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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
Objective was to implement a test in order to characterize the fin debonding resistance of new
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
This study deals with the development of a reliable corrosion test named DeFinX (Debonding
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
Figure 1: Pictures of a condenser with fin debonding issue and metallographic analysis
of the failure mode by optical observation
Objective was to implement a test in order to characterize the fin debonding resistance of new
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
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
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).
Figure 2: Illustration of ZAS coating deposit on extruded tube
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
This study deals with the development of a reliable corrosion test named DeFinX (Debonding
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
Figure 1: Pictures of a condenser with fin debonding issue and metallographic analysis
Objective was to implement a test in order to characterize the fin debonding resistance of new
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).
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
by atomisation of a
ZAS (picture 2).
Figure 2: Illustration of ZAS coating deposit on extruded tube
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.
3
tube alloy and tube surface. However, too high quantity of zinc deposited on tube surface can
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
definition. These guidelines define cycles of temperature and humidity.
In addition, the temperature profile was observed on the condenser surface during its
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
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- 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
such alkaline solution and cyclic conditions representatives of external environment and
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
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 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.
tube alloy and tube surface. However, too high quantity of zinc deposited on tube surface can
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
definition. These guidelines define cycles of temperature and humidity.
In addition, the temperature profile was observed on the condenser surface during its
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
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 -
he acidic range (pH
Then DeFinX test defines
such alkaline solution and cyclic conditions representatives of external environment and
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
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
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.
4
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.
Figure 4: Samples during immersion step and spraying step
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.
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
Figure 4: Samples during immersion step and spraying step
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 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
5
Figures 5 and 6: samples dimension for fins resistance measurement and samples placed
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
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 loss result, and acceptance level
Other technology
Figures 5 and 6: samples dimension for fins resistance measurement and samples placed
Figures 7 and 8: Dynamometer measurement device
Results are given in Newton (N) for 3 cm width, then in N/cm, then in % of resistance loss
loss result, and acceptance level
6
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).
7
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
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 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
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
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.
8
Relation between fin/tube mechanical resistance against mileage on condenser
with ZAS tube technology
test analysis
In order to characterize the fin debonding resistance several condenser parts with ZAS tube
in DeFinX test. It allowed us to draw the correlation between the
tube and fin mechanical resistance and the lifetime until failure during test (figure 11).
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
The approach of Physics of Failure consists in the reproduction of the failure mode observed
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”.
As expected, corrosion can be approximated by an exponential mechanism.
Fin resistance loss (%)
Fin resistance loss (%)
Relation between fin/tube mechanical resistance against mileage on condenser
In order to characterize the fin debonding resistance several condenser parts with ZAS tube
test. It allowed us to draw the correlation between the
tube and fin mechanical resistance and the lifetime until failure during test (figure 11).
lifetime until failure after
The approach of Physics of Failure consists in the reproduction of the failure mode observed
up. As the corrosion mechanism is not a
esents the curves called “vehicle failure law” and “Test bench failure law”.
As expected, corrosion can be approximated by an exponential mechanism.
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
9
Fin resistance loss (%)
Vehicle failure law and DeFinX test failure law
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.
target is 220.000 km, that means we have to find a technology which has to
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 “DeFinX
proposed in the figure 13 with the equation: Y=0.063X.
test failure law
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.
hnology which has to
To validate a new design, we can impose basically a zone which is not acceptable in terms of
DeFinX reliability law” as
Fin resistance loss (%)
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
10
Fin resistance loss (%)
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
the bench is above this red line, we can expect to pass the vehicle requirements.
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
the bench is above this red line, we can expect to pass the vehicle requirements.
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
11
A weibull analysis on bench data and on vehicle data defines beta equal
mechanism of “wear-out”.
Following the equation (1) and all parameters in our case, the new T time to pass is 5 weeks.
the new zone which verifies the reliability of the design including the
Fin resistance loss (%)
Figure 14: updated criterion integrated reliability target
validated with the results proposed in the figure 15
equal 2.5. This value
Following the equation (1) and all parameters in our case, the new T time to pass is 5 weeks.
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
Fin resistance loss (%)
Figure 15: Validation of new design
r reproducing exposure to strong alkaline media and cyclic
temperature and humidity conditions representative of condenser usage in its car environment.
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
Accelerated corrosion test involving exposure under controlled conditions of humidity cycling
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
temperature and humidity conditions representative of condenser usage in its car environment.
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 –
Accelerated corrosion test involving exposure under controlled conditions of humidity cycling
Engineering, John Wiler & Son, 2007