Sacrificial Corrosion Protection Ability ofAluminum Alloys for Automobile Heat Exchangers
by Yoshiaki Ogiwara * and Kouji Okada *Aluminum is widely used in automobile heat exchangers in order to decrease their

ABSTRACT weight and improve efficiency. Normally aluminum corrodes by pitting, and this cancause leakage, which is fatal to the heat exchanger. Mechanisms of sacrificial corrosion are therefore used to suppress pitting in heat exchangers made of aluminum. The ability scope of such
sacrificial materials in suppressing corrosion is investigated using scanning electrodes, and theresults of the evaluation are reported.

1.

INTRODUCTION

Automobiles are equipped with a variety of heat exchangers for such systems as radiators and air conditioners,
and aluminum alloys are often used making them light inweight. Typical heat exchangers include radiator, condenser, evaporator, and heater, and they tend to have apotential problem of corrosion when aluminum is used.
These products have heat exchanging medium (long life
coolant) flowing inside and their outside is exposed to,depending on the position of attachment, different environments such as anti-freezing mixture and dew condensation. In the case of heat exchangers for air conditioners
where the contained medium like chlorofluorocarbon is
non-corrosive, the corrosive environment inside can beneglected. Generally, however, materials for heat exchangers should be provided with anti-corrosion measures, both
inside and outside, to assure the life of the products.
It is characteristic of aluminum materials that they often
suffer from corrosion pitting in the depth direction. If pitting
proceeds in a heat exchanger and penetrates its medium
path, the exchanger falls out of use due to leakage of themedium. As a means to protect pitting in aluminum, it isnormal practice to adopt sacrificial corrosion protection,
whereby a material that is electrochemically less noble
than the medium path material is provided for in order toprotect the medium path by feeding electrons. The outersurface of products are usually protected, in the case ofheat exchangers using butt seam welded tubes, by making the radiating fin material less noble than the tube material to effect sacrificial protection. In the case of extruded
tubes, because it is difficult to form a solder layer onto thetubing, the fin material is clad with solder, while the outer* 1st Products Development Sec., Metal Research Lab.,
R & D Div.

surface of the tube is covered with a sacrifice layer formedby Zn thermal spraying or the like. In both cases, the innersurface, being always in contact with a cooling medium, iscovered with a cladding layer which is less noble than thetube core material.
As mentioned, sacrificial protection based on potential
difference is extensively utilized in aluminum heat exchangers and accordingly, at the time of product design, it isvery important to optimize the ability of protection as wellas to understand quantitatively the extent of protection.
The authors evaluated the extent of sacrificial corrosion
protection by using the scanning electrode method andinvestigated its correspondence with the corrosion test.
The results will be presented in this report.

2.

EVALUATION OF SACRIFICIAL PROTECTION ABILITY

Since sacrificial protection makes use of material's potential difference to suppress corrosion, surface analyses atthe time of design such as potential distribution1)~2) andcorrosion current distribution would be very efficient to realize a material development with a reduced cycle time.
Analysis methods include scanning vibration electrodemethod, Kelvin probe method, and scanning electrodemethod and others, each having characteristics of its own
as described below.
2.1 Scanning Vibration Electrode Method3)~4) (Figure 1)
A micro electrode made of Pt or such is driven to vibratemechanically in a test solution, electrode potentials at
amplitude peaks are measured, and electrode potentialdifferences corresponding to vibration amplitude are calculated. The current flowing in the direction of vibration isthen determined by using the electrode potential differ-31

v

Detecting electrode∆E

Referenceelectrode

Current = ∆E / solution resistanceVibration
SampleFigure 1

Schematic view of scanning vibration electrodeR

Measuring cell
Vibration
E

SampleFigure 2

Schematic view of Kelvin probe

ence thus obtained together with the solution resistance.
This method is capable of directly determining the amountand distribution of effective current that is working for sacrificial protection, since it measures the current that flows
into or out of the portion of interest.
2.2 Kelvin Probe Method5) (Figure 2)
A non-contact electrode is positioned above a sample
constituting a capacitor with it. As the electrode is made tovibrate, the capacitance changes accordingly inducing
electrical charges to flow, the amount of which is measured to represent the surface potential of the region under observation. Since the measured potential is not apotential in thermodynamic equilibrium, calibration is required using a reference electrode.
When this method is applied to corrosion analysis, it
should be noted, it is sometimes influenced by the changesof the capacitor caused by geometrical changes of thesurface as well as by deposition of corrosion products,
which may take place during corrosion analysis.
6)

