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http://www.iaeme.com/IJMET/index.asp 315 [email protected]
International Journal of Mechanical Engineering and Technology (IJMET) Volume 7, Issue 6, November–December 2016, pp.315–322, Article ID: IJMET_07_06_032
Available online at
http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=7&IType=6
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
ELECTROCHEMICAL CORROSION PROPERTIES
FOR DIFFERENT ZONES OF AA 6061-T6 GTAW
WELDMENTS
Hemadri Naidu T
Research Scholar, KSIT, VTU, Bangalore, Karnataka, India.
Channakeshavalu K
Principal, EWIT, VTU, Bangalore, Karnataka, India.
ABSTRACT
AA 6061-T6 Aluminum alloy is medium high strength heat treatable wrought structural
material used for various versatile applications ranging from truck and marine frames to missile
propellant storage tanks. It possesses good corrosion resistance coupled with excellent workability
when welded with suitable filler material. AA 6061 possess good weldability with ER 4043 filler
wire due to Si rich composition. AA 6061 weldments mostly fail in heat affected zone (HAZ) due to
dissolution of metastable Mg2Si strengthening phases by weld thermal energy. The degree of
dissolution of metastable Mg2Si phases dictated the corrosion resistance of the HAZ. The
dissolution of metastable Mg2Si phases can be retarded by weld thermal management. By suitable
weld process and parameters combinations the heat energy can be controlled and sound weldments
with good corrosion resistance can be obtained. In practice welding being a localized thermal
process, residual stresses will be induced invariably into the integrated structure and influence the
corrosion properties. This paper delineates the electrochemical corrosion of the various zones of
AA 6061/ER 4043 weldments joined using GTAW process variants of conventional AC, un-pulsed
Direct Current Straight Polarity (DCSP) and current pulsed DCSP. The present study established
that the effective heat extraction from the HAZ region has bearing on the corrosion resistance of
the HAZ compared to the fusion zone. Present analysis proves that weld metal is more corrosive
resistance compared to base metal and HAZ. By employing composite back-up bar of copper and
SS during high heat input AC welding process to extract heat has increased corrosion resistance
along with improved weldments properties without impairing the fabrication weldability. Also by
adopting less heat input by pulsed DCSP welding results, retards the coarsening of Mg2Si particles
and thereby improves the corrosion resistance by 2.3%. It is found that corrosion rate at HAZ is
decreased to 9.5%. Hence, it is concluded that corrosion rate for AC with composite backup bar at
HAZ (39.8 mpy) is reduced to (38 mpy) for pulsed DCSP SS backup bar.
Key words: TIG welding, direct current straight polarity (DCSP), residual stresses, corrosion
resistance, polarization, thermal energy, electrochemical reactions, tafel extrapolation method
Cite this Article: Hemadri Naidu T and Channakeshavalu K, Electrochemical Corrosion Properties
for Different Zones of AA 6061-T6 GTAW Weldments. International Journal of Mechanical
Engineering and Technology, 7(6), 2016, pp. 315–322.
http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=7&IType=6
Hemadri Naidu T and Channakeshavalu K
http://www.iaeme.com/IJMET/index.asp 316 [email protected]
1. INTRODUCTION
Aluminum alloys continues to be the primary candidate material for structural applications of aerospace
industries due to their excellent workability coupled with attractive specific strength. AA 6061-T6
Aluminium alloy is a work horse alloy among Aluminium alloys used for structural applications. Due to its
balanced amounts of magnesium and silicon made AA 6061-T6 a quasi-binary Al-Mg2Si system [1]. It
possesses good corrosion resistance coupled with excellent workability when welded with suitable filler
material. AA 6061 possess good weldability with ER 4043 filler wire due to Si rich composition [2]. AA
6061 possess good weldability with ER 4043 filler wire even though it has post anodisiation color
mismatching. Where anodisiation color matching is important normally AA 5356 are preferred. AA6061
weldments mostly fail in heat affected zone (HAZ) due to dissolution of metastable Mg2Si strengthening
phases by weld thermal energy [3]. Weldments are basically composite structure with cast structure in
fusion zone, highly epitaxial grain at fusion line, coarse and over aged HAZ and wrought structured base
material [2]. Hence it is found that the corrosion resistance of the welded material at different zones is
invariably not the same. This is influenced by the temper condition of the parent metal, filler wire used,
process and parameters employed and post weld heat treatment imparted, if any. The degree of dissolution
of metastable Mg2Si phases dictated the corrosion resistance of the HAZ. The dissolution of metastable
Mg2Si phases can be retarded by weld thermal management. By suitable weld process and parameters
combinations the heat energy can be controlled and sound weldments with good corrosion resistance can
be made. In practice welding being a localized thermal process, residual stresses will be induced invariably
into the integrated structure and influence the corrosion properties.
