E-Coating Chassis Parts: Eliminating Internal Corrosion
Internal Corrosion in Stamped Control Arms
A tier-1 automotive supplier producing stamped steel front lower control arms for a major SUV platform discovered a critical corrosion issue during salt spray validation testing. The control arms, fabricated from 2.5 mm HC420LA high-strength low-alloy steel, were designed with closed-section stampings that formed a welded box structure. The liquid spray-applied epoxy primer used in the original paint line could not reach the internal surfaces of these box sections. After 240 hours of neutral salt spray per ASTM B117, corrosion products exuding from the interior weep holes contaminated the exterior coated surface, and 72-hour salt spray showed active red rust inside the box sections that were completely uncoated.
The customer, a major North American OEM, issued a corrective action request (CAR) stating that internal corrosion propagation could weaken the structural section and create a safety concern over the 10-year / 150,000-mile corrosion perforation warranty. The supplier needed a coating solution that could achieve complete coverage of all internal surfaces, including the tight crevices at spot-weld joints and the narrow channels between the double-layered stampings.
Cathodic E-Coating: The Complete Coverage Solution
Cathodic electrodeposition coating (CED), commonly called e-coating, was identified as the only finishing method capable of coating surfaces that are physically inaccessible to spray or dip systems. In e-coating, the parts are submerged in a bath of waterborne epoxy resin and then connected to a DC electrical circuit. The negatively charged paint particles are attracted to the positively charged part, depositing evenly on all conductive surfaces. Because the deposition is electrically driven, paint reaches every surface that is electrically connected, including internal cavities, box sections, and spot-weld gaps.
The process sequence was: alkaline degrease → rinse → zinc phosphate treatment → rinse → deionized water rinse → cathodic e-coat bath at 28±1 °C → ultrafiltration rinse → deionized water rinse → cure oven at 180 °C for 20 minutes.
| Parameter | Spray Epoxy Primer | Cathodic E-Coat |
|---|---|---|
| Coating method | Manual or robotic spray | Submerged electrolytic deposition |
| Internal box section coverage | 0–5% (line-of-sight only) | 100% (electrically driven) |
| Film thickness uniformity | ±30–50% on complex geometry | ±5–10% over entire surface |
| Minimum film thickness at edges | 10–20 µm (thin at edges) | 15–25 µm (consistent at edges) |
| Transfer efficiency | 40–60% (overspray loss) | 95–99% (recycled through UF) |
| VOC content | ~380 g/L | <50 g/L |
| Cure temperature | 140–160 °C (if baked) | 175–185 °C |
Validation Test Results
The e-coated control arms were tested alongside the previous spray-primed parts under identical conditions. The corrosion protection improvement was dramatic across every metric.
| Test | Standard | Spray Primer | E-Coat |
|---|---|---|---|
| Neutral salt spray (ASTM B117) | 500 h, scribe creep | 4–7 mm creep at 500 h | 0.5–1.5 mm creep at 1,000 h |
| Internal box section corrosion | 240 h NSS, section cut | Red rust on 70% of internal surfaces | No corrosion visible |
| Cyclic corrosion (SAE J2334) | 80 cycles (≈10 years field) | Perforation at spot welds at cycle 45 | No perforation at cycle 80 |
| Stone chip resistance (VDA 230-213) | Gravel impact at 2 bar | Coating removal at 60% of impact sites | Coating removal at 5% of impact sites |
| Adhesion (crosshatch, ASTM D3359) | Dry and wet adhesion | 4B (good) dry, 2B (poor) wet | 5B (best) dry and wet |
Production Implementation and Quality Monitoring
The e-coat line was designed for a throughput of 120 control arms per hour, with a 1,200-liter bath volume and continuous ultrafiltration to maintain solids content at 18–22%. The deposition voltage was ramped from 150 V to 280 V over 120 seconds to prevent film rupture and ensure uniform thickness on the complex stamped geometry. Film thickness was monitored at 12 locations per part using a magnetic induction gauge, including two internal box section points reached through pre-cut inspection holes.
In production, the e-coated parts consistently showed 22–26 µm coating thickness on exterior surfaces and 18–22 µm on internal surfaces, exceeding the OEM's 15 µm minimum requirement for internal box sections. The salt spray performance improved from failing at 240 hours to passing 1,000 hours with less than 2 mm scribe creep, meeting the 1,000-hour OEM requirement with margin.
The per-part coating cost increased by $0.80 due to the higher capital amortization of the e-coat line, but warranty claim projections dropped by an estimated $1.20 per part, resulting in a net cost reduction of $0.40 per part. More importantly, the CAR was closed with full customer approval, and the supplier secured an additional 200,000-unit-per-year contract extension based on the demonstrated corrosion performance.
Cathodic e-coating provides unparalleled uniform coverage for complex stamped automotive chassis components. For engineers designing steel structures with enclosed cavities, box sections, or complex internal geometries, e-coating is the only liquid coating technology that achieves complete internal protection without the need for secondary operations or sealants.
