How to Choose a Structural Adhesive for Bonding Metal Parts

Choose a toughened epoxy when the metal joint sees high load or heat, a methacrylate when surfaces are lightly oiled or prep is minimal. It holds for rigid steel and aluminum joints. Above about 121 °C continuous service, past typical acrylic limits (3M, 2015), use a heat-cured epoxy (European Aluminium, 2015); confirm any choice with a lap-shear test.

• Epoxy lap-shear strength on as-received aluminum rose from about 6.9 MPa (1,000 psi) with no surface preparation to nearly 38 MPa (5,500 psi) after a chemical etch, while an acrylic stayed essentially constant (3M, 2015).

• European Aluminium recommends an adhesive glass transition temperature 10 to 15 °C above the highest expected operating temperature for a thermally stable structural joint (European Aluminium, 2015).

• For many adhesives, stressed bonded joints are limited in practice to 60 to 80 °C, while heat-curing adhesives reach about 150 to 250 °C (European Aluminium, 2015).

• Toughened acrylic joints on 6082-T651 aluminum lost more strength at 95% relative humidity than when immersed in deionized water or 3 wt% salt solution (del Real-Romero et al., 2006).

This answer covers rigid metal-to-metal joints in steel, stainless steel and wrought aluminum bonded with two-part or heat-cured structural adhesives. It does not cover metal to plastic or composite, cast metals, tapes, or aerospace joints qualified to their own specifications. The values are typical maker and industry data, not design allowables.

Which conditions decide the structural adhesive for metal?

Five conditions decide the structural adhesive for a metal joint: surface condition, service temperature, bond gap, peel and impact loading, and the cure time the line allows. TWI, the UK joining research organization, lists six selection factors: the materials joined, the quality of the substrate surfaces, the forces on the joint, the operating environment, the required durability, and process factors such as cure time and dispensing (TWI, accessed 2026). Those factors carry no ranges, so this guide attaches a measurable condition to each. Surface condition means whether mill or stamping oil stays on the part and which cleaning or pretreatment the line can run. Service temperature means the highest temperature the joint sees under load, in °C. Bond gap means the adhesive thickness in mm that the parts actually produce, including poor fit-up. Loading means whether the joint sees peel, impact or vibration, or mainly shear. Cure time means how soon the part must be handled. For its white paper, the adhesive maker 3M counts as structural those adhesives routinely capable of overlap shear strengths above 6.9 MPa (1,000 psi) on metal at room temperature, and groups them as epoxies, acrylics, urethanes and cyanoacrylates (3M, 2012). For load-bearing metal joints, the practical choice is usually between a toughened epoxy and a methacrylate, which suppliers sell as an acrylic structural adhesive; the MMA (methyl methacrylate) type is the one aimed at metals (3M, 2015).

Which structural adhesive suits oily or lightly prepared steel?

A two-part methacrylate structural adhesive suits oily or lightly prepared steel, because two-part acrylics tend to bond well through many common stamping and forming oils (3M, 2012). 3M reports that acrylic structural adhesives often give the same bond strength with no or minimal surface preparation, including on the slightly oily mill finish of cold rolled steel (3M, 2015). European Aluminium describes two-step acrylics as bonding to lightly contaminated, oily surfaces and two-part acrylics to moderately contaminated ones (European Aluminium, 2015). Room-temperature epoxies behave differently: 3M states that epoxies generally require rigorous cleaning of oils from metal joint surfaces for room temperature bonding (3M, 2012). The exception is a line with an oven. In automotive body shops, heat-cured epoxies are applied over press lubricants, with the adhesive, pretreatment and lubricant chosen as one compatible system (European Aluminium, 2015). Where the steel can be prepared, ASTM D2651-01(2024), a guide, lists mechanical methods for carbon steel such as wire brushing, hand sanding and grit blasting. Etch solutions, abrasive dust and epoxy hardeners carry their own hazards; read each SDS and follow your workplace PPE, ventilation and dust rules before running a preparation step. Methacrylates add a safety condition: the common MMA type has a strong odor and needs good ventilation, is flammable and should be kept in a flammable storage cabinet (3M, 2015; European Aluminium, 2015). Read the flash point on each component's SDS before storage and use. US OSHA treats any liquid with a flashpoint at or below 93 °C as a flammable liquid under 29 CFR 1910.106 (OSHA, accessed 2026); apply that rule, or your jurisdiction's equivalent, to the adhesive and to any acetone or isopropyl alcohol wipe solvent.

Which adhesive suits aluminum joints that see humidity?

