ASTM D2651-90(1995)

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: D 2651 – 90 (Reapproved 1995)
Standard Guide for
Preparation of Metal Surfaces for Adhesive Bonding
This standard is issued under the fixed designation D 2651; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
INTRODUCTION
It is impossible and impractical to present a single surface-preparation method for a metal group,
such as aluminum alloys, stainless steels, magnesium alloys, etc. The wide variety of individual alloys
and heat treatments under each group, the fact that certain adhesives exhibit specific compatibilities
with surface preparation and the complexity and nature of parts being bonded preclude the use of an
all-inclusive procedure for a metal group. Procedures for aluminum alloys are well standardized,
possibly because more bonding has been done with these alloys. Preliminary tests should be conducted
with the specific adhesive and the exact lot of metals to determine performance. This is especially true
for stainless steel. The adhesive manufacturers’ recommendation should also be considered.
1. Scope D 907 Terminology Relating to Adhesives
D 1125 Test Methods for Electrical Conductivity and Re-
1.1 This guide covers procedures that have proved satisfac-
sistivity of Water
tory for preparing various wrought metal surfaces for adhesive
D 2674 Methods of Analysis of Sulfochromate Etch Solu-
bonding. It does not address the preparation of cast metals.
tion Used in Surface Preparation of Aluminum
1.2 Surface preparation methods are included for:
D 3933 Guide for Preparation of Aluminum Surfaces for
Metal Section
Structural Adhesives Bonding (Phosphoric Acid Anodiz-
Aluminum Alloys 7
Stainless Steel 8
ing)
Carbon Steel 9
2.2 ASM International Document:
Titanium Alloys 10
Metals Handbook Volume V
Magnesium Alloys 11
Copper and Copper Alloys 12
2.3 Military Specifications:
MIL-A-8625 Type II, Anodic Coatings for Aluminum and
1.3 Procedures included herein are based on the commercial
Aluminum Alloys
practice of numerous agencies and organizations. The methods
MIL-M-3171 Magnesium Alloy, Processes for Pretreatment
may be revised or supplemented, as necessary, to include
and Prevention of Corrosion
methods based on proven performance.
MIL-M-45202 Magnesium Alloy, Anodic Treatment of
1.4 The surface preparation of metal systems used for
2.4 SAE Standards:
qualification and quality control testing of the adhesive should
Aerospace Material Specification 2470—Anodic Treatment
be agreed upon by both manufacturer and user.
of Aluminum Base Alloy, Chromic Acid Treatment
1.5 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3. Terminology
responsibility of the user of this standard to establish appro-
3.1 Definitions—Many of the terms in this guide are defined
priate safety and health practices and determine the applica-
in Terminology D 907.
bility of regulatory limitations prior to use.
4. Apparatus
2. Referenced Documents
4.1 Trays or Pans, suitable for preparation of laboratory
2.1 ASTM Standards:
specimens and small parts. Generally in production, a series of
This guide is under the jurisdiction of ASTM Committee D-14 on Adhesives
and is the direct responsibility of Subcommittee D14.80 on Metal Bonding Annual Book of ASTM Standards, Vol 15.06.
Adhesives. Annual Book of ASTM Standards, Vol 11.01.
Current edition approved Nov. 30, 1990. Published January 1991. Originally Available from ASM International, Metals Park, OH 44073.
published as D 2651 – 67. Last previous edition D 2651 – 79 (1984). Available from Standardization Documents, Order Desk, Bldg. 4, Section D,
A surfactant that has been found suitable for this purpose consists of a 50 % 700 Robbins Ave., Philadelphia, PA 19111, Attn: NPODS.
mixture of a cationic and a fluochemical surfactant such as Zonyl FSC manufactured Available from the Society of Automotive Engineers (SAE), 400 Common-
by E.I. DuPont, Wilmington, DE 19898. wealth Dr., Warrendale, PA 15096.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 2651
tanks containing the necessary solutions is arranged with Usually, the time should not exceed 8 h and parts should be
overhead cranes to transport parts to be prepared. Tanks should covered or wrapped in Kraft paper. Prepared surfaces can
be lined with suitable materials to resist the solutions, and change their characteristics on standing and adhesives vary
methods for circulating the solutions and heating with tempera- widely in their tolerance of adherend surface conditions. In
ture controls should be included if required. production bonding of complicated or large parts, the time
4.2 Hooks, Baskets, Clamps, or other holding devices, may interval may exceed 8 h. Investigation should be made to
be used to convey parts being prepared. The use of a 300 Series determine the time limitation of the adhesive and the rate of
stainless steel for such equipment is suggested. Other metals change on the surface of the adherend. On the basis of the
may be used if they are resistant to the solution used. results of such an investigation, it may be possible to extend
the length of time and establish standards for the interval
5. Conditions
between surface preparation and adhesive application to fit
5.1 Handling—All parts during and after processing and
production schedules. Contaminating operations in the area
before bonding should be handled as little as possible and only
should be avoided. Especially detrimental are paint or other
with clean, lint-free cotton or nylon fabric gloves.
