The core method for determining if a hot runner has experienced corrosion failure is to combine macroscopic morphological observation, microscopic detection, and operating condition analysis. Chemical corrosion mainly manifests as uniform thinning and a fluoride reaction layer, commonly seen in the processing of fluorine-containing materials such as PFA. Electrochemical corrosion presents as localized pitting corrosion, crevice corrosion, or galvanic corrosion, often occurring in areas of dissimilar metal contact or near cooling water channels.
I. Identification and Judgment of Chemical Corrosion
Chemical corrosion is a direct chemical reaction without the participation of electric current. In hot runners, it is most typically initiated by hydrofluoric acid (HF) produced by the high-temperature decomposition of fluorine-containing materials such as PFA and PTFE.
1. Typical Characteristics
Macroscopic Manifestations: Uniform thinning or pitting corrosion (0.05~0.2mm in diameter) appears on the surface of the cavity or runner.
The surface loses its luster, appearing dull or powdery.
Microscopic Evidence: SEM/EDS analysis reveals Cr and Mo loss, with F enrichment on the surface.
XRD analysis detects metal fluorides (such as FeF₂, CrF₃).
Operating Conditions: Use of fluoroplastics such as PFA and FEP.
Processing temperature > 350℃, residence time > 10 minutes.
Key On-Site Motto: "High temperature and no water indicate chemical corrosion," meaning that corrosion occurring in a dry, high-temperature environment is primarily considered chemical corrosion.
2. Judgment Procedure: After shutdown, disassemble the nozzles and manifold for visual inspection. Use an endoscope to observe the inner wall of the flow channel for grayish-white deposits or pitting. Take samples for EDS composition analysis to confirm the presence of fluorine enrichment.
II. Identification and Judgment of Electrochemical Corrosion:
Electrochemical corrosion is a galvanic cell reaction triggered by a potential difference in an electrolyte environment, accompanied by current generation. It is commonly found in cooling systems or at the contact points of different metals.
1. Typical Characteristics
Macroscopic Manifestations:
Localized deep pits or grooves of corrosion, concentrated at the junctions of dissimilar metals (e.g., copper cooling pipes and steel molds).
A loose, reddish-brown rust layer (Fe₂O₃·nH₂O) or green rust (Cu₂(OH)₃Cl) surrounds the corroded area.
"Through-wall" corrosion channels appear in the crevices.
Microscopic Evidence: Metallographic analysis reveals intergranular corrosion or selective dealloying (e.g., zinc stripping in brass).
Electrochemical testing shows a self-corrosion potential difference > 50mV, indicating a risk of galvanic corrosion.
Operating Condition Clues:
Cooling water pH < 6.5 or conductivity > 1000 μS/cm.
Direct contact between dissimilar metals exists (e.g., copper-steel, aluminum-stainless steel).
Judgment Mnemonic: "Where there is water, it is electrochemical," meaning that corrosion occurring in humid or liquid environments is primarily considered an electrochemical mechanism.
2. Judgment Process
Inspect cooling water interface and insert mating surfaces for water seepage, rust, or bulges.
Measure the potential difference between different metal components (using a multimeter and reference electrode).
Sample corrosion products for XPS or EDS analysis to assess Cl⁻, O²⁻, and other ion enrichment.
III. Rapid On-Site Judgment Checklist
Judgment Dimensions:
Chemical Corrosion
Electrochemical Corrosion
Environment:
High temperature, dry, anhydrous
Humid, with electrolytes (water, salt spray)
Corrosion Morphology:
Uniform thinning, pitting
Local deep pits, grooves, galvanic corrosion
Typical Locations:
Nozzle head, flow divider
Cooling water connector, insert gaps
Accompanying Phenomena:
Material embrittlement, F enrichment
Obvious rust, Cl⁻ enrichment
Key Detections:
EDS for F;
Potential difference measurement;
Cl⁻ detection Engineering recommendations: For PFA machining molds, perform an endoscopic inspection every 300 mold cycles, paying particular attention to the area near the gate; for water cooling systems, test the pH and conductivity of the cooling water quarterly to prevent excessive chloride ions from causing corrosion.
