Corrosion Science 51 (2009) 1901–1906

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Corrosion Science

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The effect of water content on the electropolishing behavior of Inconel 718 alloyin perchloric–acetic acid mixtures

Ching An Huang *, Yu Chen ChenDepartment of Mechanical Engineering, Chang Gung University, Taoyuan 333, Taiwan

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 August 2008Accepted 26 March 2009Available online 28 May 2009

Keywords:A. Inconel 718 alloyB. ElectropolishingC. Electron beam weldingPerchloric and acetic acid mixture

0010-938X/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.corsci.2009.03.039

* Corresponding author. Tel.: +886 3 2118800x534E-mail address: gfehu@mail.cgu.edu.tw (C.A. Huan

The electropolishing behavior of Inconel 718 alloy was studied in perchloric–acetic acid mixtures using arotating disc electrode. The electropolishing behavior of an Inconel 718 weld, which was prepared withelectron beam welding, was also investigated. A leveled but not brightened surface can be achieved whenInconel 718 alloy is potentiostatically polished in the acid mixture with 20 vol.% perchloric acid. Interest-ingly, a brightening effect could be obtained in this acid mixture by adding 10–50 ml l�1 water or bybeing at rest at room temperature for several days. When electropolishing in acid mixture with40 vol.% perchloric acid, leveling and brightening of the Inconel 718 surface can be detected. When elec-tropolished in this acid mixture, the fusion zone of the Inconel 718 weld cannot be leveled together withits nearby base metal. Nevertheless, a good polished surface of the Inconel 718 weld can be achieved withthe acid mixture with 20 vol.% perchloric acid by adding 40 ml l�1 of water. Electropolishing was per-formed in the limiting diffusion current region where the transport of water to the anode seemed tobe the rate-determining process.

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1. Introduction

Inconel 718 alloy is one of the most widely used Ni-basedsuperalloys due to its superior mechanical properties and corro-sion resistance at high temperatures. Therefore, Inconel 718 alloyis often used in the fabrication of components that require a highthermal resistance, such as gas turbine engines and blades, andcombustion chambers [1–5]. It is well known that the fatiguestrength or even the corrosion resistance of an alloy can be im-proved through electropolishing (EP) [6,7]. It is generally believedthat welding with a relatively low heat input, such as electronbeam welding (EBW), is suitable to join Inconel 718 parts withouta great loss in mechanical properties [8]. In an electron beam (EB)weld, the fusion zone and heat-affected zone (FZ and HAZ) havedifferent phases or grain structures from those in the base metal(BM). Therefore, the leveling and brightening effect on the FZ,HAZ and BM in a weld is dissimilar. In this study, the EP behaviorof an EB-welded Inconel 718 specimen in perchloric–acetic acidmixtures is also investigated.

Normally, EP on a metallic component could be performed byanodic dissolution under a mass-transfer-controlled mechanismin a suitable electrolyte [9–15]. That is, EP on a metallic componentis carried out by polarizing it at a potential located in the limiting-current plateau of its anodic polarization curve. Regardless of thedifferent phases, grain types, and crystalline imperfections in a

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metallic component, leveling and brightening of the surface canbe obtained by EP. It is generally accepted that the chemical spe-cies governing the mass-transfer-controlled mechanism dependssignificantly on both the metallic electrode and electrolyte[13,16,17].

The commonly used solutions for EP are composed of one ormore concentrated acids, such as perchloric, sulfuric, phosphoricand acetic acids [13,16]. The EP behavior of a metallic componentcan be markedly affected by adding a small amount of H2O to theEP solution. Water added would combine with dissolving cationsor act as an oxidant during EP [13,18]. Electropolishing is generallyperformed in the potential region where the limiting diffusion cur-rent is observed. If the diffusion of H2O to the anode is the rate-determining process, the limiting diffusion current should increasewith increasing H2O content in the electropolishing solution. Inthis work, the EP behavior of as-rolled and EB-welded Inconel718 specimens in the perchloric–acetic acid mixtures is investi-gated by using a rotating disc electrode (RDE). Meanwhile, the ef-fect of adding H2O to the acid mixture with 20 vol.% HClO4 on theEP behavior of Inconel 718 alloy is also studied.

2. Experimental procedure

An as-rolled Inconel 718 sheet with dimensions 120 �40 � 3 mm3 was used in this study, and its chemical compositionis given in Table 1. Disc-type specimens with a diameter of 6 mmwere cut from the as-rolled Inconel 718 sheet by wire electricaldischarge machining to a make rotating disc electrode (RDE) with

Table 1The chemical composition of the as-rolled Inconel 718 sheet used in this study.

