Corrosion Science 50 (2008) 3308–3313

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Corrosion Science

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Heat-transfer corrosion behaviour of cast Al alloy

Wei Zhou a,*, Naing Naing Aung a, Anil Choudhary b, Mouhcine Kanouni c

a Precision Engineering and Nanotechnology Centre, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue,Singapore 639798, Singaporeb Ciba Specialty Chemicals (India) Limited, Mumbai 400 053, Indiac Global Applications Center, Ciba Specialty Chemicals Corporation, 540 White Plains Road, Tarrytown, NY 10591, USA

a r t i c l e i n f o

Article history:Received 9 February 2008Accepted 29 August 2008Available online 11 September 2008

Keywords:A. AlloyA. AluminumB. SEMC. Hot corrosionC. Crevice corrosion

0010-938X/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.corsci.2008.08.047

* Corresponding author. Tel.: +65 6790 4700; fax: +E-mail addresses: mwzhou@ntu.edu.sg, WZhou@C

a b s t r a c t

Heat-transfer corrosion behaviour of an ISO 2379 cast Al alloy was studied in antifreeze radiator coolantunder heat-rejecting condition. Extensive analyses of microstructures and corroded surfaces were carriedout under the optical microscope, scanning electron microscope equipped with energy dispersive spec-troscopy and X-ray diffractometer. Heat-rejecting condition led to a cavitation process and cavities wereobserved within the a-Al matrix. Crevice corrosion was predominant at oxygen depleted regions in heat-transfer corrosion cell. Al2Cu, Al15(Fe,Mn)3Si2 dendrites, Al4Cu2Mg8Si7 and Si phases served as the effec-tive cathodes resulting microgalvanic corrosion at the anodic site of a-Al matrix.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Aluminum alloys are increasingly used in automotive applica-tions because of the constant drive to reduce automobile partsweight for better fuel efficiency. The use of a cast aluminum cylin-der block is one of the key weight saving features in the engine de-sign. An engine block transfers heat from metal to coolant. Metalthat is heat-rejecting may suffer higher corrosion rate than metalthat is heat-absorbing or heat-neutral. The main reason for thisaccelerated corrosion is coolant boiling at the surfaces. Thusheat-transfer corrosion is usually a major issue in cooling systemof an engine block.

The corrosion behaviour of an alloy is influenced by its micro-constituents [1–3] and the environmental conditions to which itis exposed [4,5]. Due to the high content of alloying elements likesilicon, copper and iron, the cast Al alloy has complex microstruc-tures containing intermetallics which vary from binary to quater-nary phases in the aluminum matrix. These intermetallicparticles can be either cathodic or anodic with respect to the ma-trix phase or they may be initially anodic but due to dealloying, la-ter become cathodic to the aluminum matrix. Such complexitieshave made it challenging to understand heat-transfer corrosionbehaviour of cast aluminum alloys in various cooling systems.

Coolant plays a critical role in sustaining overall engine heatbalance by removing heat. However, it is essential that enginecoolants prevent heat-transfer corrosion of aluminum cylinder

ll rights reserved.

65 6791 1859.antab.Net (W. Zhou).

heads during engine operation. Any corrosion products formedmay deposit on interior radiator surfaces, reducing heat-transferefficiency of the radiator. Overheating and boil-over of the coolingsystem may then occur.

ASTM D4340 is the standard test to cover laboratory screeningprocedure for evaluating the effectiveness of engine coolants incombating corrosion of aluminum casting alloys under heat-trans-fer conditions [6], but the test can only provide the weight changeof the test specimen due to heat-transfer corrosion.

Although some researches have been carried out in the area ofengine coolant corrosion under heat-transfer condition [7–11],the heat-transfer corrosion mechanism in the cooling system of en-gine materials is not well understood. The objective of this studywas to gain a better understanding of the heat-transfer corrosionbehaviour of ISO 2379 cast Al alloy and to provide essential knowl-edge for the development of better antifreeze radiator enginecoolants.

2. Experimental

2.1. Material

The material studied was an ISO 2379:1972 cast Al alloy withthe following chemical composition (in wt%): Si-6.0, Cu-3.5, Fe-1.0 (max), Mn-0.9 (max), Mg-0.1 (max), Zn-1.0 (max), Ni-0.35(max), Ti-1.0 (max) and Al-the rest. Specimen size is 6.5 cm (2.6in.) in diameter, 1.3 cm (0.5 in.) in thickness. Two small holes hav-ing 0.325 cm diameter were made in the specimen to locate thethermocouple. The test specimen was polished with emery papers

Pressure gauge

Pressure relief valve

To pressure source

Fill plug

Top assembly plate

Threaded rod

Test coolant level, 500 ml

Glass cell

O-ring

Thermocouple hole

Aluminium test specimen

Heat transfer bar

Bottom assembly plate Band heater

Thermal out-off

Fig. 1. Schematic of heat-transfer corrosion test assembly.