spraying operations, processes using powdered materials, oil
5.2 Water used for preparing solutions should be distilled,
vapors from pumps and other machinery and spraying of mold
steam condensate, demineralized, deionized or otherwise
release agents.
treated, if necessary, to obtain a condition of not more than 50
5.5 Common Steps:
ppm of solids and have a pH between 5.5 and 10, or not more
5.5.1 Remove ink markings or stamped identifications from
than 50 ppm of dissolved solids and not over 30 micromhos
the metals. This can be accomplished by wiping with a cloth
conductance (see Test Method D 1125). Rinsing may be done
wetted with a variety of commercial solvents, such as acetone,
by spray or by dipping in a tank in which the water is circulated
methyl ethyl ketone, lacquer thinner and naphtha.
and constantly being renewed by an overflow method. After
5.5.2 The preferred degreasing method is vapor degreasing
removing from any rinse the water-break test is commonly
with 1,1,1-trichloroethane. Commercial vapor degreasing
used. If the water film is discontinuous, it indicates that the
equipment is usually used. This equipment has a deep tank
process is unsatisfactory and must be repeated (see 5.5.4).
with provision for heating a chlorinated solvent to 82 to 87°C
Other methods, such as indicator paper and contact angle, have
(180 to 190°F). Parts are suspended in the vapor zone above
been used to determine the effectiveness of the process.
the hot liquid and allowed to remain until there is condensation
Rinsing should be sufficiently long and vigorous to assure
and run off (approximately 5 min). This step is repeated if
removal of soluble residue chemicals, or particles. Critical
necessary until all visible contamination is removed. The parts
specific temperatures are specified for the rinse water in some
are raised above the tank and allowed to dry thoroughly.
processes. Where not specified, temperature is not critical. The
Previously bonded parts, such as honeycomb core materials,
number of times a process may be repeated depends upon the
are usually not degreased due to difficulty in removing residue.
amount of metal removed in relation to the tolerance require-
5.5.3 Following vapor degreasing, mechanical or chemical
ments for the part and whether clad or bare metal is used.
cleaning methods are usually employed.
Generally, not more than two repeat treatments are permitted.
5.5.4 A water-break test is a common method used to
NOTE 1—No entirely foolproof method exists to determine bondability analyze surface cleanliness. This test depends on the observa-
of a metal surface after preparation. Trial bonding followed by testing and
tion that a clean surface (one that is chemically active or polar)
evaluation against mechanical strength standards should be conducted to
will hold a continuous film of water, rather than a series of
determine initial and continuous effectiveness of a method.
isolated droplets. This is known as a water-break-free condi-
5.3 Solutions—The life of solutions should be standardized.
tion. A break in the water film indicates a soiled or contami-
The useful life of solutions depends upon the number and size
nated area. Distilled water should be used in the test, and a
of the parts being prepared. Solutions should be sampled
drainage time of about 30 s should be allowed. Any trace of
periodically and analyzed for materials pertinent to the particu-
residual cleaning solution should be removed or a false
lar treatment method, such as, titration for hexavalent chro-
conclusion may be made. If a water-break-free condition is not
mium (CrO ), iron, chlorides, aluminum, etc. A pH reading or
3 observed on the treated surface, it should not be used for
acid content of trichloroethane vapor degreasing solution, or
bonding. The surface should be recleaned until the test is
both, should be obtained. Standards should be established and
passed. If continuous failures occur, the treating process itself
records kept for such tests on each lot or vat of solution.
should be analyzed to determine the cause of the problem.
Records should indicate the continuous conditions of solutions,
6. Report
such as amount of constituents added to bring solutions within
The report for all methods shall consist of flow charts, record
limits, amount of contaminants present and date prepared (see
sheets or other methods that indicate all steps in the surface
Methods D 2674).
preparation used, with spaces for entries made upon comple-
5.4 Room Conditions for surface preparation as well as
tion of each step.
other adhesive bonding operations should be controlled for
temperatures of 18 to 24°C (65 to 75°F), relative humidity of
ALUMINUM ALLOYS
40 to 65 %, air-filtered to remove dust and pressure maintained
7. Procedure
at slightly higher than ambient.
