I am not sure the requirements of current JIS pressure vessel codes. Traditionally there are no specific requirements for corrosion allowance in pressure vessel codes (ASME, KS, BS, etc.). However if you design the heat exchanger with pressure vessel code, you also have to confirm the code of heat exchanger. Normally the design codes for heat exchangers, boilers, furnaces, and stacks do have some specific requirements for the corrosion allowance. See attachment.
Meanwhile, my memory (more than 10 years ago) indicates the joint efficiency per weld joint detail was somewhat different between JS and KS/ASME. But not sure the current version.
Material Engineer가 해당 운전 조건과 설계 기준을 검토하고 재질별 특성을 고려하여 설계 수명에 안정적일 수 있도록 부식 여유를 결정하지요
따라서 일부 Code의 세부 내용에 의해 최소 두께를 언급할 수는 있겠으나, 그런 내용이 전반적으로 볼때에는 부식 여유는 설계 사항에 따라 갑니다.