LUBE OIL COOLER Brass Tube Failure_technical Paper

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Failure analysis of the brass tubes in a lubricating oil cooler

S. Qu , G. Yao, J.F. Tian, Z.F. Zhang

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China

a r t i c l e i n f o

Article history:

Received 28 December 2010Received in revised form 29 June 2011

Accepted 19 July 2011

Available online 6 August 2011

Keywords:

Lubricating oil cooler

Brass tubes

Pit

Dezincification

Stress corrosion cracking (SCC)

a b s t r a c t

Failure analysis was carried out on leaked brass tubes of a lubricating oil cooler. Direct evi-

dences of dezincification and stress corrosion cracking (SCC) were observed by scanningelectron microscope (SEM) and energy dispersive spectroscopic (EDS) analysis. It is found

that there are many small pits distributed on the fracture surface and EDS analysis revealed

the occurrence of dezincification in the small pits. SCC was observed on the cross-sectional

plane of the fracture by SEM. Ammonia test has proved the existence of residual stress in

the as-received tubes. It is determined that the brass tubes have been suffered from the co-

action between dezincification and SCC.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

A lubricating oil cooler are usually used to cool the hydraulic liquid of a hydraulic power assistance system. The

cooler is shell and U-shaped tube type heat exchanger with hydraulic liquid on the shell side and seawater on the tube

side.

Leaks occurred at two U-shaped sections of cooling brass tubes in the lubricating oil cooler. The performed anal-

yses allow us to indicate the failure cause of the cooling tubes. The failed tubes were examined visually and by

scanning electron microscope (SEM) and X-ray mapping, respectively. The results of these analyses are presented

in this study.

2. Visual observation and experimental procedure

One of the failed cooling brass tube is shown in Fig. 1, it is apparent that there is a transverse crack at the U-shaped

section of cooling brass tube and no evident corrosion phenomenon can be found on the outside of tube.In order to find out the corrosion extent along the thickness direction of the wall, the cracked tube was cut along the

direction perpendicular to the crack and the metallographic section plane of the fracture was obtained. SEM observations

were performed on the surface and the cross-sectional plane of the fracture to detect the failure mode. The results will

be shown in Sections3.2 and 3.4, respectively. EDS analysis was made to identify the elemental composition of different

corrosion regions. Meanwhile, elemental X-ray mapping was made on the cross-sectional plane of the fracture to determine

the distribution of the elements Cu and Zn. In addition, ammonia test was performed to find out whether or not there is

residual stress leading to the SCC.

1350-6307/$ - see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.engfailanal.2011.07.018

Corresponding author. Tel.: +86 24 83978776.

E-mail address: [email protected](S. Qu).

Engineering Failure Analysis 18 (2011) 22322239

Contents lists available atSciVerse ScienceDirect

Engineering Failure Analysis

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l oc a t e / e n g f a i l a n a l
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3. Experimental results

3.1. Chemical composition analysis

The failed tubes were made of the arsenic brass. The chemical composition of the as-received brass tube was examined by

chemical analysis, and the results are shown inTable 1. In order to compare with the examined results, the standard valuesof the arsenic brass are also listed in Table 1. It can be seen that the chemical compositions of as-received brass tube are quite

close to the standard values, indicating that the compositions are in consistent with the requirement.

3.2. SEM observations of fracture surface and the inner wall

Fig. 2a is a low-magnification image of the fracture surface. The outer and inner wall surfaces of the brass tube are indi-

cated by the black arrows 1 and 2. The part between the two arrows is the cracking path. It can be seen that there are some

small pits on the inner wall surface near the fracture surface as indicated by the white arrow. An ordinary shape of fracture is

shown inFig. 2b and the partial enlarged view in mourning border ofFig. 2b is expressed inFig. 2c. Both figures show that

there are a lot of pits on the fracture surface. Sometimes, larger-size pits can also be seen on the fracture surface as indicted

by the white arrows inFig. 2d. The magnificated image of these larger pits is shown in Fig. 2e and the fracture surface is

crude with lots of smaller pits. It indicates that the larger pits have the same formation mechanisms as the whole fracture.

Moreover, some large-size pits can be seen on the inner wall surfaces adjacent to the cracking fracture (seeFig. 2f) and thesepits connect directly with the fracture. This fracture surface image is similar to the failure surface of a muntz tubesheet

which was induced by dezincification[1]. In addition, a number of grooves in pit-like forms can be seen on the etching sur-

face of the failure muntz tube sheet.

