reliable corrosion test development for heat exchanger fin...

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Reliable corrosion test development for heat exchanger fin debonding

evaluation

Mr Vincent Renault, Dr Ing Anne-Gaëlle Villemiane, Mrs Maryse Philippe, Mr Arnaud

Dubois, Dr Christian Casenave, Mr David Delaux

VALEO Thermal System, 8 Rue Louis Lormand, 78321 La Verrière Cedex, France,

+33.6.20.57.29.57, [email protected]

Abstract

This study is devoted to the development of a reliability corrosion test for heat exchanger

named DeFinX (Debonding Fin of Heat Exchangers). It was focused on a heat exchanger product

integrated within vehicle Air Cooling (A/C) loop, the condenser. In the field, some case of fin

debonding issues was noticed on this product (see picture 1). This degradation led to the loss of

efficiency of the A/C loop.

Objective was to implement a test in order to characterize the fin debonding resistance of new

condensers to avoid any issue in the field, to enhance material and product resistance versus corrosion

phenomenon. We took into account several conditions as car washing products, vehicle exposition to

de-icing salt on the road and environmental humidity and temperature variations. The test procedure

precises how to determine the force characterizing the fin debonding. The test also allowed evaluating

the product component corrosion sensitivity by doing microscopic analyses.

The test has been dependable by making a complete analysis of parts recovered from the field.

We used DeFinX test also on new parts with same technical definition for corrosion resistance

evaluation until failure. We correlated results by reliability study to define a criterion avoiding the risk

of fin debonding on new Valeo technical part definition.

Keywords

Aluminium, exchanger, fin debonding, corrosion, test development

Introduction

This study deals with the development of a reliable corrosion test named DeFinX (Debonding

Fin of Heat Exchangers). This test concerns

vehicle Air Conditioning (A/C) loop, the condenser. In the field,

issues were noticed on it (Picture 1). This degradation led to the loss of efficiency of the A/C

loop.

Figure 1: Pictures of a condenser with fin debonding issue and metallographic analysis

of the failure mode by optical observation


condensers to avoid any issue

versus corrosion phenomenon. Several service conditions, such as

products, to de-icing salt on the road

were taken into account.

Condenser technology studied is defined by extruded tubes, manifolds and

Assembly of these components is made by controlled atmosphere brazing process, which

ensures tightness and mechanical resistance. In order to increase the corrosion protection of

the tube, a zinc metal deposit is performed prior to brazi

zinc wire on aluminium surface

Figure 2: Illustration of ZAS coating deposit on extruded tube

The main advantage of the ZAS is to promote the tube protection

playing the sacrificial anode role

Manifold

2


his test concerns a heat exchanger product integrated within

vehicle Air Conditioning (A/C) loop, the condenser. In the field, some cases of fin debonding

(Picture 1). This degradation led to the loss of efficiency of the A/C


of the failure mode by optical observation


condensers to avoid any issue on the field, so as to enhance material and product resistance

non. Several service conditions, such as exposure to

icing salt on the road, to environmental humidity and temperature variations

Condenser technology studied is defined by extruded tubes, manifolds and



inc metal deposit is performed prior to brazing. This is done by atomisation of a

zinc wire on aluminium surface. This process is called Zinc Air Spray - ZAS (picture 2).


The main advantage of the ZAS is to promote the tube protection against pitting corrosion

role. The zinc ensures an electrochemical potential gap between

Fins Tube


a heat exchanger product integrated within

some cases of fin debonding

(Picture 1). This degradation led to the loss of efficiency of the A/C



the field, so as to enhance material and product resistance

exposure to car washing

environmental humidity and temperature variations

Condenser technology studied is defined by extruded tubes, manifolds and fins (see fig.1).



by atomisation of a

ZAS (picture 2).


against pitting corrosion by

. The zinc ensures an electrochemical potential gap between

tube alloy and tube surface. However, too high quantity of zinc deposited on tube surface can

lead to bad fin/tube brazed joint corr

we named fin debonding issue.

Considering this issue we faced

developed.

DeFinX test presentation

The DeFinX test conditions are based on

definition. These guidelines define cycles of temperature and humidity.

In addition, the temperature profile was observed on the condenser surface during its

operation (table 1).

Table 1: European profile (a

temperature selection for test.