1.1. Corrosion
Corrosion of industrial metals is an invariable natural process as they are extracted from their stable state
by imparting external energy. Hence any little opportunity they try to come back to thermodynamically
stable state. Corrosion process of metallic material involves the destructive attack of metal by chemical or
electrochemical reaction with its service environment. Usually corrosion consists of a series of redox
reactions that are basically electrochemically in nature [4]. The metal is oxidized to corrosion products at
anodic sites:
M ↔ M +2 + 2e- (1)
and hydrogen is reduced at the cathode sites,
2H++2e- ↔ H2 (2)
Because of the electrochemical nature of most corrosion processes, electrochemical methods are
invariably used for corrosion studies [5-8]. Electrochemical techniques can be used to measure the kinetics
of corrosion rates. The polarization resistance method measures the instantaneous corrosion rates as
compared to other methods on which metal loss is measure over a finite period of time. This technique
may be used for measuring even very low corrosion rates (less than 0.1mpy). Tafel Extrapolation
technique uses data obtained from cathodic and anodic polarization measurements. At relatively high-
applied current densities, the applied current density and that corresponding to hydrogen evolution have
become virtually identical. To determine the corrosion rate from such polarization measurements, the Tafel
region is extrapolated to the corrosion potential and corrosion rate are determined [4]. At the corrosion
potential, the rate of hydrogen evolution is equal to the rate of metal dissolution, and this point corresponds
to the corrosion rate of the system expressed in terms of current density. Under ideal conditions, the
accuracy of the Tafel extrapolation method is almost at par with conventional weight loss methods.
NASA report [9] on general section on special material addressed the corrosion issue. It states that
deterioration processes which impair the life expectancy of parts include galvanic corrosion, stress
corrosion etc. Precautionary measures to prevent deterioration shall include consideration of such controls
as limitation of operating stresses, application of protection coatings, and use of special heat treatment.
Electrochemical Corrosion Properties for Different Zones of AA 6061-T6 GTAW Weldments
http://www.iaeme.com/IJMET/index.asp 317 [email protected]
In the present investigation the corrosion properties of the different zones of the weldments made in
AA6061-T6 Aluminum alloys welded using various processes and parameters are studied using
polarization technique.
1.2. Aluminium Alloy AA 6061
AA 6061 Aluminum has the “balanced “amount of Mg and Si to form quasi-binary Al-Mg2Si with Mg to
Si ration of 1.73:1. AA 6061 is strictly binary Al-Mg2Si alloy with 1.4% Mg2Si [10-11]. It contains minor
addition of 0.3% cooper to improve strength, but has negative impact on corrosion resistance. The
chemical composition of AA 6061 and ER 4043 are given in table 1.
2. EXPERIMENTATION AND METHODOLOGY
In present work 3.2mm thick 300mm x 400mm length coupons of chemical milled quality AA 6061 alloy
in T6 temper condition imported from M/s ALCAN is welded using mechanized GTAW process
employing ER 4043 filler wire of 1.2mm diameter in the form of pools of 7.0 kg. AA 6061 long sheets of
1500mm x 3000mm are sheared to size of 300mm x 400mm using power shearing machine. The sheared
coupon edges are milled on conventional milling machine.
AA 6061-T6 material selected
Coupons prepared for welding
Deoxidized and neutralized in
diluted NaOH and Nitric acid
Coupons cleaned and dried
Coupons are welded using
GTAW in AC and DCSP
Balanced wave AC with
SS/composite backup bar Direct current straight polarity
(DCSP) with / without pulsation
and with SS backup bar.
Coupons for corrosion (10mm x10mm)
Corrosion potential and potentio-dynamic
polarizations were evaluated
Corrosion rate is measured
Edges prepared for welding
Comparative analysis
Optimized welding process
Hemadri Naidu T and Channakeshavalu K
http://www.iaeme.com/IJMET/index.asp 318 [email protected]
The edge milled coupons are vapor degreased to remove the surface contaminants and organic matter.