5.4.1 The time and conditions between surface preparation 7.1 Aluminum alloys may be pretreated with many different
and applying adhesive primer, if used, and bonding are critical. methods, using both acidic and basic cleaners.
D 2651
7.2 A typical pretreatment would normally involve the usually impossible and undesirable to immerse these parts in
following steps:
surface-preparation solutions without destroying or harming
7.2.1 Vapor degrease or non-etching alkaline cleaner, the original adhesive bond. Satisfactory surface preparations
7.2.2 Rinse,
have been accomplished by using paste-type etchants. These
7.2.3 Acid or alkaline, and paste systems can be made by adding fumed silica to the
7.2.4 Rinse.
sulfuric-dichromic acid solutions. The application of these
7.3 After 7.2.4, the parts may be dried or they may be given paste systems allows for their use in fairly controlled areas. The
an anodizing treatment.
etching time is generally 10 to 20 min. Exercise extreme care
7.4 Typical pretreatments are listed in 7.4.1. Refer to the
to ensure removal of all traces of the etchant. A thorough
appropriate literature for details on cleaning techniques and
rinsing with distilled water may be necessary to ensure
procedures.
complete removal. Several commercial products are available.
7.4.1 Alkaline degreasing solutions may be used instead of,
Some commercial compounds under the classification of “wash
or in addition to, vapor degreasing. Commercially available
primers” have been found to perform satisfactorily in place of
proprietary solutions may be used. A common degreasing
the paste type systems. Bond strengths obtained using either of
method is to immerse for 10 min at 70 to 82°C (160 to 180°F)
these surface preparations are somewhat lower than those from
in the following solution by weight: 3.0 parts sodium metasili-
the immersion processes.
cate, 1.5 parts sodium hydroxide, 0.5 parts sodium dodecyl-
7.4.7 Brush Plate Bond Etch (Stylus Method):
benzene sulfonate, such as Nacconol 90G , to 133.0 parts
7.4.7.1 General—This method of electrochemical surface
water.
preparation, although not widely known, has been effectively
7.4.2 Acid etching solutions typically used are the follow-
demonstrated for small batch runs where tanks and other
ing:
capital-intensive methods might have been used. Almost all of
7.4.2.1 Sulfuric Acid/Sodium Dichromate (optimized FPL
the solutions, “brushes” (or styli), and electrical power supplies
etch).
are proprietary. When this method is used, the entire “family”
Material Concentration
of proprietary items should be used; mixed usage of items
Sulfuric Acid (66°Be or Baume) 38 to 41.5 oz/gal (287.9 to 310.0 g/L)
Sodium Dichromate 4.1 to 9.0 oz/gal (28–67.3 g/L) between vendors should not occur.
Aluminum Alloy–2024 bare 0.2 oz/gal (1.5 g/L), min
7.4.7.2 General Precleaning—Removal of grease, oil, and
Temperature 66 to 71°C (150 to 160°F)
other organic soils can be accomplished with virtually any
Immersion Time 12 to 15 min
solvent, including trichlorotrifluoroethane and similar non-
7.4.2.2 P-2 Etch (Sulfuric Acid/Ferric Sulfate).
flammable types.
Material Concentration
Sulfuric acid (6.5 to 9.5 N) 27 to 36 % by weight 7.4.7.3 Mechanical Preparation—Surfaces which are
Ferric sulfate 135 to 165 g/L (18 to 22 oz/gal)
scaled, corroded, or otherwise oxidized should be abraded
Temperature 60 to 65°C (140 to 150°F)
using a nonmetallic abrasive. Aluminum oxide-impregnated
Immersion Time 10 to 12 min
nylon matting, glass-bead blasting and aluminum oxide cloth
7.4.3 Phosphoric Acid Anodizing can be performed in ac-
are typical acceptable abrasives. Abrasive and debris should be
cordance with Guide D 3933.
wiped or water rinsed from the surface.
7.4.4 Sulfuric Acid Anodizing—A method found suitable for
7.4.7.4 Electrochemical Cleaning—This is accomplished
bare aluminum (nonclad), machined, or chemically milled
by immersing the wrapped electrode in an alkaline
parts which must be corrosion protected
...