In the field, condensers are submitted to following factors:

- seasonal effect inducing for example de

- car user habit as vehicle external cleaning (products used and cleaning frequency)

- pollutant coming from environment as SOx, NOx, exhaust gas, brake dust

- water retention capacity which is a specific factor common to the exchangers

As a consequence, condensers need to resist against corrosion in the wide range of pH from

acid (close to 2) to alkaline (close to 12 for cleaners). The current available tests as SWAAT

sea water acetic acid test according to ASTM G85

=2.8). No corrosion norms define test in the range of alkaline pH.

such alkaline solution and cyclic conditions representatives of external environment and

condenser usage.

Cycle and pollutant selection

The DeFinX test is built as follows:

- The range of temperature applied is between 35°C to 45°C which is in line with the

average temperature values seen at the skin surface of the condenser.

increase the test severity, temperature could have been se

example 60°C but it was demonstrated that this condition was giving less corrosion

effect due to less efficiency on the humidity parameter. Humidity parameter is applied

to maintain external pollutant accumulation and corrosiv

climatic phase. This is considered as an ac

the time the condenser is warm and should be dry.

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lead to bad fin/tube brazed joint corrosion resistance because of zinc diffusion. This is what

we named fin debonding issue.

Considering this issue we faced on the field, a new reliable corrosion test needed to be

The DeFinX test conditions are based on [1] guidelines for accelerated corrosion test



Table 1: European profile (air temperature), condenser temperature profile and

In the field, condensers are submitted to following factors:

seasonal effect inducing for example de-icing salt introduction on the car front face

le external cleaning (products used and cleaning frequency)

pollutant coming from environment as SOx, NOx, exhaust gas, brake dust

water retention capacity which is a specific factor common to the exchangers


acid (close to 2) to alkaline (close to 12 for cleaners). The current available tests as SWAAT

sea water acetic acid test according to ASTM G85- A3 is working in the acidic range (pH

=2.8). No corrosion norms define test in the range of alkaline pH. Then DeFinX test defines


Cycle and pollutant selection

eFinX test is built as follows:

The range of temperature applied is between 35°C to 45°C which is in line with the

average temperature values seen at the skin surface of the condenser.

increase the test severity, temperature could have been selected at higher level as for



to maintain external pollutant accumulation and corrosive atmosphere during the

climatic phase. This is considered as an accelerating factor versus the field as most of

the time the condenser is warm and should be dry.


osion resistance because of zinc diffusion. This is what

the field, a new reliable corrosion test needed to be

[1] guidelines for accelerated corrosion test



ir temperature), condenser temperature profile and

icing salt introduction on the car front face

le external cleaning (products used and cleaning frequency)

pollutant coming from environment as SOx, NOx, exhaust gas, brake dust

water retention capacity which is a specific factor common to the exchangers


acid (close to 2) to alkaline (close to 12 for cleaners). The current available tests as SWAAT -

he acidic range (pH

Then DeFinX test defines


The range of temperature applied is between 35°C to 45°C which is in line with the

average temperature values seen at the skin surface of the condenser. In order to

lected at higher level as for



e atmosphere during the

celerating factor versus the field as most of

Figure 3: DeFinX test cycle definition

Pollutants selected are:

- Kärcher ™ solution, to represent the chemical basis applied for car cleaning product

with a pH=11.7 +/-0.5.

- This pollutant is applied once a week during 30 min by total immersion of the

condenser sample. The sample is removed and

main part of the cleaning product

- A second step of pollutant is applied by spraying NaCl 5% demineralised water

solution at the sample surface.

Figure 4: Samples during immersion step and spraying step

Sample selection

To be able to evaluate the corrosion resistance of the brazing joint, the criteria selected is the

fin resistance loss or the strength needed to tear off the fins from the tube surface. For this

measurement, it is necessary to prepare several tubes and fins sample with

cm length as in the pictures 5 and 6.

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Figure 3: DeFinX test cycle definition

Kärcher ™ solution, to represent the chemical basis applied for car cleaning product

0.5.

This pollutant is applied once a week during 30 min by total immersion of the

condenser sample. The sample is removed and shaked in order to drain

main part of the cleaning product

A second step of pollutant is applied by spraying NaCl 5% demineralised water

solution at the sample surface.


evaluate the corrosion resistance of the brazing joint, the criteria selected is the


measurement, it is necessary to prepare several tubes and fins sample with

cm length as in the pictures 5 and 6.

