ELECTROCHEMICAL CORROSION PROPERTIES FOR …€¦ · · 2016-12-07dissolution of metastable Mg 2Si strengthening phases by weld thermal energy. The ... process and parameters employed

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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

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ISSN Print: 0976-6340 and ISSN Online: 0976-6359

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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


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


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



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



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


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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


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

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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



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.

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ELECTROCHEMICAL CORROSION PROPERTIES FOR …€¦ · · 2016-12-07dissolution of metastable Mg 2Si strengthening phases by weld thermal energy. The ... process and parameters employed