Cracking and low thickness are major obstacles to the high corrosion performance of conversion coating on magnesium alloy. In this work, new principles regarding the design of conversion bath were proposed. The conversion bath should contain two categories of species: G-I species and G-II species. G-I species reacts directly with Mg substrate, forming a cracked inner layer. The pH will be increased due to the reaction of G-I species, promoting the nucleation of supersaturated G-II species. The rapid nucleation of G-II species will form a compact and refined PCC. Due to the high density of nucleation, the PCC will be rich with grain boundaries, which will release the local stress and mitigating the formation of cracks. The ionic product (J_sp) and the critical ionic product (J_(C,sp)) of G-II species are crucial to the designing of conversion bath. The closer J_sp to J_(C,sp), the bigger nucleation density and the smaller grain size one can obtain on Mg substrate, and the better quality PCC one can acquire. The nature of tuning TA/pH is to increase J_sp to the level of J_(C,sp), thus promoting the cluster size of G-II species to its critical nucleus size. This is very similar to the concept of constitutional supersaturation and critical nucleus size. The adjustment of TA/pH should be accompanied by the addition of complexing agent; the acceleration of reaction rate should be achieved by adding oxidizing agent and catalysts. The conversion bath was optimized by adding 0.06 M EDTA4-, 0.1 M NO3- and 0.18 M NH4+ into the benchmark bath, the TA/pH value was further adjusted using NaOH, corresponding to a value of 11.80 and a J_sp value of approximately 1.0 × 10-3. The PCC prepared using the optimized conversion bath shows significant improvement of thickness (ca. 10.0 μm) and corrosion resistance.

Dr. Tao Zhang has been engaged in Corrosion and Protection of Materials for decades. TA/pH theory was founded to present a guideline for designing conversion coating on Mg alloy to overcome two traditional problems, i.e., coating cracking and low thickness. The corrosion performance of conversion coating can be significantly improved based on TA/pH theory instead of empirical method, leading to alternating the polluting chromate pickling by the environment-friendly phosphate conversion coatings. Moreover, the idea of corrosion resistance-conductive coating for Mg alloy was presented, and a series of multifunctional coatings were developed, which have been widely applied in the national economy and national defense. Dr. Tao Zhang has first authored or corresponding-authored 60 SCI scientific publications. He has received several awards and honors including first-class award for science & technology from the Chinese Society of Corrosion and Protection in 2018, the outstanding young scholar award from the Chinese Society of Corrosion and Protection in 2015, second-class award for National defense invention in 2016, Young Top-notch Professionals from the Organization Department of the Central Committee of the CPC in 2015, Hundred Talents from Chinese Academy of Sciences in 2012.