0 2 4 6 8 10

Si phase

Al2Cu phase

Al4Cu2Mg8Si7 phase

Al15

(Fe, Mn)3Si

2 phase

Mn

Fe

Si

Al

Si Cu

Cu

MgAl

CuCu

CuAl

Si

Cou

nts

(Arb

itary

Uni

t)

Energy (keV)

Al2Cu

Al4Cu2Mg8Si7

Al15(Fe, Mn)3Si2

Si phase

α-Al matrix

a

b

Fig. 2. (a) Optical micrograph showing the microconstituents of ISO 2379 cast Alalloy microstructure and (b) energy dispersive X-ray spectrums of themicroconstituent.

0 20 40 60 80 100

0

10000

20000

30000

Al4Cu2Mg8Si7

Al; Al15(Fe,Mn)3Si2

AlAl4Cu2Mg8Si7

Al4Cu2Mg8Si7

Al2Cu

Al2Cu

SiSi: Al2Cu

Si Al

Al

Al15(Fe,Mn)3Si2

Al; Al15(Fe,Mn)3Si2

Al

Inte

nsity

Angle (2 Theta)

Fig. 3. XRD pattern showing the peaks of microconstituents.

Original alloy surface Corroded area under glass O-ring

Corroded area under coolant

Aluminum alloy test specimen

Heat-transfercorroded area

a

b

100 µm

Fig. 4. (a) Photograph and (b) SEM micrograph showing heat-transfer corroded areaand crevice corrosion area under glass O-ring in heat-transfer corrosion cell.

W. Zhou et al. / Corrosion Science 50 (2008) 3308–3313 3309

and washed with warm tap water. The specimen was then rinsedwith de-ionized water and finally with acetone. In order to makesure thermocouple holes were free of metal chips and polishingdebris, the holes were flushed with acetone and excess liquid inthe holes was removed with a glass capillary pipette.

3310 W. Zhou et al. / Corrosion Science 50 (2008) 3308–3313

2.2. Antifreeze radiator coolant test solution

A new type of antifreeze radiator coolant was selected for test-ing to evaluate its effectiveness in combating corrosion of cast alu-minum alloys under heat transfer conditions. The coolant with pH8.3 was formulated with 25.7% monoethylene glycol (MEG), 70%

50 µm

10 µm

c d

a

Fig. 5. (a) SEM micrograph showing formation of protective oxide film on the heat-transfdifferent degree of cavitation corrosion attack within a-Al matrix (arrowed).

Bubbles grow iSmall bubbles form in coolant at high pressure and temperature

a b

Coolant

Metal

193 kPa 193 kPa

135 ˚C 135 ˚C

Fig. 6. Schematic illustrations of cavitation

water, and 4.3% corrosion inhibitor of the following composition:40.7% KOH, 3.5% tolytriazole derivative, and 55.8% organic carbox-ylic acids (15.1% C7-organic acid, 29% C8-organic acid, and 11.7%C10-organic acid). ASTM D4340 [6] specifies that the ‘‘engine cool-ant concentrate shall be sampled in accordance with Test MethodD1176” [12]; therefore, the test solution specified in ASTM D4340

0 5 10 15 20C

ount

s (A

rbita

ry U

nit)

O

Al

Energy (keV)

10 µm

b

er corroded surface, (b) energy dispersive X-ray spectrum of the oxide film, (c and d)

n size Bubbles burst and exert a hammering action over protective film

c

Coolant Coolant

Metal MetalCavities

193 kPa

135 ˚C

process in the heat rejecting condition.

W. Zhou et al. / Corrosion Science 50 (2008) 3308–3313 3311

[6] was prepared according to ASTM D1176 [12]: 165 mg of re-agent grade sodium chloride was first dissolved in 750 ml of de-ionized water, and then 250 ml of the antifreeze radiator coolantwas added.

2.3. ASTM D4340 test

ASTM D4340 test simulates the real world operating conditionof heat rejecting cast aluminum alloys. Thermally influenced ef-fects such as changing thermal coefficient of expansion and ratesof mass transport can be reflected in this test. It covers a laboratoryscreening procedure for evaluating the effectiveness of enginecoolants in combating corrosion of cast aluminum alloys underheat transfer conditions that may be present in aluminum cylinderhead engines [6].

