Electrodeposition coating method using lead-free cationic electrodeposition coating composition

An electrodeposition coating method comprising the steps of:

• (a) providing a lead-free cationic electrodeposition coating composition containing an aqueous medium, a binder resin dispersed or dissolved in the aqueous medium, a neutralizing acid, an organic solvent, and a metal catalyst, wherein
• the binder resin contains a sulfonium-modified epoxy resin having a milliequivalent number of sulfonium salt group per 100 g resin of 7 to 45, an amine-modified novolak epoxy resin having a molecular weight of 700 to 6000, an amine-modified bisphenol epoxy resin, and a blocked isocyanate curing agent,
• the lead-free cationic electrodeposition coating composition has a volatile organic content of 1% by weight or less,
• the lead-free cationic electrodeposition coating composition has a coating liquid conductivity of 1000 to 2500 μS/cm,
• said sulfonium-modified epoxy resin is obtained by allowing an epoxy resin to react with a sulfide compound represented by the following formula:
• HO—R—S—R′—OH
• wherein R and R′ are each independently a linear or branched alkylene group having 2 to 8 carbon atoms,
• the ratio by weight of said amine-modified bisphenol epoxy resin to said sulfonium-modified epoxy resin is within a range from 30/70 to 70/30, and
• a coated layer electrodeposited on an object to be coated has a minimum film-forming temperature of 20 to 35° C.;
• (b) adjusting the lead-free cationic electrodeposition coating composition to a temperature of from lower by 2° C. than the minimum film-forming temperature to higher by 6° C. than the minimum film-forming temperature;
• (c) immersing the object to be coated in the lead-free cationic electrodeposition coating composition;
• and
• (d) carrying out electrodeposition using the object to be coated as a cathode under the temperature of step b).