Element Ni Fe Cr Nb Mo Ti Al Mn Co Si

wt.% bal. 19.5 17.65 5.14 2.94 0.99 0.60 0.1 0.14 0.15

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an exposed area of 0.28 cm2. The disc specimen cut from the Inco-nel 718 sheet was tightly embedded with M-bond epoxy (M-bond610 ADHESIVE KID, Measurement Group Inc., USA) in a polypropyl-ene sleeve and connected to the spindle of the RDE-cell kid (EG&GModel 616) with a flexible spring and a stainless steel bar. The RDEwas used for electropolishing and electrochemical tests. The rotat-ing speed of RDE was kept constant to be 1000 rpm during electro-polishing and electrochemical test. The RDE surface wasmechanically ground with 600 grit emery paper, cleaned with dis-tilled water, dried with a cold air blow dryer, and then prepared forelectropolishing and electrochemical tests.

In this work, the EP behavior of Inconel 718 alloy after EBW wasalso studied. The EB-welded Inconel 718 specimen was joined bytwo welding processes, as given in Table 2, in which penetrationwelding was used to join two Inconel 718 plates in the first processand then cosmetic welding was performed to trim surface under-cut in the second process. As shown in Fig. 1, an EB-welded speci-men with a disc shape was mounted in a polypropylene sleeve andshaped to be a RDE. The welded portions, including FZ, HAZ, andBM, were electropolished at the same time. Therefore, the EPbehavior of the EB-welded Inconel 718 specimen can be evaluatedby investigating its surface morphology after electropolishing inthe acid mixtures.

Electropolishing and electrochemical tests were conducted at27 + 1 �C in an electrochemical three-electrode cell with potentio-stat/galvanostat (EG&G Model 2263A). As mentioned above, theRDE made of Inconel 718 was used as the working electrode. Aplatinum plate with an exposed area of 1 cm2 was used as thecounter electrode and set parallel to the working electrode witha distance of 1 cm. The Ag/AgCl electrode in the saturated KCl solu-tion was used as the reference electrode. The anodic polarizationcurve of the RDE specimens was measured potentiodynamicallywith a scan rate of 5 mV/s, from �250 mV (vs. open circuit poten-tial) to 9 V (vs. Ag/AgClsat.). Concentrated perchloric acid (70 wt.%)and acetic acid (99.7 wt.%) in analytical grade were mixed in vari-ous volume ratios for electrochemical polishing and testing.According to the anodic polarization behavior, the potential range

Table 2The welding schedule for EBW used in this study.

Weldingschedule

Voltage(kV)

Current(mA)

Focus depth(mm)

Feed rate(mm/min)

Heat input(J/mm)

Penetration 50 55 3.45 1016 162Cosmetic 50 100 3.46 2173 138

Fig. 1. Schematic of the RDE spec

corresponding to the limiting-current plateau was determined, andEP was conducted in this range.

After EP, the RDE was cleaned with distilled water, ultrasoni-cally cleaned in ethanol for 5 min, dried with a cold air blaster,and then examined with an optical microscope (OM, OlympusBH2-UMA) and a surface profiler (Hommel Werk Model T2000).

3. Results and discussion

3.1. Potentiostatic etching in the limiting-current plateau

Fig. 2 shows the anodic polarization curves of the Inconel 718RDE in acid mixtures with 20 and 40 vol.% HClO4. It can be seenthat a limiting-current plateau was detected when the anodic po-tential was higher than 4.2 V in the acid mixture with 20 vol.%HClO4. The limiting-current plateau in the acid mixture with40 vol.% HClO4 was found in a potential range between 3.5 and4.5 V. According to the anodic polarization curves shown inFig. 2, potentiostatic etching was conducted at 7 V in the acid mix-ture with 20 vol.% HClO4, and at 4 V with 40 vol.% HClO4. Afterpotentiostatic etching, the surface morphologies of Inconel 718were studied and are presented in Figs. 3(a) and (b). Leveling with-out brightening of the Inconel 718 surface was observed for thespecimen that was potentiostatically etched in the acid mixturewith 20 vol.% HClO4; on the other hand, a well electropolished sur-face, that is, a leveled and brightened surface, can be obtained inthe acid mixture with 40 vol.% HClO4.

As shown in Fig. 4, leveling and brightening of the surface can beachieved when the Inconel 718 RDE is potentiostatically etched inacid mixture with 20 vol.% HClO4 after being at rest for about 10 daysat room temperature. That is, there is an aging effect for the acid mix-ture with 20 vol.% HClO4 on the EP behavior of the Inconel 718 RDE. Itimplies that the acid mixture with 20 vol.% HClO4 gradually changesits chemical reactivity. From the anodic polarization curves shown inFig. 5, the limiting-current density measured in the aged acid mix-ture with 20 vol.% HClO4 is obviously higher than that in the as-pre-pared one. The aging effect of the acid mixture with 20 vol.% HClO4

on the EP behavior could be resulted from the decomposition ofHClO4 with a high oxidation capacity in the acid mixture.