Kärcher ™ solution, to represent the chemical basis applied for car cleaning product

This pollutant is applied once a week during 30 min by total immersion of the

in order to drain off the

A second step of pollutant is applied by spraying NaCl 5% demineralised water


evaluate the corrosion resistance of the brazing joint, the criteria selected is the


measurement, it is necessary to prepare several tubes and fins sample with 3 cm width and 20

Figures 5 and 6: samples dimension for fins resistance measurement and samples placed

Dynamometer device for fin resistance measurement

The dynamometer device will allow tearing off fins. In

order to get a statistical batch of measurements, it is

necessary to repeat at least 5 times the measurement.

Table 2: Example of measurement obtained

Results are given in Newton (N) for 3 cm width, then in N/cm, then in % of resistance loss

compared to the sample taken from not tested part.

This table will allow drawing the following graph (figure 9)

Figure 9: Fin resistance

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inside CCT bench

Dynamometer device for fin resistance measurement

The dynamometer device will allow tearing off fins. In

order to get a statistical batch of measurements, it is

necessary to repeat at least 5 times the measurement.

Table 2: Example of measurement obtained

Figures 7 and 8: Dynamometer measurement


compared to the sample taken from not tested part.

This table will allow drawing the following graph (figure 9)

Figure 9: Fin resistance loss result, and acceptance level

Other technology


Figures 7 and 8: Dynamometer measurement device


loss result, and acceptance level

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Chemical solution correlation between Kärcher™ solution and Soda + Glycerin

One of the most representative cleaning agents used for car body is generally Kärcher™

solution. The efficiency of this cleaner can be attributed to the low pH as a 0.5% volume

solution reach pH=11.7 +/-0.5. Moreover, it contains some additives as described in the

material safety data sheet that gives tensid properties either wetting the surface or for allowing

contact with foreign matter (dust).

Using such formulation in a corrosion test requires knowing the composition for achieving

reproducible conditions.

Formulations are generally not fully disclosed through the material safety data sheet or the

technical data sheet. Moreover, the formulations may be changed for reaching REACH

compliance in case formulation components become ruled.

In order to avoid these drawbacks for defining reproducible test, Valeo designed a

composition in order to reproduce close effect on the condenser as the one given by the

original Kärcher ™ solution. For achieving this aim, three targets were defined

- same pH as original Kärcher ™

- close wetting property as original Kärcher ™

- close fin loss resistance and brazing joint degradation as demonstrated on the

condenser

pH=11.7 +/-0.5 was reached by using NaOH pellets 5mM in demineralised water solution

Wetting property was achieved by using glycerin, well known available chemicals, without

any restriction of use. The concentration of glycerin was adjusted to 5% for the wetting

property, without pH modification.

In order to demonstrate the equivalence of corrosion attack on the condensers, two

comparative DeFinX tests were performed, first by using Kärcher ™ solution, second by

using the reproduced composition (NaOH 5mM +glycerin 5%). The tests were performed on

the technology of condensers, called ZAS on which fin debonding was reported. ZAS defines

the AlMn0.8% extruded tube covered with Zinc metallic layer. This definition requires

cladded fin for creating brazing joint between tube and fin. As a countermeasure for this fin

debonding phenomenon, an alternative technology of coating called “other technology” was

evaluated in the DeFinX test. The so called “other technology” defines the AlMn 0.8%

extruded tube coated with an alternative film. This film allows the brazing between tube and

fin without using cladded fin material.

Table 3 hereunder reports pH, surface tension and % of fin resistance loss after 5 weeks

DeFinX test

Karcher ™ 0.5% NaOH 5mM + Glycerin 5%

pH 11.7 +/-0.5 11.7 +/-0.5

Surface tension mN/m 35.3 +/- 1.2 38 +/-2

% fin resistance loss

ZAS technology

98% 98%

% fin resistance loss “other

technology”

29% 27%

Table 3: DeFinX test result comparison pH, surface tension, fin resistance loss

Analysis of the condenser tubes were performed after the

Kärcher ™ Solution and the NaOH 5mM + 5% glycerin. Very similar degradation

tube fin area were observed on both condenser tube technologies.

