Nickel (Ni) is not much effective for the resistance whilst Cr, Mo. and N are very effective for the chloride SCC resistance. The ppm limitation is based on the worst case (chloride concentration will be greatly raised) after/during drying the facilities.
Please find the following references.
http://www.technonet.co.kr/qa/view.html?id=9615&code=4100&start=60
http://www.entegris.com/pdf/tech/3000-0458.pdf