3.3. Energy spectrum analysis

In order to find out the formation mechanisms of the pits above, different parts of the fracture surfaces were chosen to do

the energy spectrum analysis, and the results are shown inFig. 3. A local enlarged image of fracture is shown inFig. 3a. There

is a pit with diameter of about 200 lm which is denoted by number one on the fracture surface. Fig. 3b is the intensified

image of this pit and the EDS analysis curve inFig. 3b1. The corresponding numerical results are expressed inTable 2. Both

the curves and the numerical results indicate that the content of zinc element is extremely scarce in all pits. It is reported

that dezincification occurred mostly on the grain boundary in muntz tube sheet[1].Fig. 3c is the local enlarged image of the

fracture surface denoted by number two inFig. 3a. Local area without pits is chosen to do the EDS analysis as indicated by# inFig. 3c. The EDS analysis curve of this area is shown inFig. 3c1and the corresponding numerical results are expressed

inTable 2. Both the curve and the numerical results demonstrate that the chemical compositions of this area are basically in

accordance with the normal material composition. In other words, zinc element is not absent in this area.

Fig. 1. An example of a leaked tube (the leaked point as indicated by the white arrow).

Table 1

Chemical compositions of the as-received tube.

Chemical compositions

Cu Al As Fe Sb Bi Pb P Zn Impurity

Testing value (wt%)

76.9 2.06 0.04 0.031


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The results above indicate that the formation of those pits would be attributed to the dezincification because the chemical

compositions of the fracture area with pits are zinc deficiency. On the other hand, the chemical compositions of the fracture

areas without pits are in accordance with normal compositions of the brass tube. Therefore, this gives rise to an open ques-

tion: how and why did the dezincification occur in the local areas of the brass tube?

3.4. SEM observation of metallographic section plane of fracture

Fig. 4a is a local enlarged image of the metallographic section plan of fracture. The intersecting line between fracture sur-

face and section plan of fracture is indicated by arrow one. However, the intersecting line between inner wall of tube and

section plan of fracture is denoted by arrow two. The letters A and B indicate mounting material. It can be seen that

lots of small pits distributed at the region as denoted by the white arrow in Fig. 4a and 4b is a backscattered electron image

with the same region asFig. 4a. It is clearly revealed that the delta region near the fracture was severely eroded with the pits(white arrow). In addition, one linear vein can be observed to extend from the delta region as indicated by the black arrow.

Fig. 2. SEM observation of fracture surface: (a) low-magnification observation; (b) fracture morphology; (c) local enlarged image ofFig. 2b; (d) large pits on

the fracture surface; (e) morphology in the large pits; and (f) some large pits on inner wall connected with fracture.

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The linear vein inFig. 4b is proved to be a crack as indicated by the white arrow in Fig. 4c after etched by the ferric-alcohol

reagent.Fig. 4d is a local enlarged image of the crack denoted by the white arrow in Fig. 4c. Some branches can be observed

on the lead crack (white arrows). This kind of crack corresponds to the character of the stress corrosion cracking [2].

3.5. Elemental X-ray mapping on the cross-sectional plane of the fracture

The distribution of two main elements (Cu and Zn) on the cross-sectional plane near the fracture by energy spectrum

analysis is shown inFig. 5. As inFig. 4b, Fig. 5a is a backscattered electron image of the cross-sectional plane near the

fracture. The fracture surface lies on the bottom of image. Evidently, two regions denoted by black arrows are dark becausethere are lots of small pits in them. The distribution image of element Cu on the cross-sectional plane of the fracture is

Fig. 3. Energy dispersive spectroscopic (EDS) analysis result of small pits on the fracture surface; (a) two regions were chosen: a pit (point as indicated by

one) and matrix (point as signed by two; the enlarged image of point one (b) and corresponding EDS analysis curve (b 1); the enlarged image of point

two (c) and corresponding EDS analysis curve (c1).

Table 2

Energy dispersive spectroscopy (EDS) analysis results on fracture surface.

Measured position Chemical composition (wt%)

Al Cu Zn

Fracture position 1 inFig. 3a 0.28 99.22 0.50

Fracture position 2 inFig. 3a 1.92 76.56 21.53

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shown inFig. 5b. It is clearly shown that the distribution of element Cu corresponds totally with the whole surface of the

cross-sectional plane, which indicates that the distribution of element Cu on the cross-sectional plane is normal.Fig. 5c isthe distribution image of element Zn on the surface of the cross-sectional plane of the fracture. It is apparent that the dis-

tribution of element Zn only corresponds with the brighter part inFig. 5a, but not with the whole cross-sectional plane. It is

indicated that the dark parts inFig. 5a are the regions of zinc deficiency because these areas are composed of many small pits

with less zinc content.

The EDS analysis curves of main elements (Cu, Zn and Al) on the dark and bright areas in Fig. 5a are shown inFig. 6and

the corresponding numerical results are listed in Table 3. Both of results indicate that the Zn content on the dark region in

Fig. 5a is far less than the average value of the matrix.

3.6. Ammonia test

In order to find out the source of residual stress, a U-shape section of brass tube without cracks was used to do the ammo-

nia test at room temperature (2225 C) for 16 h with 25% ammonia solution.Fig. 7a is the piece of brass tube before ammo-nia test. After ammonia test, some cracks can be clearly seen on the tube surface (see Fig. 7b). As a result, it can be proved

that there was residual stress in the brass tube, which supplied a condition to form the SCC.