3.2. The EP behavior in the acid mixture with 20 vol.% HClO4 withdifferent amounts of H2O added

Fig. 6 shows the anodic polarization curves of the Inconel 718RDE in the acid mixture with 20 vol.% HClO4 with the H2O added,from 2 to 80 ml l�1. The anodic polarization curve obtained fromthe aged acid mixture is also presented. Obviously, the limiting-current density increases, and the threshold potential for the lim-iting-current plateau shifts toward more active potential withincreasing H2O concentration in the acid mixture with 20 vol.%HClO4. As expected, a similar anodic polarization effect for the acidmixture with 20 vol.% HClO4 being at rest for ca. 10 days on theanodic polarization behavior was observed.

imen of an Inconel 718 weld.

Fig. 3. Surface morphologies of Inconel 718 alloy potentiostatically polished in acid mixtures with (a) 20 and (b) 40 vol.% HClO4.

Fig. 4. Surface morphology of Inconel 718 alloy potentiostatically etched in the acidmixture with 20 vol.% HClO4 being at rest for ca. 10 days.

0 2 4 6 8-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

Cur

rent

den

sity

(Acm

-2)

Potential (V vs. Ag/AgCl sat.)

as-prepared

being at resr for ca. ten days

Fig. 5. The anodic polarization curves of Inconel 718 in the as-prepared acidmixture with 20 vol.% HClO4 and in the acid mixture being at rest for ca. 10 days.

0 2 4 6 8 10

0.00

0.01

0.02

0.03

0.04

0.05

0.06

Cur

rent

den

sity

(A c

m-2)

Potential (V vs. Ag/AgClsat.)

20 vol.% HClO 4

40 vol.% HClO 4

Fig. 2. The anodic polarization curves of Inconel 718 alloy in the acid mixtures with 20 and 40 vol.% HClO4.

C.A. Huang, Y.C. Chen / Corrosion Science 51 (2009) 1901–1906 1903

By electropolishing in the acid mixture with 20 vol.% HClO4, lev-eling without brightening of the Inconel 718 surface was achieved,with a granular surface morphology [see Fig. 3(a)]. Fig. 7(a)–(e)shows optical micrographs of the surface morphologies of Inconel

0 2 4 6 8 10

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Cur

rent

den

sity

(A c

m-2)

Potential (V vs. Ag/AgClsat. )

20 vol% HClO4+

6 ml l-1 H2O

10 ml l-1 H2O

20 ml l-1 H2O

40 ml l-1 H2O

80 ml l-1 H2O being at rest for ca. ten days

Fig. 6. The anodic polarization curves of Inconel 718 alloy in the acid mixture with 20 vol.% HClO4 containing different amounts of H2O added.

Fig. 7. The surface morphologies of Inconel 718 alloy potentiostatically etched in the acid mixture with 20 vol.% HClO4 containing (a) 4, (b) 6, (c) 10, (d) 40 and (e) 80 ml l�1

H2O added.

1904 C.A. Huang, Y.C. Chen / Corrosion Science 51 (2009) 1901–1906

C.A. Huang, Y.C. Chen / Corrosion Science 51 (2009) 1901–1906 1905

718 potentiostatically etched in the acid mixture with 20 vol.%HClO4 containing different H2O concentrations. Obviously, the EPbehavior of Inconel 718 alloy is significantly affected by the amountof H2O added to the acid mixture with 20 vol.% HClO4. By increasingthe H2O concentration from 4 to 10 ml l�1, the granular surface mor-phology was gradually diminished and replaced by a leveled andbrightened surface, as shown in Fig. 7(a)–(c). Very small precipitatesin a needle form were not leveled together with the nearby substratewhen electropolished in the acid mixture with a H2O concentrationlower than 6 ml l�1. Addition of 4–10 ml l�1 H2O to the acid mixturewith 20 vol.% HClO4 enhances the EP effect on the Inconel 718 alloy.As presented in Fig. 7(c) and (d), brightening and leveling the surface,that is, producing a well polished surface of Inconel 718 RDE can beachieved by electropolishing in the acid mixture with 20 vol.% HClO4

containing H2O added from 10 to 50 ml l�1. In Table 3, the surfaceroughness values, Ra, of electropolished surfaces are lower than0.03 lm, which is a reasonable value for the metallic surface rough-ness after EP as reported by many researchers [17,18]. However,increasing the H2O concentration higher than 60 ml l�1 in the acidmixture with 20 vol.% HClO4 leads to small precipitates in a needleform that were clearly observed on the surface after EP [seeFig. 7(e)]; that is, these precipitates can not be leveled together withthe Inconel 718 substrate. Thus, electropolishing in these acid mix-tures, the surface roughness values, Ra, increase slightly or are higherthan 0.03 lm. Some tiny white stains could be seen visually on thesepolished surfaces after EP.