2.3.1. Test procedureThe assembled heat-transfer corrosion cell is shown schemati-

cally in Fig. 1. A heat flux was established through the cast alumi-num alloy while exposed to 500 ml of antifreeze coolant testsolution under a pressure of 193 kPa using the heat-transfer corro-sion cell [6]. Internal pressure was monitored using a suitable pres-sure gage and a pressure-relief valve was installed to protectagainst bursting. Heat-resistant O-rings was used. Internal pres-sure was monitored using a pressure gage and a pressure-reliefvalve was installed to protect against bursting. The aluminumheat-transfer specimen temperature was maintained at135 ± 1 �C throughout the test by means of a thermocouple con-nected to the temperature controller. The thermocouple was in-serted into one of the thermocouple holes in the test specimen.The other thermocouple hole was used for occasional precisetemperature measurements. The test was continued for 1 week(168 h). Duplicate tests were carried out to get the reproducibleresults.

Original alloy surface

Crevice corrosion area

a

b

Fig. 7. SEM micrographs showing (a) pits (arrowed) at crevice

2.3.2. Weight loss measurementThe corroded specimen was washed with a soft bristle brush

using a mild detergent and immersed in an aqueous solution con-taining four parts concentrated nitric acid (70% by weight) plus onepart water at 25 �C for 10 min. After 10 min, an ultrasonic cleaningwas followed for 1 min using the same cleaning solution and thenflushed well with de-ionized water, and finally with acetone. Ther-mocouple holes were flushed well and excess liquid was removed.The specimen was cooled to room temperature and weighed to thenearest 0.1 mg.The heat-transfer corrosion rate (R) was calculatedas follows:

R ¼ ðWb �Wa þ BÞ � 1000A

ð1Þ

Where R is the corrosion rate, mg/cm2/week, Wb is the weight oftest specimen before test, g, Wa is the weight of test specimen aftertest, g, B is the weight loss of blank, g (the average weight loss fromthree unused and clean specimens was used as the blank correc-tion), A is the heat-flux surface area inside O-ring, cm2.

Generally, coolants that produce less than 1.0 mg/cm2/weekmay be considered as candidates for further evaluation. Thus,the value of heat-transfer corrosion rate provides essential knowl-edge for the development of better antifreeze radiator enginecoolants.

2.4. Analyses of corroded surface

Analyses of the microstructures and corroded surfaces werecarried out using optical microscope (Carl Zeiss Axioskop 2), scan-ning electron microscope (SEM, Jeol model 5600 LV) equippedwith energy dispersive spectroscopy (EDS). Various microconstit-uents in the alloy were identified using X-ray diffractometer(XRD, Philips, model PW1830) with Cu Ka radiation at 40 kVand 30 mA.

100 µm

corrosion area and (b) closed-up of crevice corrosion area.

3312 W. Zhou et al. / Corrosion Science 50 (2008) 3308–3313

3. Results and discussion

3.1. Microstructure

Microstructure of the ISO 2379 cast Al alloy consisted of a-Almatrix, Chinese script-shaped large cored dendrites, platelet-shaped feature, irregular dispersed particles, and honeycomb typestructure, as shown in Fig. 2a. Extensive EDS (Fig. 2b) and XRDanalyses (Fig. 3) were carried out to identify various types of themicroconstituents in the alloy. The Chinese script feature wasfound to be Al15(Fe,Mn)3Si2. The presence of Mn enhances the for-mation of the a-Fe intermetallic rather than the b-F‘e intermetallicphase [13,14]. In addition to the Chinese script feature, starlikecrystals were also observed. These crystals were a coarser versionof the a-Fe phase with the same stochiometry. It is reported thataddition of 0.9% Mn could result in the formation of polyhedral(starlike) intermetallics [13]. The platelet-shaped features andirregularly-shaped dispersed particles were identified as the sec-ondary phases of Si and Al2Cu. The honeycomb type morphologywas found to be Al4Cu2Mg8Si7. It has been found that the Mg con-tent of Al–Si–Cu alloy led to the precipitation of honeycomb typeintermetallics [15,16].

3.2. Rate of corrosion

The corrosion rate of the alloy from weight-loss measurementgave 0.3 mg/cm2/week. According to ASTM D 4340, a rate of weight

Fig. 8. SEM micrographs showing microgalvanic corrosion (arrowed) at a-Al matrix

loss greater than 1 mg/cm2/week for an Al alloy should be rejected[6]. The result suggested that the corrosivity of test antifreeze radi-ator coolant was acceptable.

3.3. Effect of heat rejecting condition on corrosion

The colour of heat-transfer corroded area was totally differentfrom that of original alloy as shown in Fig. 4 The heat-transfer cor-roded area and crevice corrosion area in heat-transfer corrosioncell can be discerned from the photograph (Fig. 4a) and SEM micro-graph (Fig. 4b).

