Non-chrome Zinc Flake 코팅에 과한 연구

Abstract

A Study of Non-chrome Zinc Flake Coating

Sang Soo Kim

Department of Metallurgical Engineering, Graduate school, Pukyong National University

Abstract

Zinc flake coating of magni 500 system mainly recommended for external and internal threaded fasteners, and is a chromium-free environmentally friendly duplex coating system that consists of a basecoat and topcoat. In first chapter, in addition to the basic properties of the base and top coating agents, optimum coating conditions, corrosion resistance and characteristics of coating film when coated with magni 565 to steel plate were studied. In second chapter, layer thickness and corrosion resistance of M10 bolt coated with magni 565 were investigated to find out the optimum coating conditions, and also some characteristics of the coated bolt were studied. In last chapter, schematic cross section morphology of coating film and the corrosion protection mechanism of magni 565 coating based on the cross section morphology were studied in detail.

The main results obtained in first chapter are as follows ; 1. The system showed a good wettability with contact angle(θ) of 14~32° and surface tension of 35~40 dyne/㎝ at room temperature. Moreover, both the contact angle and surface tension were affected little by the viscosity of coating agent and surface roughness of the steel substrate.

2. Optimal coating conditions for base and top coats by using steel substrate were as follows ; ① Zahn cup viscosity for base and top coats were 60 and 55 sec, respectively. ② RPM and time for base and top coats in dip/spin process were 300, 15 sec and 335, 15 sec, respectively. ③ Curing temperature and time for base and top coats were 250℃, 25 min. and 200℃, 25 min., respectively. The samples coated with optimal conditions showed a great corrosion resistance in salt spray test with 1500 hours or longer of initial appearance time of rust.

3. The coating film on steel plate was composed of overlapping layer of zinc and aluminium flakes, and the thickness of base coat increased with an increase of base coat viscosity.

4. Based on the C-F peaks of 1,1-Difluoroethen homopolymer, it was thought that the base coat was an inorganic polymer bond layer. Meanwhile, the top coat showed C-F peaks of polytetrafluoroethylene with C-H peaks of phenol in FT-IR analysis.

5. From the lower weight loss of base coat in TG analysis, it was thought that cross linking density of base coat was larger than that of top coat. And the glass transition temperature, Tg of base coat was observed at about 100℃, while that of top coat was observed at about 120℃ in DSC analysis. It was thought that the small exothermic reactions observed in DSC curves were due to the thermosetting resins contained in the coating agents and any other thermal reactions were not occurred in film forming process.

6. Compared to the non-coated specimen, the coated sample showed more higher polarization resistance and corrosion potential with lower corrosion current density. And a protection efficiency in the specimen coated with optimal conditions was about 98%.

7. The adhesion of coating layer was good enough to have A~C grade adhesion, and it was even better when coated in the optimized conditions, and influenced little by surface roughness.

8. While aging of film in the ultra violet irradiation test was not observed in one week, a considerable decrease of polarization resistance in EIS test was observed after 2 weeks.

The main results obtained in second chapter are as follows ; 1. Optimal coating conditions for base and top coats for M10 bolt were as follows; ① Zahn cup viscosity for base and top coats were 60 and 55 sec, respectively. ② RPM and time for base and top coats in dip/spin process were 260, 15 sec and 290, 15 sec, respectively. ③ Curing temperature and time for base and top coats were 250℃, 25 min. and 205℃, 25 min., respectively.

2. The samples coated with optimal conditions showed around 10㎛ layer thickness and a great corrosion resistance in salt spray test with 1500 hours or longer of initial appearance time of rust.

3. The bolts with the optimum conditions showed a proper torque in torque/tension test. Torque coefficient k increased with the number of bolting and clamping force of M10 bolt showed significantly lower than that of specified value 28.3kN. It was thought that the repeated bolting made the coating film peel off and powdery.

4. Heat resistance temperature without cracks in coating film was below than 300℃.

5. Observed coating film defects were uncoated areas, excessive coating layer, cracks and unfastened areas and those defects were found mainly in the crest and root of the bolt threads.

The main results obtained in last chapter are as follows ; The base coating film was composed of lamellar zinc flakes, which provides sacrificial cathodic protection. Meanwhile, the top coating film was composed of organic aluminium pigments layer, which provides barrier protection to the corrosion circumstances.