Project's information

Project's title Research on formulation of environmentally friendly passive solutions without hexavalent chromium ions for zinc coatings
Research hosting institution Institute for Tropical Technology
Project leader’s name Dr. Le Ba Thang
Project duration 01/01/2009 - 01/01/2010
Project’s budget 350 000 000 VND
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Goal and objectives of the project
  • Research on formulation of environmentally friendly passive solutions free Cr(VI) for zinc coatings and develop the processes for forming the passive film which possess an atmospheric corrosion resistance equating to those of the conventional passive film  and film treated in the Cr(III) imported passive solutions. The colour of the resulted passive films is white-blue and iridescent. Their costs are reasonable and could be able to apply at pilot scale  or at least in a workshop.
  • The expecting products of this project are three Cr(III) passive solutions which include two white-blue and an iridescent solutions as well as a passive engineering process for  zinc coatings.
Main results


  • The complex systems with Cr(III) to prepare the passive solutions for zinc coatings have been studied and selected. These systems are fluoride; citric acid for the 02 passive solutions giving white blue color and fluoride-malonic acid for a passive solution giving iridescent color.
  • Effects of 10 technological factors involving to the process of the formation  and properties of conversion coatings were investigated. In particular, the additional introduction of Co(II) into solutions giving white blue and iridescent colors with concentrations of 1-1,5 and of 1,5-2 g/l respectively has increased the corrosion performance of the conversion coatings up to 3-4 times.
  • Conversion coatings obtained from the solutions have white-blue and iridescent colors without Cr(VI) ions those have not cracks on the surface. The weight of the iridescent conversion coating is higher than  the white blue about 4 times.
  • Salt spray duration of conversion coatings tested are: 95-120 hours for white blue and 225 hours for iridescent (5% of white rust surface), they are beyond the criteria of registration (48 hours for white blue and 72 hours for iridescent) and can be equal to those of the conventional chromate coatings and  coatings formed from Cr(III)-based imported solutions.
  • In fact, the passive solutions of project are compatible with the zinc coatings that prepared from the different industrial plating baths. From three resulted solution of project, two are compatible with some types of Cr(III)-based passive solutions which have widely used in practice.


  • Having applied at pilot scale in two zinc plating workshops using 3 passive solutions of project.
  • These experiments have shown that the passive solutions are promising in practical applications. The use of these solutions does not change the existing technology processes and in some cases it should not need to replace the passive baths because of compatibility. Feedback from businesses is positive and these businesses are looking forward to continue the cooperation with us in developing Cr(III) passive systems and putting them into actual production.
Novelty and actuality and scientific meaningfulness of the results
  • Having researched successfully on formulation the 03 passive solutions based on Cr(III). The passive solutions are stable when working, the features of conversion coatings are enhanced. The conversion coatings obtained from the studied solutions have corrosion resistance that equals to those of the conventional conversion coatings and of the conversion coatings formed from the imported Cr(III) solutions.
  • Having proposed assumptions about the mechanism of forming the conversion coatings on zinc electroplates by electrochemical measurement.
  • It is the first time the Cr(III)-based passive solutions were prepared in a domestic study.
  • The products of project have contributed actively to the environment protection, with a reasonable price, are able to bring significant benefits for the makers. The results obtained are background for developing the black passive solution and advanced passive solutions  for zinc plating in the future.
Products of the project

Published papers: project has published 01 scientific paper in national journal

  • Le Ba Thang, Nguyen Van Khuong, Nguyen Thi Thanh Huong, Nguyen Van Chien, Truong Thi Hoa, “Influence of the Co(II) ion on properties of the Cr(III)-based onversion coating on Zn electroplating”, Journal of Science and Technology, Vol. 48 (3A), 2010, pp. 39-46.

Training: has guided 01 student for master thesis, 01 student in practice for scientific research, and 01 student for doing graduate thesis:

  • Vuong Truong Son, "Comparison of the properties of Cr(III) and Cr(VI)-based conversion coatings on Zn electroplates", Master Thesis of Science in Chemistry, Hanoi University of Education, 2010.
  • Truong Thi Hoa, "The effects of Co(II), immersion time on the formation and properties of  Cr(III)-based conversion coatings on Zn electroplates”, Course Exercise, Hanoi University of Education, 2010.
  • Truong Thi Hoa, "The effects of the transition metals (Co, Zn, Fe) on the formation and properties of Cr(III)-based conversion coatings on Zn electroplates", Graduate Thesis, Hanoi University of Education, 2011.


  • 03 types of the passive solutions based on Cr (III), 20 liters per each.
  • 03 processes for preparing the passive solutions.
  • 03 technical passive processes for zinc coatings.