Degreased coupons are deoxidized in dilute NaOH bath followed neutralization in dilute nitric acid.
Coupons are further cleaned in demineralized water and air dried.
2.1. Welding Process
The gas tungsten arc welding (GTAW) processes is selected and used in present study. M/s Hobart
Brothers make 250A automatic welding machine is used. Welding is performed using both balanced wave
AC and Direct Current Straight polarity (DCSP) cycles. DCSP welds are carried out using with and
without current pulsation. The current pulsation of 10ms-on/ 10ms-off is used with 60% back ground
current. No pulsation is used for filler addition. DCSP gives lesser heat input compared to AC cycle even
though cleaning action is relatively poor [ 12]
3. CORROSION TEST
Corrosion potential and potentio-dynamic polarizations were evaluated independently by saw cutting the
fusion zone, HAZ and base material from the welded coupons. The prepared sample 1.0cm x 1.0cm was
fixed in specimen holder. Corrosion experiment is carried out employing potentiostat. The cut samples
were kept in a cell containing sodium chloride solution. A 35gm of sodium chloride and 1000ml of
distilled water and whose pH is 6.9. The cell current readings are taken with a short and slow sweep of the
potential. A three electrode cell is used with specimen as working electrode; the reference electrode is
saturated calomel electrode (SCE) and platinum counter electrode. The recorded current density is used in
the calculation of corrosion rate by using Tafel extrapolation method [13]. The corrosion rate CR is
measured by equation (3). The results of the corrosion test are shown in table 3.
C.R= (0.13 Icorr E.W)/d .(3)
Where,
mpy = milli-inches per year
Icorr = corrosion current density (µΑ/cm3)
E.W = equivalent weight of the corroding species (g)
D = density of the corroding species (g/cm3)
4. RESULTS AND DISCUSSIONS
Aluminium alloys are generally sensitive to chloride environment and suffer pitting corrosion [14-15].
However, the degree and nature of the attack depends on many factors including the solution type,
concentration, level of aeration of the bath, temperature and temper as well as form condition of the
Aluminum alloy [16-18].
Intermetallic particles are generally grouped into coarse intermetallic particles and fine precipitates. In
Aluminium alloys, coarse intermetallic particles form during the solidification process itself, while fine
form during the aging process. The coarse intermetallic particles are further divided into two group’s
namely active and noble particles relative to the matrix. The coarse particles of Mg2Si in AA 6061 system
are active one. The corrosion potential for Mg2Si particles in chloride solution is -1590 mV SCE [19].
Welding will have a strong influence on the pitting corrosion of heat-treatable Al alloys, mainly
microstructural changes in heat affected zone and partially melted zone might lead to non-uniform pitting
potential across the weldment [20T]. The welding procedure that has the least influence on microstructure
has the least chance of reducing the corrosion resistance of Aluminium weldments. The weldment with
more negative potential will attempt to protect the other part. Thus, if the weld metal is anodic to the base
metal, the small weld can be attacked preferentially to protect the larger surface area of the base metal.
Fortunately ER 4043 filler gives more cathodic weldment compared to base material [19]. As evident from
the results obtained from table 3, the weld metal is more corrosive resistance compared to base material
and HAZ. HAZ region is more anodic compared to base material in all welded condition. This is attributed
Electrochemical Corrosion Properties for Different Zones of AA 6061-T6 GTAW Weldments
http://www.iaeme.com/IJMET/index.asp 319 [email protected]
to coarsening of Mg2Si particles in HAZ region due to weld thermal energy [21]. The potential of HAZ
was largely fluctuated during the immersion time compared to weld metal and base material. The Mg-
containing phases are in general active to the matrix and act as anode. They are susceptible to active
dissolution or Mg de-alloying when exposed in acidic solution or chloride solution [22-23]. This behavior
is attributed to the fact that the current density of Mg2Si phase is bigger than Icorr for the matrix. With the
reduced heat input, as reported in table 2, during welding the coarsening of the Mg2Si phase is reduced and
there by the corrosion extent of HAZ is retarded [24]. Even the nature of the pits is different based on the
degradation of HAZ due to weld heat.