2.3 Scanning Electrode Method (Figure 3)
The probing electrode consists of a reference electrodeand the Loggin capillary connected with salt bridge andsaturated KCL sollution. The pointed end is used as a probe
to directly measure the potential of a spot on the sample
which enables, because its space resolution is fine, measurements of surface potential distribution by moving theprobe over the sample.
Occasionally, however, thermal compensation is needed
since its reference electrode belongs to external electrodes.
Furthermore, at measurements in diluted solutions andfrom time to time, KCl dissolves from agar that comprises
the salt bridge at the capillary and influences the sample
surface to induce pitting.
2.4 Corrosion Test Method
This is a method most commonly used, in which test
samples modeling the actual products are prepared andthey are subjected to corrosion tests in corrosive environments. Typical test methods include salt spray test, CASS
test and others, and they specify their respective evaluation criteria for each product. Although corrosion test
method is indispensable for final evaluation of products,
the method takes a considerably long time to complete
making it difficult to apply for all materials at the time ofmaterial design.

3.

EXPERIMENTAL METHOD

In this study, the surface potential distribution of samples
each comprised of a sacrifice material and a tube corematerial bonded together was measured in an OY waterusing a scanning electrode, and the results were compared with those of corrosion tests.
The scanning electrode used in the measurements consisted of a glass tube having a pointed end of 100 µm in
diameter which contained a salt bridge made up of saturated KCl and agar, together with a Ag / AgCl(s) referenceelectrode jointed by saturated KCl solution. The probe wasmounted on a fixture that incorporated a micrometer,
thereby enabling scanning parallel to as well as perpendicular to the sample surface.
Test pieces were of the alloys having compositions
shown in Table 1. They were soaked at 520°C for 4 hr after

Sacrifice material
30×30×0.4mm
Al-10Si Solder
24×24×0.4mm

Material to be protected
80×30×1mm

Ag/AgCl(s) Reference electrodeV
Saturated KCl

Hole for jig attachment

Epoxy adhesive

Saturated KCl + Agar
Sample25

Figure 332

Schematic view of scanning electrode

Figure 43025

Corrosion test sample

Furukawa Review, No. 18. 1999

Table 1

Composition of samples
SiCore material 1
Core material 2
Sacrifice material 1
Sacrifice material 2

0.2
0.3
0.1
0.1

Fe0.30.20.20.2

Cu
0.150.5——

Table 2Mn
1.1
1.1
—
—

Chemical composition of OY water
Concentration

Zn—

Cl-195—

SO42-60

1.03+302+1FeCu

4.0(ppm)(wt%)

Engine block

Inner heater

Potential (mV vs Ag/AgCl(s))

Sample under test

Test chamber

Sample 1
Sample 2

DC motor

Figure 5

Schematic view of circulating corrosion test
equipment

Distance from the edge of sacrifice layer (mm)Figure 6

casting, hot rolled to 3 mm thick, intermediately annealed
at 400°C for 2 hr, and finally cold rolled to 0.4 mm thick tomake test pieces. Subsequently, they were assembled asshown in Figure 4 and brazed at 600°C for 3 min in air.
Since a small amount of solder with solder flux was seen
to leak out of the bonded area, the area was etched by 10% NaOH for 30 sec at 50°C, desmatted by 1:1 HNO3 for
30 sec, and its periphery was sealed with epoxy adhesiveso that solder leakage might not influence the measurement.
Two types of samples were prepared and tested:samples having an ordinary sacrificial ability consisting ofcore material 1 / sacrifice material 1 (hereafter calledSample 1); and samples having an enhanced sacrificial
ability consisting of core material 2 / sacrifice material 2(hereafter called Sample 2).
Samples were kept in a circulating OY water environment for one week, using a circulating corrosion test equipment illustrated in Figure 57), and were transferred fromthe circulating corrosion test equipment to a still OY water
environment after corrosion turned into a steady state. OY
water, the composition of which is shown in Table 2, wasladled out of the test equipment and was used in the still
OY water environment test.
Surface potential was measured by the scanning electrode held 100µm above the sample surface. Sample potential was picked up by an aluminum stick, 3 mm in diameter with its entire surface insulated excluding its end face,pressed against the sacrifice material to make electricalcontact. Furthermore, the mixed potential of the system
was determined with the scanning electrode fixed to aposition that is 10 mm apart from the edge of the testsample.

Surface potential distributionSample 1Sample 2
Photo 1

4.