In the heat-transfer corroded area, white colour protective filmcovered the a-Al matrix (Fig. 5a). The EDS analysis showed thatthis film was Al oxide (Fig. 5b). The anodic and cathodic reactionsof Al alloy in ethylene glycol–water solution can be characterizedby the Al oxidation and reduction of oxygen [17]. The dissolvedoxygen diffuses towards the electrode surface for the cathodicreaction which generates hydroxide ions used by the oxidation ofAl to form aluminum oxide film [17]. The magnified views of theheat-transfer corroded area clearly show that the cavities occurredpreferentially within a-Al matrix by breaking down of Al oxide film(Fig. 5c and d). Different degrees of cavitation corrosion attackwere observed and some of cavities were deep (Fig. 5d).

Cavitation is a complex phenomenon involving the interactionof hydrodynamic mechanical, metallurgical and chemical or elec-trochemical factors [18,19]. The cavitation rate is dominated bymetallurgical factors such as the phase volume fraction, morphol-

near (a) Al2Cu, (b) Al15(Fe, Mn)3Si2 dendrite, (c) Al4Cu2Mg8Si7 and (d) Si phase.

W. Zhou et al. / Corrosion Science 50 (2008) 3308–3313 3313

ogy and distribution of different phases where the hydrodynamicand mechanical parameters of cavitation and chemical factors areconstant [18].

The initiation and successive growth stages of cavitation pro-cess in the heat rejecting condition can be explained by schematicillustrations shown in Fig. 6. Under heat-rejecting condition, theantifreeze coolant could be vaporized and yielded air bubbleswhen the local pressure was lower than the saturated vapor pres-sure that corresponded to the antifreeze coolant temperature(Fig. 6a). The generated air bubbles flowed on the alloy surfaceand collided with others to form larger bubbles (Fig. 6b). Whenthe air bubbles flowed into positions of higher pressure, they mightcollapse and produce hammering action over surface of Al oxidefilm resulting mechanical damage within a-Al matrix (Fig. 6c).

3.4. Crevice attack in heat-transfer corrosion cell

Pits were observed at the area between the alloy and the glassO-ring in the heat-transfer corrosion cell (Fig. 7a). The pits initiatedat the localized sites, preferentially on the a-Al phase inside cre-vice (Fig. 7b). The crevice area was the oxygen depleted zone dueto oxygen concentration difference between inside and outsidecrevice. The region inside crevice could not then support a cathodicreaction but it could still support an anodic reaction. Chloride ionsfrom the test solution were the favoured ions to be attracted intothe crevice. Both the chloride ion concentration and the hydrogenion concentration increased within the crevice. The effect of thisacidification accelerated crevice corrosion.

3.5. Effect of microconstituents on corrosion

The Al2Cu, Al15(Fe,Mn)3Si2 dendrites, Al4Cu2Mg8Si7 and Siphases functioned as local cathodes and stimulated the formationof microgalvanic corrosion at anodic a-Al matrix (Fig. 8). More cor-rosion was observed at a-Al area around Al2Cu (Fig. 8a) comparedwith Al2Cu, Al15(Fe,Mn)3Si2 dendrites (Fig. 8b), Al4Cu2Mg8Si7

(Fig. 8c) and Si phases (Fig. 8d). However, the degree of microgal-vanic corrosion was not as high as cavitation corrosion.

Intermetallics and Si are usually cathodic with respect to the a-Al matrix. They could play a dual role in the corrosion behavioureither as a galvanic cathode or as a kinetic barrier to dissolution.If the area fraction of anodes is larger than that of cathodes, thecathodic phases along the grain boundary could not serve as the ki-netic barrier to the dissolution of anodic grain body. The cast Al al-loy had large area fraction of a-Al (Fig. 1a) compared with that ofAl15(Fe,Mn)3Si2, Al4Cu2Mg8Si7, Al2Cu and Si phases. As a conse-quence, these phases acted as the galvanic cathodes leading tothe microgalvanic corrosion at the anodic a-Al matrix.

4. Conclusions

Cavitation was the most predominant corrosion of ISO 2379cast Al alloy in antifreeze radiator coolant under heat-rejectingcondition. Formation of micro-bubbles from vaporization of anti-

freeze coolant led to cavitation process. Crevice corrosion occurredat oxygen depleted regions in the heat-transfer corrosion cell. Thealloy had the complex microstructure consisting of large coredAl15(Fe,Mn)3Si2 dendrites, Al4Cu2Mg8Si, Al2Cu and Si phases inthe a-Al matrix. The large area fraction of anodic a-Al matrix facil-itated the galvanic action of the cathodic intermetallics and Siphases resulting in microgalvanic corrosion. In order to improvethe corrosion resistance of cast Al alloy the cast microstructureshould be modified by both grain size refinement and preventionof large cored dendrites formation using suitable castingtechnique.

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