4. Discussion

4.1. The relation between small pits and dezincification

Based on the observations above, the fracture morphologies mainly have two features, one is fragmentized and the other

is composed of many small pits. Moreover, there are some large pits with the size of up to hundred of microns on the inner

wall near the fracture. SEM observations show that the large pits are composed of lots of small pits and EDS analysis results

indicate that dezincification always occurred in the small pits. As a result, it can be determined that the corrosion of the

tubes basically resulted from the small pits and the pits were formed where dezincification had occurred. Thus, the SEMobservation provided direct evidence that the formation of pits should be attributed to dezincification.

Fig. 4. SEM observation of cross-sectional plane of the fracture: (a) region with pits on the cross-sectional plane of the fracture (white arrows); (b) back-

scattered image ofFig. 4a; (c) stress corrosion cracking on the cross-sectional plane of the fracture after eroded; and (d) an enlarged image ofFig. 4c.



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Dezincification is one of typical case for the selective leaching. There are numerous theories about the mechanisms of

selective leaching, but two predominant mechanisms prevail [2]. The first mechanism states that two elements dissolve

in the alloy and then one re-deposits on the surface. The second mechanism emphasizes that one element selectively dis-

solves from the alloy, leaving the more noble elements in a porous mass. As dezincification of copper, two types of damages

can be characterized; one type of dezincification is uniform, and the second is plug-type. It is evident that the current dezin-

cification is not uniform and should be attributed to the latter. This kind of dezincification often leads to the formation of

large pits with the size of hundred of microns. With further service under the corrosion environment, it is easily understood

that the dezincification finally caused the corresponding failure in the brass material[1,3,4]and will be further discussed in

the following section.

4.2. The relation of leakage of copper tubes with SCC

The SEM observation results of the cross-sectional plane of the fracture and the ammonia test indicated that the crack is

one kind of SCC (seeFigs. 4 and 7). Except for the well-known ammoniac solutions, the previous work[58]has shown thatcopper and copper alloys are susceptible to the SCC in many industry environments which contain sulfate[5], nitrates[6],

Fig. 5. X-ray mapping of cross-sectional plane of the fracture showing dezincification: (a) back-scattered SEM microscopy image; (b) Cu X-ray mapping;

and (c) Zn X-ray mapping.



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Fig. 6. Corresponding EDS analysis curves ofFig. 5a; (a) the darker region ofFig. 5a, and (b) the brighter region ofFig. 5a.

Table 3

Energy dispersive spectroscopy (EDS) analysis results on the cross-sectional plane.

Measured position Chemical composition (wt%)

Al Cu Zn

Dark region inFig. 5a 2.27 95.87 1.84

Bright region inFig. 5a 0.93 77.78 21.29

Fig. 7. Ammonia test results (a) before test and (b) the cracks as signed by white arrow after test.



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and moist air containing either sulfur dioxide[7]or nitrogen oxides[8]. It is presumed that the sea water in the tube side can

lead to the SCC of the brass tubes. Therefore, whether the service environment or the state of these brass tubes supply con-

ditions for the formation of the SCC.

From the observation results above, the failure of the brass tube can be considered to undergo three processes. First, some

local pits were formed by the plug-type dezincification and became the stress concentration sources. Then, with the coac-

tions between the residual stress and aggressive medium of sea water in the brass tubes, the SCC was initiated from these

small pits. Last, once this kind of cracks penetrated the wall of brass tubes, leak event has occurred.

5. Conclusions

Based on the SEM observations and the analysis above, it can be concluded that the failure of the brass tubes is attributed

to the co-action between the small pits formed by dezincification and SCC. The SEM observation and EDS analysis give direct

evidences for the dezincification and SCC of brass tubes.

Acknowledgment

We highly appreciate for the financial support to Materials Failure Analysis Center (MFAC) from Shenyang National

Laboratory for Materials Science (SYNL).

References

[1] Shalaby HM. Eng Fail Anal 2006;13:7808.[2] William TB, Roch JS. Corrosion failures. In: ASM handbook: failure analysis and prevention, metals handbook, vol. 10. 8th ed. Materials park: ASM

International; 1975. p. 180.[3] Russo SG, Henderson MJ, Hinton BRW. Eng Fail Anal 2002;9:42334.[4] Carlo M, Andrea G, Mattia B. Eng Fail Anal 2010;17:4319.[5] Pickering HW, Byrne PJ. Corrosion 1973;8:325.[6] Graf L, Byrne PJ. Corrosion 1973;8:325.[7] Graf L. In: Conference on fundamental aspects of stress corrosion cracking, the Ohio State University, Columbus (OH): NACE; 1969. p. 325.[8] Johnston RG. Sheet Metal Ind 1940;14:1197.


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