Appearance of samples after cleaning

RESULTS

Figure 6 shows the results of surface potential distribution
measurements beginning from the edge of the sacrificematerial. Since the test sample consists of core materialand sacrifice material soldered together, the potentialchanges gradually from a less noble state at the sacrificematerial edge to a noble state at a distance, reaching anequilibrium potential that depends on the property of eachsample. Equilibrium potentials at a distance for Sample 1and Sample 2 are, since they have different core materi-33Potential (mV Ag/AgCl(s))

Number of pitting (count)

Sample 1
Sample 2

Core material
Sacrifice materia
Distance from the edge of sacrifice layer (mm)Figure 7

Current density (µA.cm-2)

Distribution of pitting frequency

Polarization curve of Sample 1 in OY water

Potential (mV Ag/AgCl(s))

Maximum pitting depth (µm)

Figure 9

Sample 1
Sample 2

Core material
Sacrifice materia

Distance from the edge of sacrifice layer (mm)Figure 8

Maximum pitting depth in the 5 mm range

Current density (µA.cm-2)

Figure 10 Polarization curve of Sample 2 in OY waterals, approximately -480 mV and -460 mV respectively.
Photo 1 shows the appearance of samples after they
underwent circulation corrosion tests with their corrosion
products removed. Both samples show larger numbers ofpitting on the sacrifice material but smaller numbers on
the core material. A more detailed observation on the sacrifice material indicates that Sample 1 has a localized distribution of pitting leaving some surface areas undamaged
as rolled, while Sample 2 has a uniform distribution overthe entire surface. Pitting frequency on the core materialis seen to be small near the sacrifice material and increases
at a distance. Statistics of the number of pitting counted
every 5 mm beginning from the sacrifice material edge isshown in Figure 7, where the number of counts for a range
from 0 mm to less than 5 mm is marked on the 0 mm
position. Figure 8 shows, in a similar manner, the resultsof measurements for the maximum pitting depth in therange.
The number of pitting increases as the distance fromthe sacrifice material increases, reaching a value almost
constant at a position 10 to 15 mm away. When Sample 1and Sample 2 are compared, the former has more counts
generally.
In common with the pitting count, maximum pitting depth
for ranges partitioned every 5 mm is shallower than theaverage for the 5 mm range where the distance from thesacrifice material is 0 to 10 mm, but reaches a constant

34

value farther on. With Sample 2, a tendency is seen inwhich the depth is shallow for the 0 mm range, becoming
deeper for the 20 mm range where the distance from thesacrifice material is 20 mm or more.

5.

DISCUSSION

Due to protection by sacrifice material, pitting in its neighborhood is suppressed not only in its occurrence number
but also in its maximum depth. Electrochemical states ofthese samples were estimated by measuring their polarization curves at 25°C in an OY water that is the same asused in the experiments. Core material and sacrifice material of each sample were etched by 10 % NaOH for 30
sec at 50°C, and desmatted by 1:1 HNO3 for 30 sec before measurement. Assembled samples were entirely
masked with epoxy adhesive excluding 1 cm2 , and theirpolarization curves were measured at a scanning speedof 20 mV/min. The potential was measured in terms of theequilibrium potential of each material, beginning at a time
30 min after N2 deaeration for the anode side, and also 30
min after aeration to atmosphere for the cathode side. A
reference electrode of saturated Ag / AgCl was used. From
the results shown in Figure 9 and Figure 10, electrochemical behaviors of the core and sacrifice materials in thestate of sacrificial corrosion protection can be read respec-Furukawa Review, No. 18. 1999

Surface potential
Number of pitting
Maximum pittingdepthDistance from the sacrifice layer (mm)Number of pitting (count), Maximum pitting depth (µm)

Distance from the sacrifice layer (mm)Pitting potential

Potential (mV vs Ag/AgCl(s))

Surface potential
Number of pitting
Maximum pitting
depth

Number of pitting (count), Maximum pitting depth (µm)

Potential (mV vs Ag/AgCl(s))

Pitting potential

Figure 11 Relationship between pitting and surface
potential distribution of Sample 1

