Evaluation of electrochemical and surface characteristics of conversion coatings on ZM21 magnesium alloy

Abstract

Surface of a metal or material is important and suitable modification of the surface can provide enhanced properties for different applications. Magnesium is the lightest of all structural metals. Its low density (1·8 g cc⁻¹ in alloy form) coupled with favourable casting properties, makes magnesium the material of choice for light weight components. But the metal is chemically highly reactive and prone to corrosion. Hence modification of the surface is important. The magnesium alloy under consideration is Mg–Zn–Mn, (ZM21) alloy system, and the composition is 2·03Zn–0·81Mn over which phosphate–permanganate, chromate conversion coatings were applied. The samples were subjected to surface roughness measurements, salt spray exposure tests, electrochemical potentiostatic polarisation studies in corrosive environment such as NaCl, NaOH, Na₂SO₄, NaHCO₃ and NH₄NO₃. Microstructural examination, SEM analysis (for surface morphology), Fourier transform infrared (FTIR) spectral investigation (for determining the functional group) and X-ray diffraction (XRD) measurements (for identifying the compound) have also been employed for assessing the surface characteristics of this alloy. The results, in general, indicate that coatings provide enhanced corrosion resistance. Scanning electron microscopy of chromate conversion coating on ZM21 Mg alloy shows an intense transgranular crack on the coated film surrounded by a scattered distribution of spongy nature of film product. Scanning electron microscopy of phosphate conversion coating on ZM21 Mg alloy reveals a complete coverage showing a dense population and continuous network of film product. The formation of bonds among phosphorous, oxygen and hydrogen elements was shown by FTIR studies. X-ray diffraction analysis revealed the formation of compounds such as MgMnO₃, Mg(OH)₂, Mg₂H, HPO₄ in phosphate–permanganate conversion coated alloy, and compounds such as ZnCr₂O₄, Cr–O, CrO₂, MnCrO₃ in chromate conversion coating.

Keywords

ZM21 MG ALLOY CONVERSION COATINGS PERMANGANATE-PHOSPHATE CHROMATE COATINGS

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References

Magnesium encyclopedia , available at: www.magnesium.com

Natarajan

, Sivan

, Gerald Tennyson

and Ravikiran

: Corros. Prevent. Control, 2004, 51, (4), 142–164.

Gray

J. E.

and Lu

: Alloy. Compd, 2002, 336, 88–113.

Sharma

A. K.

, Suresh

, Bhojraj

, Narayanamurthy

and Sahu

R. P.

: Met. Finish., 1998, 96, 10.

Niu

L. Y.

, Jiang

Z. H.

, Li

G. Y.

, Gu

C. D.

and Lian

J. S.

: Surf Coat. Technol, 2006, 200, (9), 3021–3026.

Lin

C. S.

, Lin

H. C.

, Lin

K. M.

and Lai

W. C.

: Corros. Sci, 2006, 48, (1), 93–109.

Rongeat

, Grosjean

M.-H.

, Ruggeri

, Dehmas

, Bourlot

, Marcotte

and Roué

: J. Power Sources, 2006, 158, (1), 747–753.

Zhao

, Wu

S. S.

, An

, Luo

J. R.

, Fukuda

and Nakae

: Mater. Chem. Phys., 2006, 99, (1), 54–60.

Blawert

, Heitmann

, Dietzel

, Nykyforchyn

H. M.

and Klapkiv

M. D.

: Surf Coat. Technol, 2005, 200, (1-4), 68–72.

10.

Zhao

, Wu

S. S.

, Luo

J. R.

, Fukuda

and Nakae

: Surf Coat. Technol., 2006, 200, (18-19), 5407–5412.

11.

Brunelli

, Dabalà

, Calliari

and Magrini

: Corros. Sci., 2005, 47, (4), 989–1000.

12.

Zucchi

, Grassi

, Frignani

, Monticelli

and Trabanelli

: Surf Coat. Technol, 2006, 200, (12), 4136–4143.

13.

Bethencourt

, Botana

F. J.

, Cano

M. J.

and Marcos

: Appl. Surf Sci, 2004, 238, (1-4), 278–281.

14.

Huo

H. W.

, Li

and Wang

F. H.

: Corros. Sci, 2004, 46, (6), 1467–1477.

15.

Kouisni

, Azzi

, Zertoubi

, Dalard

and Maximovitch

: Surf Coat. Technol, 2004, 185, (1), 58–67.

16.

Dabalà

, Brunelli

, Napolitani

and Magrini

: Surf Coat. Technol, 2003, 172, (2-3), 227–232.

17.

Umehara

, Takaya

and Terauchi

: Surf Coat. Technol., 2003, 169-170, 666–669.

18.

Chong

K. Z.

and Shih

T. S.

: Mater. Chem. Phys., 2003, 80, (1), 191–200.

19.

Rudd

A. L.

, Breslin

C. B.

and Mansfeld

: Corro. Sci., 2000, 42, (2), 275–288.

20.

Gonzalez-Nunez

M. A.

, Nunez-Lopez

C. A.

, Skeldon

, Thompson

G. E.

, Karimzadeh

, Lyon

and Wilks

T. E.

: Corros. Sci., 1995, 37, (11), 1763–1772.

21.

Skar

J. I.

, Water

and Albright

: Proc. Int. Cong. Expos., Detroit, MI, USA, March 1997, Society of Automotive Engineers, 7.

22.

Hawke

and Albright

D. L.

: Met. Finish., 1995, 93, 34.

23.

Naohiro

, Yoshiaki

, Yukio

, Yoshihiko

, Yoshinori

and Takeshi

: Japanese patent JP11323571A2, 1999.

24.

Naohiro

, Yoshiaki

, Yukio

and Kenichi

: Japanese patent JP11029874A2, 1999.

25.

Takayuki

and Isao

: Japanese patent JP6330341A2, 1994.

26.

Joesten

L. S.

: US patent 5683522, 1997.

27.

Ishizaki

, Nishida

and Sato

: US patent 5900074, 1999.

28.

Ziang

Y. F.

, Guo

, Wei

Y. H.

, Thai

C. Q.

and Ding

W. J.

: Synthet. Met., 2003, 139, 335–339.

29.

Zeng

A. P.

, Xue

, Qian

Y. F.

and Wang

Z. J.

: Acta Metall Sin. (English Lett.), 1999, 12, 946.

30.

Rivera

J. B.

, McMaster

R. L.

and Ike

C. R.

: US patent 6126997, 2000.

31.

Tomlinson

C. E.

: US patent 5964928, 1999.

32.

Tomlinson

C. E.

: US patent 5952049, 1999.

33.

Yfantis

, Paloumpa

, Schmeiber

and Yfantis

: Surf Coat. Technol, 2002, 151-152.

34.

Simaranov

A. U.

, Marshakov

S. L.

and Mikhailovskii Yu

: Protect. Met., 1992, 25, 611.

35.

Eppensteiner

F. W.

and Jenkins

M. R.

: Met. Finish Guidebook Direct., 1992, 90, 413.

36.

Umehara

, Terauchi

and Takaya

: Mater. Sci. Forum, 2000, 350-351, 273.

37.

Soosai

M. M.

, Pandarinath

B. R.

and KUMAR

: Proc. Int. Conf. GLOCORR, Feb 2002, New Delhi, India, NCCI, paper No. 2.29, published in CD.

38.

Natarajan

, Sivan

and Tennyson

P. G.

: Steel Grips, 2004, 2, (5), 399.