Identifying Organic Contaminants in Sub Tier Castings Using FTIR Fingerprinting
Attenuated total reflectance FTIR identifies organic parting agents on raw castings before expensive coating processes fail on customer lines.
Conversion Coating Failure
Meaning
Surface treatment defects that occur when a chemical layer fails to adhere to a metal substrate or provide corrosion resistance constitute a quality issue in finishing operations. A conversion coating failure leads to the premature oxidation of the base metal and the peeling of subsequent paint or powder layers. This condition is often detected through salt spray testing or cross-hatch adhesion evaluations.
Physical Symptom
Visual inspection of the treated part reveals uneven coloration, powdery residues or bare spots where the chemical reaction did not take place. A conversion coating failure often appears as blistering under the topcoat when the part is exposed to moisture. In some cases, the metal surface remains shiny and bright, indicating that the acid etch or phosphate solution was ineffective.
These signs indicate that the protective barrier is missing or non-uniform.
Chemical Cause
Contamination of the treatment baths with heavy metals or oil residues prevents the formation of a stable crystalline structure. When conversion coating failure occurs, the root cause is frequently traced to improper pH levels or incorrect chemical concentrations in the dipping tanks. Poor rinsing between stages allows alkaline cleaners to carry over into acidic baths, neutralizing the active ingredients.
Temperature fluctuations in the processing line also disrupt the kinetics of the surface reaction.
Legal Liability
Responsibility for the resulting damage is determined by the quality agreement between the metal finisher and the original equipment manufacturer. Under Chinese commercial law, a conversion coating failure is treated as a latent defect that the supplier must rectify at their own expense. The finisher is liable for the cost of stripping the defective parts, reapplying the coating and any associated assembly delays.
Detailed records of bath chemistry and processing times are necessary to defend against claims of negligence or breach of warranty.