Figure 9: Metallographic analysis of the tube/fin area for ZAS tube condensers a

technology” tube condensers after 5 weeks

0.5% and NaOH 5mM + Glycerin 5%

The 5mM NaOH and glycerin 5% show close chemical characteristics

properties than the original 0.5% Kärcher

5 weeks DeFinX test are comparable, 5mM NaOH + 5% glycerin can replace the original

Kärcher ™ solution in this test definition.

Parts from field analysis

In order to characterize the fin debonding resist

sampled from the field. The technology of tube was ZAS on all the parts analyzed. Mileage of

the parts was included between 10.000 km and 320.000 km. Lifetime on the field was

included between 9 and 27 months in servi

No real tendency has been established between the lifetime and the f

were only able to characterize the evol

(figure 10).

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Analysis of the condenser tubes were performed after the DeFinX test performed with the

Kärcher ™ Solution and the NaOH 5mM + 5% glycerin. Very similar degradation

observed on both condenser tube technologies.

Figure 9: Metallographic analysis of the tube/fin area for ZAS tube condensers a

technology” tube condensers after 5 weeks DeFinX tests performed with Kärcher

0.5% and NaOH 5mM + Glycerin 5%

The 5mM NaOH and glycerin 5% show close chemical characteristics

properties than the original 0.5% Kärcher ™ solution. As the degradations observed after the

test are comparable, 5mM NaOH + 5% glycerin can replace the original

Kärcher ™ solution in this test definition.

In order to characterize the fin debonding resistance several condenser parts have been



included between 9 and 27 months in service in France.

No real tendency has been established between the lifetime and the fin resistance lost. We

able to characterize the evolution of the fin resistance loss against the mileage

test performed with the

Kärcher ™ Solution and the NaOH 5mM + 5% glycerin. Very similar degradations of the

Figure 9: Metallographic analysis of the tube/fin area for ZAS tube condensers and “other

Kärcher ™ solution

The 5mM NaOH and glycerin 5% show close chemical characteristics - pH and wetting

. As the degradations observed after the

test are comparable, 5mM NaOH + 5% glycerin can replace the original

ance several condenser parts have been



in resistance lost. We

against the mileage

Figure 10: Relation between fin/tube mechanical resistance against mileage on condenser

Parts after DeFinX test analysis

In order to characterize the fin debonding resistance several condenser parts with ZAS tube

definition have been tested in

tube and fin mechanical resistance and the lifetime until failure during test (figure 11).

Figure 11: Relation between fin/tube mechanical resistance against

DeFinX test on con

Reliability correlation between field and test

The approach of Physics of Failure consists in the reproduction of the failure mode observed

in the field on the bench via an ac

linear degradation law, we have to go up to failure.

The figure 12 presents the curves called “vehicle failure law” and “Test bench failure law”.

As expected, corrosion can be approximated by an exponential mechanism.

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Relation between fin/tube mechanical resistance against mileage on condenser

with ZAS tube technology

test analysis


in DeFinX test. It allowed us to draw the correlation between the


Figure 11: Relation between fin/tube mechanical resistance against lifetime until failure after

test on condenser with ZAS tube technology

Reliability correlation between field and test


in the field on the bench via an accelerated test set-up. As the corrosion mechanism is not a

linear degradation law, we have to go up to failure.

esents the curves called “vehicle failure law” and “Test bench failure law”.


Fin resistance loss (%)


Relation between fin/tube mechanical resistance against mileage on condenser


test. It allowed us to draw the correlation between the


lifetime until failure after


up. As the corrosion mechanism is not a

esents the curves called “vehicle failure law” and “Test bench failure law”.


Figure 12: Vehicle

A singular point is defined at 100% of fin resistance loss between the bench and the vehicle.

At this point, we obtain an equivalency of 5.3 weeks of test and 186.000 Km in the field.

If the carmaker target is 220.000 km, that means we have to find a tec

pass 6.3 weeks.