Figure 12 Relationship between pitting and surface
potential distribution of Sample 2

tively, since the figures represent both materials of eachsample together in a sheet.
Pitting possibility depends on the surface potential andits anodic polarization curve. Thus, the relationship between pitting and surface potential distribution was studied for the two samples, and the results are shown in Figure 11 and Figure 12.
The state of electrochemical corrosion protection forSample 1 is estimated as follows. The surface potential of
the core material near the sacrifice material is maintainedat around -500 mV, which is less noble than the corrosion
potential of core material 1 in OY water (-480 mV) by 20mV. As the distance from the sacrifice material increases,
the surface potential approaches the corrosion potentialof core material 1, reaching quite near the pitting potentialof core material 1. With Sample 2, on the other hand, thesurface potential of the core material near the sacrificematerial is around -520 mV, which is less noble than thecorrosion potential of core material 2 (-456 mV) by 64 mV,
providing a larger potential difference than Sample 1.
Pitting occurrence declines when the surface potentialof core material becomes less noble than the corrosion
potential or the pitting potential. With the materials ofpresent study, in which the corrosion and pitting potentials
of core material are almost equal, creation of corrosion
nuclei is supposed to be suppressed due to the surface
potential kept less noble than the pitting potential. The
nearer to the sacrifice material, the less noble the surface
potential of the core material is kept, thereby increasing
the difference with respect to the pitting potential. Pitting
occurrence decreases as the difference between the surface potential and the pitting potential increases, and in
the case of Sample 1, minimizing to one fifth of that at aposition 20 mm or farther from the sacrifice material where
sacrificial corrosion protection is no longer effective. The
tendency is similar in the case of Sample 2, although pitting occurrence is smaller in number than Sample 1 at aposition 20 mm or farther. This suggests that the pitting
potential of a material influences not only the potential atwhich pitting begins to grow, but also the occurrence frequency of pitting nucleus.
Maximum pitting depth is seen to become shallower
when the surface potential of core material is less noble

than the corrosion potential by 10 mV or more. This fact issupposed to be the result of suppression of pitting growth,similar to suppression of pitting occurrence, which iscaused by a surface potential that is less noble than thepitting potential. The effect of depth suppression becomes
distinct with the larger potential difference: while maximum
pitting depth of Sample 1, which has a maximum potential
difference of about 20 mV between the core and sacrificematerials, is 42 µm at a position closest to the sacrificelayer, that of Sample 2 having a difference of 60 mV is12µm.
In terms of sacrificial corrosion protection, the ability ofSample 1 is effective to suppress pitting occurrence considerably but is insufficient to stop efficiently the growth ofpitting that has once occurred. Its pitting depths become
shallower near the sacrifice material than at a distance,
but their absolute values are larger than those of Sample
2. With Sample 2 in contrast, as shown in Figure 7 andFigure 8, core material 2 has a lower pitting occurrence
than core material 1, but pitting that has once occurred
there has a possibility of growing to the same depth. However, as indicated by the polarization curves in Figure 9and Figure 10, the difference between the corrosion potentials of the core and sacrifice materials is larger, resulting in an augmented protection ability of the sacrifice material than Sample 1 to make pitting near the sacrificematerial much shallower.

6.

CONCLUSIONS

We have studied the surface corrosion potentials of corrosion samples that are modeling combinations of sacrificematerial / core material in OY water utilizing a scanningelectrode. The following conclusions have been drawn.
1) Pitting occurrence of core materials can be efficiently
suppressed by incorporating sacrifice materials, making the surface potential of core material less noble thanthe corrosion potential. The larger the potential difference with respect to the corrosion potential, the greater
the suppression effect becomes.
2) To suppress pitting growth, it is necessary to makethe potential less noble than the corrosion potential ofcore material by 10 mV or larger. The larger the poten-35

tial difference, the greater the suppression effect becomes.
3) Sacrifice protection can be analyzed by the use ofscanning electrode. By using this method, the extent ofeffective sacrificial protection is clarified.

REFERENCES

1) G.Y. Gao, S.C. Dexter; Corros. Control Offshore Marine Constr.
(1989), 405.
2) Kato, Yamauchi; Summaries of Japan Inst. of Light Metals,(1992), 257. (in Japanese)3) Fushimi, Seo; Proceedings of Forum on Corrosion and Protection, (1996), 303. (in Japanese)
4) Hayase, Fukuizumi, Aoki; Proceedings of Forum on Corrosion and Protection, (1996), 305. (in Japanese)5) Tawara, Kodama, Fujimoto; Proceedings of Forum on Corrosion and Protection, (1996), 367. (in Japanese)
6) Masuda, Nishijima; Corrosion Protection Engineering, 36,(1987), 710. (in Japanese)7) R.R. Wiggle, V. Hospadaruk; SAE Tech. Paper Series, (1980),No.800800.
Manuscript received on July 9, 1998.

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Furukawa Review, No. 18. 1999