Coupon welded with AC using composite back bar shown less corrosion rate compared to conventional
back up bar. The copper back up near HAZ facilitated faster heat extraction and thereby lesser Mg2Si
particles coarsening [25]. This aided in increased corrosion resistance as evident in table3. Similar trend
can be seen AC welds Vs DCSP welds. Less heat input also aide, of course marginally, weldment
corrosion resistance further [26].
Hence, by adopting good thermal management during welding, as evident from composite back up bar
AC and pulsed DCSP welding results, retards the coarsening of Mg2Si particles and thereby improves the
corrosion resistance.
Table 1 Chemical composition (in wt%) of AA 6061- T6 and filler wire 4043
Material Si Mg Cu Fe Mn Zn Ti Cr Al
AA
6061
0.65 1.1 0.3 0.5 0.1 0.12 0.1 0.1 Balance
ER
4043
5.1 0.05 0.17 0.05 0.23 0.10 0.04 0.06 Balance
Table 2 Weld parameters for AC and DCSP welds with filler wire 4043
Process Current,
A
Voltage,
V
Weld
speed
mm/ min
Pulse
cycle
Heat
input
J/mm
Filler feed rate
mm/minutes
AC GTAW 120 16 200 - 585 2200
DCSP
GTAW
90 18 250 -
388 2200
PULSED
DCSP
GTAW
90
18
300 20ms60
% Ib
325 2400
Hemadri Naidu T and Channakeshavalu K
http://www.iaeme.com/IJMET
Sample Temperat
ure (
Un-
welded
Base
material
300
AC with
SS back
up bar
Weldment 300
HAZ 300
AC with
composit
e back up
bar
Weldment 300
HAZ 300
DCSP
SS
backup
bar
Weldment 300
HAZ 300
PULSED
DCSP -
SS
backup
bar
Weldment 300
HAZ 300
5. FIGURE
Hemadri Naidu T and Channakeshavalu K
IJMET/index.asp 320
Table 3 Corrosion results
Temperat
ure (0K)
Ecorr
( mV)
Icorr(µ
A/cm2)
Corrosion
rate (mpy)
300 -720 85 36.7
300 -620 30 12
300 -769 99 43
300 -610 30 12
300 -758 92 39.8
300 - 600 28 11.6
300 -740 90 38.9
300 -600 26 11.3
300 -725 88 38
Figure 1 Gill AC Electrochemical System
Corrosion
rate (mpy)
Nature of pits
Fine pores widely
and evenly
distributed
Fine pores.
Unevenly
distributed
Coarse pores
Unevenly
distributed
Fine pores.
Unevenly
distributed
Less Coarse pores
Unevenly
distributed
Fine pores. Less
evenly distributed
Less Coarse pores
Unevenly
distributed
Fine pores. More
evenly distributed
medium pores
Unevenly
distributed
Electrochemical Corrosion Properties for Different Zones of AA 6061-T6 GTAW Weldments
http://www.iaeme.com/IJMET/index.asp 321 [email protected]
6. CONCLUSION
In this proposed work the author exploited some of innovative characteristics for AA 6061/ER 4043
corrosion resistance characteristics as follows
In AA 6061/ER 4043 weldment, fusion zone more nobles compare to HAZ and base material. In AA
6061/ER 4043 weldment, HAZ is more anodic compared to weldment and base material. Heat input,
thereby the modifications of Mg2Si, influences the corrosion resistance of Heat effected Zone. AC
weldments HAZ is more prone to corrosion compared to DCSP HAZ approximately by 4.5%. Good
thermal management (Less heat input in pulsed DCSP (325 j/mm) to high heat input in AC welding (585
j/mm)) during welding improves the corrosion resistance of weldments. DCSP current pulsed welding
process offers best corrosion resistance for all zones of the weldment.
REFERENCE
[1] ASM specialty hand book for Aluminium and Aluminium Alloys:- ISBN:13-978-0871704962.
[2] Welding fundamentals and process Volume 6A ASM international handbook, ISBN:978-1-61503133-
7,oct31, 2011.
