Sandwich Corrosion Test
The Sandwich Corrosion Test is a common method used to evaluate the corrosion resistance of aircraft maintenance materials, particularly those used in the repair and overhaul of aircraft structures. The test involves sandwiching a sample of the material being tested between two metal panels, which are then exposed to a corrosive environment for a specified period of time.
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Sandwich Corrosion Test
- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
Sandwich Corrosion Test Overview
The sandwich corrosion test evaluates the susceptibility of materials – typically metallic alloys and coated surfaces – to corrosion when two similar or dissimilar surfaces are held in close contact with each other. The test simulates conditions found in stacked sheet assemblies, layered structural panels, and close-fitting joints where moisture can become trapped between surfaces and cannot easily drain or evaporate. This type of corrosion geometry is particularly relevant to aircraft fuselage skin assemblies, automotive body panels, and any application where flat metal surfaces are fastened or bonded in face-to-face contact.
The test procedure involves placing material specimens face to face with a defined gap maintained by a controlled spacer or moisture-retaining medium, then subjecting the assembly to cyclic or continuous exposure conditions. The exposure may include immersion in saline or acidified solution, salt spray cycling, or humid aging depending on the applicable specification. After exposure, the specimens are separated and the corroded surfaces are examined for pitting, uniform corrosion, intergranular attack, or exfoliation.
Sandwich corrosion testing is applied during alloy development, coating qualification, and surface treatment evaluation where the closed-face geometry represents a realistic worst-case condition not captured by standard open-surface corrosion tests. It is particularly useful for identifying alloy tempers and surface treatments that resist crevice-type corrosion in aero-structural assemblies.
Sandwich Corrosion Test Scope, Applications, and Benefits
Scope
Sandwich corrosion testing is primarily applied to aluminum alloys, titanium alloys, and their coated or surface-treated variants used in aerospace, automotive, and industrial structural applications. The test simulates the close-contact crevice geometry that traps moisture and accelerates localized corrosion. The test evaluates:
- Susceptibility to crevice and contact corrosion in sandwiched assemblies
- Performance of surface treatments and primers in contact geometry
- Alloy temper selection for corrosion-sensitive structural applications
- Effect of fastener and bondline materials on adjacent metal corrosion
- Comparative ranking of alloy and treatment combinations under identical conditions
Applications
- Aerospace fuselage and wing skin material qualification
- Automotive body panel and closure material evaluation
- Marine structural panel and hull material assessment
- Surface treatment and primer qualification for layered assemblies
- Alloy and temper selection for corrosion-critical structural designs
- Incoming material quality verification for corrosion-sensitive programs
Benefits
- Captures corrosion behavior specific to close-contact geometry
- More representative of service conditions than open-surface corrosion tests
- Identifies material combinations that perform poorly in sandwiched configurations
- Accelerated timeline relative to natural field corrosion exposure
- Supports alloy and treatment down-selection with comparative ranking data
- Generates documentation needed for aerospace and automotive material qualification
Sandwich Corrosion Test Process
Specimen Preparation
Specimens are cut to specified dimensions, cleaned, and prepared with the required surface condition.
1Assembly
Specimens are assembled face to face with the defined gap maintained by a spacer or moisture medium.
2Corrosive Exposure
The assembled specimens are subjected to the specified corrosive environment - saline immersion, salt spray cycling, or humid aging
3Disassembly and Evaluation
Specimens are separated, cleaned, and examined for corrosion type, depth, and distribution.
4Sandwich Corrosion Test Technical Specifications
| Parameter | Details |
|---|---|
| Applicable Materials | Aluminum alloys, titanium alloys, coated metals, surface-treated structural materials |
| Geometry | Face-to-face stacked specimen assembly with controlled gap |
| Exposure Conditions | Saline immersion, salt spray, or humid cycling per specification |
| Applicable Standards | ASTM G78, aerospace and OEM-specific procedures |
| Evaluation Criteria | Visual examination, mass loss, pit depth, intergranular attack assessment |
| Output | Corrosion morphology classification, depth measurements, pass/fail |
Instrumentation Used for Sandwich Corrosion Test
- Corrosion exposure chambers (salt spray, immersion, humidity)
- Precision spacers and assembly fixtures for specimen geometry control
- Analytical balance for mass loss measurement
- Optical microscope for corrosion morphology characterization
- Profilometer or depth gauge for pit depth measurement
- Photographic documentation system
Sandwich Corrosion Test Results and Deliverables
- Photographic documentation of corroded surfaces post-disassembly
- Corrosion morphology classification (pitting, uniform, intergranular, exfoliation)
- Mass loss data where specified by the test procedure
- Pit depth or intergranular penetration depth measurements
- Comparative ranking of material or treatment combinations where multiple specimens tested
- Quality assurance documentation
Frequently Asked Questions
The Sandwich Corrosion Test evaluates the susceptibility of materials or assemblies to corrosion in confined interfaces where moisture and contaminants can become trapped between adjoining surfaces.
The test is often used for coated metals, bonded assemblies, laminated materials, fastened joints, and components used in automotive, aerospace, marine, and industrial applications.
Depending on the material system, the test can help identify crevice corrosion, galvanic corrosion, coating breakdown, and corrosion occurring at bonded or overlapping interfaces.
Following the exposure period, specimens are inspected for signs of corrosion such as staining, pitting, coating degradation, metal loss, or corrosion propagation within the interface.
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