Failure analysis on metal automotive production parts

Importance of the topic

Automotive body panels rely on thin multilayer coatings (electro-galvanized zinc, zinc phosphate, primer, and paint) to provide corrosion protection and long-term structural integrity. Local coating failures lead to paint delamination, accelerated corrosion and potential safety and warranty issues. Rapid and accurate identification of surface and subsurface contaminants that compromise coating adhesion is therefore essential for quality control in steel production and automotive manufacturing, and for root-cause failure analysis that enables corrective actions upstream in the supply chain.

Objectives and overview of the study

This application note demonstrates a streamlined SEM+EDS workflow to identify the root cause of paint peeling on an automotive door panel. The goals were to (1) locate and characterize surface defects, (2) determine their chemical composition, (3) check for subsurface inclusions that may prevent proper coating adhesion, and (4) illustrate how an integrated SEM/EDS platform accelerates failure analysis compared with traditional separate-acquisition approaches.

Methodology

The sample was a car door made from ultra-low-carbon, titanium-stabilized steel with a standard electro-galvanized zinc coating (typical zinc thickness 5–8 µm) topped by a porous zinc-phosphate conversion layer and primer. Initial investigation targeted visibly peeling regions on the component surface.

Key experimental steps:

• Low- and higher-magnification SEM imaging to document morphology and identify candidate particles and uncoated areas.
• Concurrent EDS mapping and point/area analyses to obtain qualitative elemental distributions and semiquantitative compositions directly linked to the SEM image.
• Cross-section preparation and imaging to assess the coating stack and presence of subsurface inclusions beneath the zinc layer.

Used instrumentation

The study used a Thermo Scientific Axia ChemiSEM featuring an integrated, always-on EDS detector that provides immediate elemental data during imaging. Typical operating conditions reported in the analysis included acceleration voltages of 12–15 keV, beam currents of ~0.44–0.85 nA, and acquisition times from 30 to 60 seconds for area analyses. Backscattered electron (BSE) imaging and elemental maps (Ca, Si, F, Fe, C, O, etc.) were collected to correlate contrast with chemistry.

Main results and discussion

Imaging of the peeling region revealed localized areas with absent coating and visible foreign particles. On-surface and subsurface analyses consistently showed particles and inclusions dominated by calcium, silicon and fluorine, with additional signals of oxygen and carbon. A representative 30-second area quantification indicated large atomic percentages of oxygen, calcium and silicon, with notable fluorine content.

Interpretation:

• The elemental signature (Ca, Si, F, variable C and O) is consistent with residues of mold flux (mold powder) used in continuous steel casting. Mold fluxes are complex synthetic slags composed mainly of SiO2 and CaO with additions such as CaF2 and carbonaceous material to tune viscosity and lubrication.
• Surface inhomogeneities or entrained droplets of mold powder on the steel strip interfere with the formation and adhesion of the electrogalvanized zinc layer and the subsequent phosphate/primer layers, producing weak spots that lead to paint flaking and loss of corrosion protection.
• Cross-sectional imaging confirmed subsurface inclusions beneath the zinc coating, demonstrating that contamination can be embedded during casting and later manifest as coating adhesion failures at the component stage.

Benefits and practical applications of the method

Using an integrated SEM/EDS (Axia ChemiSEM) delivered rapid, image-linked elemental information without separate EDS re-acquisition, minimizing operator steps and turnaround time. Practical advantages include:

• Faster root-cause identification of coating defects for production troubleshooting and supplier feedback.
• Ability to distinguish exogenous inclusions (e.g., mold flux) from other contamination types (oils, salts, oxides) based on elemental fingerprinting.
• Improved quality control workflows: targeted cross-sectioning and verification following surface mapping to confirm subsurface sources.

Future trends and potential uses

Opportunities to enhance and scale this approach include:

• Automated SEM/EDS workflows and AI-driven image/chemistry classification to speed defect detection on production samples and enable statistical process control.
• Correlative microscopy combining higher-resolution TEM, Auger, or ToF-SIMS for nanoscale characterization of thin interfacial films and contaminants.
• In-situ or environmental SEM studies to observe coating adhesion and delamination mechanisms under controlled humidity/temperature or electrochemical exposure.
• Integration of failure-analysis feedback into continuous-casting process control to reduce mold flux entrainment and improve strip surface cleanliness prior to galvanizing.

Conclusion

The combined morphological and elemental analyses identified mold-flux-derived Ca–Si–F inclusions as the likely root cause of localized coating detachment on an electro-galvanized automotive panel. The case illustrates how image-linked, always-on EDS in an SEM reduces analysis time and supports effective failure diagnosis and corrective action in steel and automotive coating supply chains. Proactive detection of such inclusions and closed-loop process improvements at the casting or strip-cleaning stages can prevent downstream paint delamination and corrosion failures.

Reference

Thermo Fisher Scientific. Application Note AN0166-EN-03-2021: Failure analysis on metal automotive production parts — Coating defect assessment using the Axia ChemiSEM.