[3] Mechanical Behavior of Precipitation Hardened Aluminum Alloys Welds R.R. Ambriz and D. Jaramillo,
published by Intech, 2004, DOI:10.5772/58418
[4] Corrosion: Materials ASM Handbook Volume 13B, Editor: Stephen D. Cramer and Bernard S. Covino,
Jr ISBN:978-0-87170-707-9. Jan1, 2005
[5] Alkier RC et al, Journal of Eelectrochemical Society, 125 (1978 )1382
[6] Tester JW et al, Journal of Eelectrochemical Society, 122 (1975 )1438
[7] Akiyama E et al, Journal of Eelectrochemical Society, 146 (1999 ) 4095
[8] Ramgopal et al, Corrosion 57(2001) 702.
[9] NASA report no SE-R-0006 rev C “General specification requirements for materials and process”.
[10] Non- ferrous metal hand book Volume 2 ASM international Swearengen,
[11] J.C., 1972. The thermo mechanical behavior of 6061 aluminum magnesium–silicon alloy. Mater. Sci.
Eng. 10,103–117.
[12] Welding metallurgy Sindo Kou second edition published by John Wiley and sons Inc publication,
ISBN:0-471-43491-7,oct 2002, Inter-science.
[13] Musa Abdl Rahim Khudadad, Corrosion of TIG welding joint for Aluminium alloy 6061-T6 in sea
water at different velocities, J of Engr and Dev, Vol 18, Nov 2014, ISSN:1813-7822.
[14] Galvele JR J. Eelectrochem. Soc 123 (1976) 464 DOI:10.1149/12132857 J.Electrochem. Soc 1976
vol123, issue 4, 464-474.
[15] Galvele JR J. Eelectrochem. Soc 21 (1981) 551.
[16] Frankel et al J. Corrosion 43(1987) 429.
[17] Frankel et al Corrosion 30 (1990) 1203.
Hemadri Naidu T and Channakeshavalu K
http://www.iaeme.com/IJMET/index.asp 322 [email protected]
[18] Frankel et all J. Eelectrochem. Soc 140 (1993) 2192.
[19] K.Srinivasa Rao and Prasad Rao, pitting corrosion of heat-treatable Aluminium alloys and welds: a
review Trans. Indian Inst. Met.Vol.57, No. 6, December 2004, pp. 593-610.
[20] T senthil Kumar et all Effect of pulsed current TIG welding parameters on pitting corrosion Behaviour
of AA 6061 Aluminium alloy, J Mater. Sci. Technol, Vol 23 No2, 2007.
[21] B.F. Jogi et al Some studies on fatigue crack growth rate of aluminium alloy 6061 journal of materials
processing technology 2 0 1 ( 2 0 0 8 ) 380–384
[22] Buchheit RG J. Electrochem. Soc 142 (1995) 3994.
[23] Buchheit RG Corrosion Science 13 (1973) 853 ISBN:9780824799175-CAT#DK5545, sep 13,2002 by
CRC press.
[24] EO Eltai and E Mahdi, Electro chemical corrosion and mechanical properties of welded AA 6061 joined
by TIG and MIG welding methods. Proc. of Intl. Conf. on FTMPAE 2014
[25] Lu M and Kou S Weld. J 68 452s, 1989
[26] Huang C and Kou S Weld. J 83 (2004) 50s.
[27] Hemadri Naidu.T, Dr. K.Channakeshavalu, P.Srinivasa Rao “Investigation on the mechanical properties
of TIG welded AA 6061/T6 alloy weldments before and after heat treatment”. Published in
IJAETMAS-Journal vol-3, special-01, april-2016 pg-no[292-296]
[28] Hemadri Naidu.T, Dr. K.Channakeshavalu, P.Srinivasa Rao, “A comparative study of mechanical
properties in joining of aluminium alloy AA 6061-T6 material using TIG welding process with and
without pulsation”. Presented a paper on 5th national conference on emerging trends in engineering
technologies (ETET 2016) 11th
and 12th march 2016 JIT, Bangalore.
[29] Dr. S. Ravichandran, Analyzing Defects in Weldments Using Metallography Examination. International
Journal of Mechanical Engineering and Technology (IJMET), 1, 2008, pp. 06–13.
[30] Dr. Ali Hoobi Haleem, Dr. Nawal Mohammed Dawood and Wafa Mahdi Jodia. Investigation of
Oxidation Resistance of Ni - Ti Electrochemical Codeposition Coating of Austenitic Stainless Steel (316
L , International Journal of Mechanical Engineering and Technology (IJMET) , 6( 11 ), 2015, pp. 229 -
243 .