To validate a new design, we can impose basically a zone which is not acceptable in terms of

degradation. This zone is defined by a linear relationship called “

proposed in the figure 13 with

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Vehicle failure law and DeFinX test failure law



target is 220.000 km, that means we have to find a technology which has to


degradation. This zone is defined by a linear relationship called “DeFinX

proposed in the figure 13 with the equation: Y=0.063X.

test failure law



hnology which has to


DeFinX reliability law” as


Figure 13: Definition of the acceptance criterion based on

At this point, all degradations observed on the

permitted if we want to avoid

the bench is above this red line, we can expect to pass the vehicle requirements.

Moreover, we do not tolerate more

accelerated test and evaluate rapidly some designs.

This criterion is not sufficient. Indeed, we have a limited nu

reliability target to demonstrate (Reliability = 90

15 years and 220.000 Km on vehicle. To prove this target, we use a binomial law,

“Bogey testing”.

The formula is:

(1)

Parameters are:

C: Level of Confidence (80%)

α: 1-C (20%)

n: number parts tested (4)

R: Reliability target (90%)

β: beta - slope of Weibull distribution

t: test time (3 weeks)

T: new test time which includes the reliability targets

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Figure 13: Definition of the acceptance criterion based on DeFinX degradation law

At this point, all degradations observed on the bench which are below this red line can not

permitted if we want to avoid a risk of failure on the car. If the degradation law observed on


Moreover, we do not tolerate more than 45% of loss at 3 weeks. This helps us to set up an

accelerated test and evaluate rapidly some designs.

is criterion is not sufficient. Indeed, we have a limited number of part tested (n=4) and a

reliability target to demonstrate (Reliability = 90% and Level of Confidence = 80%) to cover

15 years and 220.000 Km on vehicle. To prove this target, we use a binomial law,

C: Level of Confidence (80%)

eibull distribution

T: new test time which includes the reliability targets

degradation law

bench which are below this red line can not

f the degradation law observed on


at 3 weeks. This helps us to set up an

mber of part tested (n=4) and a

% and Level of Confidence = 80%) to cover

15 years and 220.000 Km on vehicle. To prove this target, we use a binomial law, also called

6.3 weeks =

220.000km

A weibull analysis on bench data and on vehicle data defines

confirms the failure mechanism of “wear

Following the equation (1) and all parameters in our case, the new T time to pass is 5 weeks.

The figure 14 proposes the new

reliability targets.

Figure 14: updated criterion integrated reliability target

A new design can validated with the results proposed in the figure 15

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A weibull analysis on bench data and on vehicle data defines beta equal

mechanism of “wear-out”.


the new zone which verifies the reliability of the design including the


Figure 14: updated criterion integrated reliability target

validated with the results proposed in the figure 15

equal 2.5. This value


the reliability of the design including the

Figure 15: Validation of new design

Conclusion

DeFinX test is defined for reproducing exposure to strong alkaline media and cyclic

temperature and humidity conditions representative of condenser usage in its car environment.

Then the failure mode - fin debonding between extruded tube with zinc metallic coating and

cladded fin - has been successfully reproduced

A criterion has been defined including reliability R90C80 15 years or 220.000kms

New designs of extruded tube and fin are now proposed respecting such criterion

References

1. TR-597 Guideline for selection of

( 2006-04) - ISO 16701 : Corrosion of metals and alloys

Accelerated corrosion test involving exposure under controlled conditions of humidity cycling

and intermittent spraying of a salt solution

2. Guangbin Yang, Life Cycle Reliability

12


Figure 15: Validation of new design

r reproducing exposure to strong alkaline media and cyclic


fin debonding between extruded tube with zinc metallic coating and

has been successfully reproduced

been defined including reliability R90C80 15 years or 220.000kms

of extruded tube and fin are now proposed respecting such criterion

597 Guideline for selection of accelerated corrosion test for product qualification

ISO 16701 : Corrosion of metals and alloys – Corrosion in artificial atmosphere


nt spraying of a salt solution

2. Guangbin Yang, Life Cycle Reliability Engineering, John Wiler & Son,

r reproducing exposure to strong alkaline media and cyclic


fin debonding between extruded tube with zinc metallic coating and

been defined including reliability R90C80 15 years or 220.000kms.

of extruded tube and fin are now proposed respecting such criterion.

accelerated corrosion test for product qualification

Corrosion in artificial atmosphere –


Engineering, John Wiler & Son, 2007

reliable corrosion test development for heat exchanger fin...

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