

3 Rinse in running water and transfer immediately to the next solution out allowing the parts to dry Immerse for 2
3 minutes at 93C
102C

with-in a bath (see data with-inTable 7.11) This solution should not be boiled

4 Rinse thoroughly in running water until a neutral test is produced whenindicated by pH indicator paper

5 Air-dry

6 Bond as soon as possible (no later than 12 hours after surface treatment).7.7.2 Nitric Acid
Ferric Chloride

The procedure is as follows:

1 Degrease (seeSection 7.2.2)

2 Immerse for 1
2 minutes at room temperature in the following solution

by weight:

197 parts water

30 parts nitric acid (sp gr 1.42)

15 parts ferric chloride solution (42%)

3 Rinse thoroughly

4 Dry as quickly as possible

5 Apply adhesive immediately

7.7.3 Nitric Acid

The procedure is as follows:

1 Bright dip in concentrated nitric acid at 16C
21C for 15 seconds or

until all corrosion has disappeared

2 Rinse thoroughly

3 Dry as quickly as possible

4 Apply adhesive immediately

7.7.4 Acid Etch (Sulfuric Acid
Dichromate
Ferric Sulfate)

The procedure is as follows:

1 Remove surface contamination by sanding, wire brushing, or sand ing, if necessary

blast-2 Degrease

3 Immerse for 10 minutes at 66C in the solution given inTable 7.12.

4 Rinse (in water at or below room temperature)

5 Dry

6 Immerse the parts until a bright, clean surface has been obtained in thesolution described inTable 7.13

7 Rinse using cold tap water

8 Dip in concentrated ammonium hydroxide

9 Rinse in cold tap water

TABLE 7.11 Composition of Copper Stabilization Bath

Solution Component Concentration (g/l)

10 Dry quickly.

11 Apply adhesive immediately

7.8 GOLD

Use methods given for platinum

7.9 MAGNESIUM AND MAGNESIUM ALLOYS

The surface preparation methods for magnesium alloys are closely associatedwith corrosion prevention Magnesium is highly reactive, so corrosion-preventive coatings must be applied for most service applications The majorproblem is to apply a sufficient thickness of coating to prevent corrosion, butnot so thick that the bond fails cohesively in the coating[8] (See Chapter 5.)

7.9.1 Alkaline-Detergent Solution

The procedure is as follows:

1 Degrease using procedure described inSection 7.2.2

2 Immerse for 10 minutes at 60C
71C in an alkaline-detergent solution,

described inTable 7.14

3 Rinse thoroughly

4 Dry at a temperature below 60C.

TABLE 7.12 Composition of Copper Etching Solution

Sulfuric acid (specific gravity 1.84) 75

Ferric sulfate (commercial grade) 1

TABLE 7.13 Composition of FPL Etching Solution

Sulfuric acid (specific gravity 1.84) 2

Sodium dichromate (Na2Cr2O7  2H2O) 1

7.9.2 Hot Chromic Acid

1 Degrease as perSection 7.2.2

2 Immerse for 10 minutes at 71C
88C in a solution comprised of 1 part

chromic oxide (CrO3) and 4 parts distilled water

3 Rinse thoroughly

4 Dry at a temperature below 60C.

The methods inSections 7.9.1and7.9.2can be run consecutively with anaqueous wash in between to give improved bond strengths

7.9.3 Sodium Hydroxide
Chromic Acid

1 Degrease (as perSection 7.2.2)

2 Immerse for 5
10 minutes at 63C
79C in a solution of:

12 parts by weight water

1 part sodium hydroxide (commercial grade)

3 Rinse in water at or below room temperature

4 Immerse for 5
15 minutes at room temperature in the solution whosedetails are shown inTable 7.15

5 Rinse thoroughly

6 Dry at a temperature below 60C.

TABLE 7.14 Composition of Magnesium Etching Solution

TABLE 7.15 Composition of Magnesium Etching Solution

7.9.4 Anodic Treatments

Light anodic treatment and various corrosion-preventive treatments producegood surfaces for adhesive bonding These treatments have been developed

by magnesium alloy producers such as Dow Chemical Company, Midland,

MI, and others Details are available from the ASM Metals Handbook, Vol V

[32]and MIL-M-45202, Type I, Classes 1, 2, and 3[33]

7.9.5 Conversion Coatings and Wash Primers [8]

Some dichromate conversion coatings and wash primers designed for sion protection can be used for adhesive bonding Preliminary tests should

corro-be carried out to determine the suitability of the process corro-before its tance Details can be found in the ASM Metals Handbook, Vol V [32] andMIL-M-3171[34]

accep-7.10 NICKEL AND NICKEL ALLOYS

The steps for surface treatment of nickel and its alloys are described below

7.10.1 Abrasive Cleaning [2]

1 Solvent clean, preferably using vapor degreasing

2 Abrade with 180
2grit paper, or grit-blast with aluminum oxide mesh abrasive

40-3 Solvent clean again, according to step 1

7.10.2 Nitric Acid Etch [27]

1 Vapor degrease in trichloroethylene

2 Etch for 4
6 seconds at room temperature (B20C) in concentrated

nitric acid (sp gr 1.41)

3 Wash in cold and hot water, followed by a distilled-water rinse

4 Air-dry at 40C.

7.10.3 Sulfuric
Nitric Acid Pickle [2,35]

1 Immerse parts for 5
20 seconds at room temperature in the solutionshown inTable 7.16

2 Rinse in cold water

3 Immerse in a 1%
2% ammonia solution for a few seconds

4 Rinse thoroughly in distilled water

5 Dry at temperatures up to 65.5C.