As shown in Fig. 6, increasing water content in 20 vol.% HClO4

acid mixture increases the limiting diffusion current. This suggests

Table 3The surface roughness, Ra, of Inconel 718 alloy polished in the acid mixture with 20 vol.%

H2O content (ml l�1) 0 2 6 10 20Ra (lm) 0.19 0.12 0.04 0.02 0.0

Fig. 8. Surface morphologies of EB-welded Inconel 718 specimens after elec

Fig. 9. Surface morphologies of EB-welded Inconel 718 specimens after electropolishing40 ml l�1 H2O added.

that the transport of water to the anode surface is the rate-deter-mining process. Thus, for leveling and brightening of the surface,the optimal rate of water transport, that is, the optimal concentra-tion of water such as 10–50 ml l�1 in 20 vol.% HClO4–CH3COOHmixture is required.

3.3. The EP effect on an Inconel 718 alloy after EBW

According to our experimental results, a well polished surface ofInconel 718 alloy can be achieved by potentiostatic polishing inthree kinds of acid mixtures; that is, the acid mixture with40 vol.% HClO4, the acid mixture with 20 vol.% HClO4 aged for ca.10 days, and the acid mixture with 20 vol.% HClO4 with 10–50 ml l�1 H2O added. As shown in Fig. 1, an EB-welded RDE speci-men was prepared to evaluate its EP behavior in the three kinds ofacid mixtures mentioned above.

Fig. 8(a) and (b) shows the surface morphologies of EB-weldedInconel 718 specimens after EP in the acid mixtures with 20 and40 vol.% HClO4, respectively. As expected, the BM region in theEB-welded specimen was leveled and brightened in the acid mix-ture with 40 vol.% HClO4, but not leveled and brightened in the acidmixture with 20 vol.% HClO4. Contrary to the EP effect on the BMregion, the FZ in EB-welded specimen was not well electropolishedin either of these acid mixtures. As shown in Fig. 8(b), an obvioussurface step between the FZ and BM can be seen after EP. This indi-cates that the anodic dissolution rates between the FZ and BM of anEB-welded Inconel 718 specimen are different. As shown inFig. 9(a) and (b), leveling and brightening of the surface of EB-

HClO4 containing different H2O concentrations.

30 40 50 60 70 802 0.01 0.01 0.02 0.05 0.07 0.08

tropolishing in acid mixtures with (a) 20 vol.% and (b) 40 vol.% HClO4.

in acid mixtures with 20 vol.% HClO4 (a) being at rest for ca. 10 days and (b) with

1906 C.A. Huang, Y.C. Chen / Corrosion Science 51 (2009) 1901–1906

welded Inconel 718 specimen could be achieved when electropo-lished in the 20 vol.% HClO4 acid mixture aged for ca. 10 days orwith 40 ml l�1 H2O added. These results indicate that the afore-mentioned acid mixtures are suitable for obtaining a well polishedsurface of Inconel 718 alloy after EBW regardless of the differentgrain types or phases among the FZ, HAZ and BM in a weld. Thismeans that EP of an Inconel 718 weld depends strongly on theelectrolyte used.

4. Conclusions

The electropolishing behavior of Inconel 718 alloy and Inconel718 weld was studied in HClO4–CH3COOH acid mixtures using arotating disc electrode. From the experimental results, a leveledsurface without brightening could be achieved when Inconel 718alloy was potentiostatically polished in the acid mixture with20 vol.% HClO4. But a brightening effect could be observed whenelectropolishing in this acid mixture being at rest for ca. 10 daysat room temperature or with 10–50 ml l�1 H2O added. The increasein the water content of 20 vol.% HClO4 solutions resulted in the in-crease in limiting diffusion current. This fact indicates that thetransport of water to the anode is rate-determining process in elec-tropolishing. By electropolishing in the acid mixture with 40 vol.%

HClO4, leveling and brightening of the surface can be detected inthe base metal of the Inconel 718 weld, but the fusion zone cannotbe leveled together with the base metal. Nevertheless, a well pol-ished surface for the Inconel 718 weld can be achieved in the acidmixture with 20 vol.% HClO4 containing 40 ml l�1 H2O added.

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