For aluminum joints that see humidity, the surface pretreatment decides durability more than the choice between a toughened epoxy and a methacrylate. European Aluminium states that bonding to the naturally formed aluminum oxide does not give the required long-term strength, and that with water or high air humidity in service the natural oxide must be replaced by a stable oxide formed under controlled conditions (European Aluminium, 2015). In practice that means specifying a conversion coating or a thin anodized film, which mill or part suppliers often apply before delivery (European Aluminium, 2015). The same manual limits simple degreasing to moderately stressed joints in a dry environment and generally does not recommend abrasive methods for aluminum (European Aluminium, 2015), although ASTM D2651-01(2024) lists mechanical abrasion among its aluminum options. A study of 6082-T651 aluminum bonded with a toughened acrylic adhesive found that joints at 95% relative humidity lost more strength than joints immersed in deionized or salt water, and that a silane treatment and a sulfo-ferric (P2) etch improved durability in all environments tested (del Real-Romero et al., 2006). To screen a pretreatment and adhesive pair before production, use the wedge test, ASTM D3762-24, which ASTM describes as very discriminating in determining variations in surface preparation and adhesive environmental durability. Ask for wedge test results on your alloy and pretreatment rather than lap-shear values on a generic etched panel.

How does service temperature narrow the choice of metal adhesive?

Service temperature narrows the choice of metal adhesive by setting a floor for the adhesive's glass transition temperature (Tg), the temperature above which the cured adhesive softens from glassy toward rubbery and, under further heating or load, deforms permanently (European Aluminium, 2015). European Aluminium recommends a Tg 10 to 15 °C above the highest expected operating temperature for a thermally stable structural joint (European Aluminium, 2015). The same manual puts the practical limit for stressed joints with many adhesives at 60 to 80 °C, notes that two-part epoxies cured at room temperature can rarely be loaded above 80 °C, and gives about 150 to 250 °C for heat-curing adhesives (European Aluminium, 2015). 3M reports typical maximum continuous operating temperatures of 93 to 121 °C (200 to 250 °F) for acrylics and 149 to 177 °C (300 to 350 °F) for epoxies (3M, 2015). The two sets of figures likely differ because the European Aluminium values concern joints carrying load, while the 3M values are continuous operating limits by chemistry, so check both against the Tg and creep data on the TDS. Moisture lowers the limit further, because absorbed water can plasticize the adhesive and lower its Tg (European Aluminium, 2015). Cold service matters too: an adhesive that behaves like rubber at room temperature can become glassy at subzero conditions, and sensitivity to shock loads rises at low temperatures (European Aluminium, 2015).

Metal bonding adhesive decision table by condition

The metal bonding decision table below matches each joint condition to a structural adhesive chemistry; read every row that applies to your joint and keep the answer that satisfies all of them. Values are typical maker and industry data, not design values, so confirm the chosen chemistry with lap-shear tests on your own metal and preparation:

The row that most often changes the answer in this metal adhesive table is service temperature. A joint that runs continuously above about 121 °C (250 °F) is outside the typical range of acrylics (3M, 2015), so a heat-cured epoxy with a Tg 10 to 15 °C above the maximum operating temperature becomes the candidate whatever the surface condition (European Aluminium, 2015). When two rows conflict, such as oily steel in a hot location, either clean the steel so a toughened epoxy can bond, or test an oil-tolerant heat-cured epoxy; the methacrylate's tolerance for oil does not raise its temperature limit.

What should you ask an adhesive supplier about metal bonding?

Ask an adhesive supplier about metal bonding for data measured on your metal, surface condition and service temperature, because data sheet values come from standard specimens. ASTM D1002-10(2019) warns that using its single-lap shear strengths as design-allowable stresses could lead to product failure, and ISO 4587:2003 states that its lap-shear procedure does not provide design information. Questions to ask:

1. On which metal, pretreatment and bond gap was the published lap-shear value measured, and to which method, ASTM D1002 or ISO 4587?

2. What is the cured Tg, and what strength remains at my maximum service temperature?

3. Has the adhesive been tested over my stamping or forming oil?

4. What durability data exist after humidity or salt exposure on my aluminum pretreatment, such as ASTM D3762-24 wedge test results?

5. What are the open time and fixture time at my shop temperature? 3M gives a rough rule that each 10 °C warmer halves the open time and each 10 °C cooler doubles it (3M, 2012).

6. What are the shelf life and storage conditions? One maker lists 6 to 12 months for acrylics against 15 to 24 months for epoxies, with refrigerated storage required or recommended for many MMA acrylics (3M, 2015).

7. Which flash point and hazard classification does the SDS give for each component?

3M advises that the exact test details be based on the particular project and that buyers not rely solely on the results in the maker's technical data sheets (3M, 2012).

When should you call a bonding specialist about a structural metal joint?

Call a bonding specialist about a structural metal joint when the bond carries primary load, runs hot under sustained stress, joins dissimilar metals, or sees peel that the design cannot remove. European Aluminium states that single-lap shear data taken from a technical data sheet are not valid for design, and that design data must come from actual peel and shear tests that reflect the surface condition used in practice (European Aluminium, 2015). Joints under long-term tensile load at elevated temperature often need application-specific creep tests (European Aluminium, 2015). Mixed metals add thermal strain: most adhesives expand at 30 to 40 ppm/°C, aluminum alloys at about 20 to 25 ppm/°C and steel at 12 ppm/°C, and the joint fails if thermal strain exceeds the bond shear strength (European Aluminium, 2015). Adhesives behave relatively poorly under peel and cleavage loading, so a specialist will often redesign the joint toward shear and compression before changing the adhesive (European Aluminium, 2015). 3M advises that no final decision be made without specific validation testing (3M, 2012). Before that call, Lubinpla can organize the joint conditions from your own TDS, SDS and site history and return one cited document for the specialist to review, with your approval at each step. Bring the drawings, the surface preparation record and the service temperature history to the first meeting.

FAQ

Is a methacrylate as strong as an epoxy on metal?

A methacrylate comes close to an epoxy on metal but typically does not exceed a well-bonded epoxy. 3M states that if an epoxy adheres well to the materials with the given surface preparation, it will typically provide the highest performance and durability of any structural adhesive, and describes acrylics as having performance and durability approaching that of epoxies (3M, 2015). On unprepared aluminum, the acrylic is the more forgiving choice, because its bond strength stayed essentially constant across surface preparations in 3M tests (3M, 2015).

Can a cyanoacrylate bond structural metal parts?

A cyanoacrylate is a poor fit for load-bearing metal joints that must last. 3M describes cyanoacrylates as rigid, with low peel and impact resistance, and not good for long-term applications on metals (3M, 2012). European Aluminium considers them not appropriate for bonding metal parts in automobiles (European Aluminium, 2015).

Does an oil-tolerant adhesive still need surface preparation?

Yes. 3M states that the end user should always clean and prepare surfaces before applying adhesive, and that an acrylic is much more forgiving when proper preparation was not done (3M, 2015). ISO 17212:2012 gives guidelines for preparing metal and plastic surfaces before bonding, for both laboratory evaluation and production.

Which lap-shear test should confirm a metal adhesive choice?

Either ASTM D1002-10(2019), for single-lap metal-to-metal specimens, or ISO 4587:2003, for rigid-to-rigid bonded assemblies, can compare adhesives on your own metal and preparation. Both give comparative values, not design values. For aluminum in humid service, add the ASTM D3762-24 wedge test, which ASTM states correlates with service performance more reliably than conventional lap shear or peel tests.

Working a case like this? Lubinpla works it from your own documents and site history, step by step with your approval. Free to start, no card.

Lubinpla does not sell chemical products and names no brands in recommendations. Our reviewer also leads a specialty chemicals distribution company.

Sources checked 2026-10-13.

Sources

1. TWI. FAQ: What factors affect the choice of adhesive for a specific application? Accessed 2026-10-13. https://www.twi-global.com/technical-knowledge/faqs/faq-what-factors-affect-the-choice-of-adhesive-for-a-specific-application.aspx

2. 3M. Choosing and Using a Structural Adhesive (white paper). 2012. https://multimedia.3m.com/mws/media/1453168O/white-paper-choosing-and-using-a-structural-adhesive-uk-signature.pdf

3. 3M. Acrylic Structural Adhesives: Features and Recent Advancements (white paper). 2015. https://multimedia.3m.com/mws/media/1054052O/acrylic-adhesives-recent-advancements-white-paper.pdf

4. European Aluminium Association. Aluminium Automotive Manual, Joining, 9. Adhesive bonding. Version 2015. https://european-aluminium.eu/wp-content/uploads/2022/11/9-adhesive-bonding_2015.pdf

5. ASTM International. ASTM D2651-01(2024) Standard Guide for Preparation of Metal Surfaces for Adhesive Bonding. 2024. https://www.astm.org/d2651-01r24.html

6. ASTM International. ASTM D1002-10(2019) Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal). 2019. https://www.astm.org/d1002-10r19.html

7. ASTM International. ASTM D3762-24 Standard Test Method for Adhesive-Bonded Surface Durability of Aluminum (Wedge Test). 2024. https://www.astm.org/d3762-24.html

8. ISO. ISO 4587:2003 Adhesives: Determination of tensile lap-shear strength of rigid-to-rigid bonded assemblies. Edition 3, 2003, confirmed 2021. https://committee.iso.org/standard/34852.html

9. ISO. ISO 17212:2012 Structural adhesives: Guidelines for the surface preparation of metals and plastics prior to adhesive bonding. Edition 2, 2012, confirmed 2022. https://committee.iso.org/standard/50596.html

10. del Real-Romero J.C., Cano M., Abenojar J., Martinez M.A. Adhesive bonding of aluminium with structural acrylic adhesives: durability in wet environments. Journal of Adhesion Science and Technology 20(16), 1801-1818. 2006. https://www.iit.comillas.edu/publicacion/revista/en/598/Adhesive%20bonding%20of%20aluminium%20with%20structural%20acrylic%20adhesives:%20durability%20in%20wet%20environments

11. US OSHA. 29 CFR 1910.106 Flammable liquids. Accessed 2026-10-13. https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.106

Written by the Lubinpla team from the sources listed and checked against them before publication.