Corrosion of a Leaking Marine Oil Cooler
Posted on June 13th, 2019 by Met-Tech
Analysis Of Condensate Induced Pitting Corrosion of a Leaking Marine Oil Cooler
Summary:
Two cores from a marine diesel engine oil cooler were received for analysis to investigate the cause of failure via leaking. Results indicate that the cooler core failed due to pitting corrosion initiated on the interior (oil side) at the edge of the fins. The orientation of the leaks was consistent with corrosion due to drop out of condensed moisture or sludge at the bottom interior edges of the fins.
The cooler is manufactured from 0.014-in. thick ferritic chromium stainless steel fins assembled by copper brazing to a low carbon steel mounting plate. No manufacturing defects were noted.
ANALYSIS:
Two cores from a marine engine oil cooler were received for analysis. The cores were identified for purposes of this report via letters “A” and “B”. Reportedly the coolers are from a twelve cylinder Detroit Diesel two-stroke marine engine. The engine was in service for eleven years. Reportedly the running time on the engine is 770 hours.
Figure 1 shows a side view of oil cooler core “B” as received for analysis. The two stacks on the core are oriented horizontally (as shown) when installed on the engine. Figure 2 shows an oblique view of core “B”. There are two stacks of hollow cooling fins attached to a mounting plate. Each stack on each core has an inlet and an outlet hole. Oil circulates inside the fins while the coolant circulates outside. Stamped identification marks on the mounting plates of both cores read:
* **
*********
**********
**********
← IN OUT →
The “9 94” identification mark probably indicates a manufacture date of September 1994 consistent with the coolers being original equipment for this engine.
Spectrographic chemical analysis of the mounting plates was performed in accordance with ASTM E415-99a. Results indicate the plates are manufactured from low carbon steel with low impurity levels, likely for increased forming properties. Chemical analysis results are provided in Table 1. No unusual conditions were observed in the compositions.
The cores were leak tested by pressurizing with air. Figure 3 presents a bottom view of one stack (“B2”) of fins from oil cooler core “B”. Leak testing revealed the leak was located in the boxed area, and stereomicroscope examination revealed multiple pinhole leaks at edges of the fins. Core “A” did not leak during testing.
Figure 4 shows a close-up side view of a typical pinhole leak location on stack “B2”. The leak is located at a bottom edge where the fin overlaps for a braze joint. One pinhole leak location on a fin was examined using a scanning electron microscope (SEM). A low magnification SEM overview of the leak, Figure 5, reveals a roughly rectangular hole with irregular edges. Figure 6 is an increased magnification SEM view of the lower edge of the hole that reveals the metal has an extremely thin edge consistent with corrosion. Figure 7 shows an increased magnification SEM view of the upper edge of the hole, which has an irregular contour.
The fin was cut open for examination of the interior. Figure 8 is a close-up interior view of the pinhole leak. The leak site is a corrosion pit. Corrosion has exposed the edge of a brazed lap joint. Several reinforcing ribs also are noted. Figure 9 is an oblique SEM interior view of the pinhole leak, which more clearly reveals both the corrosion pit and the reinforcing ribs. Figure 10 is another SEM interior view of the pinhole leak that reveals the oval shape of the corrosion pit. Two boxed areas are shown at higher magnification in Figures 11 and 12. Figure 11 shows the bottom of the pit in the fin and braze material where etching from corrosion has revealed the grain boundaries in both materials. Figure 12 is a high magnification SEM view of the outer edge of the pit, which reveals grain boundary facets with additional smaller pits. The intergranular pit morphology confirms corrosive attack as the pitting mechanism.
The fin metal and braze material were analyzed by energy dispersive spectroscopy (EDS) per ASTM E1508 at the locations indicated in Figure 11. Figure 13 presents an EDS spectrum of the fin material. Large iron peaks, and a smaller chromium peak are detected along with small amount of manganese, silicon, and copper indicating the fin material is a chromium stainless steel. Figure 14 presents an EDS spectrum of the braze material. Large copper peaks and smaller iron and chromium peaks are detected, indicating a copper based braze alloy was used for assembly. No zinc or phosphorus were noted (these elements tend to form brittle compounds in stainless steel braze joints).
A cross-section was taken through the pinhole leak in another fin and a second comparison fin in the same stack at the location shown in Figure 3. The cross-section samples were prepared for metallographic analysis in accordance with ASTM E3-01. Etching in accordance with ASTM E407-99 revealed the microstructures that were examined using an optical microscope in accordance with ASTM E883-02.
Figure 15 shows a low magnification optical microscope view of a cross-section through a comparison fin. In the cross-section, the ends of the fin are J-shaped and overlap at the brazed lap joint. Additional copper braze joints attach the steel ribs to the fins.
Figure 16 shows a low magnification view of a cross-section through one of the pinhole leaks on the oil cooler core. Note the thinning of the stainless steel due to internal pitting corrosion. The leak is revealed at increased magnification in Figure 17, which shows that pitting corrosion adjacent to the leak has thinned the fin to less than a tenth of its original thickness. Figure 18 shows the overlap area adjacent to the pinhole leak. There is also thinning of the stainless steel due to internal pitting corrosion. The micro structure of the stainless steel fins is polygonal ferrite grains, which is normal.
The orientation of the leaks was consistent with corrosion due to drop out of condensed moisture or sludge at the bottom corners of the fins when the oil was not flowing. Although stainless steels resist general corrosion, they are susceptible to failure by pitting or crevice corrosion.
CHEMICAL ANALYSIS
Table 1
Chemical Analysis of Mounting Plates from Cores A1 and B1
| Element |
Core A1
% wt. |
Core B1
% wt. |
AISI/SAE
1005 Specification % wt. |
| Carbon |
0.01
|
0.01
|
0.06 max
|
| Manganese |
0.31
|
0.30
|
0.35 max
|
| Phosphorus |
0.005
|
0.004
|
0.030 max
|
| Sulfur |
0.004
|
0.003
|
0.050 max
|
| Silicon |
<0.002
|
<0.002
|
–
|
| Nickel |
<0.002
|
<0.002
|
–
|
| Chromium |
0.010
|
0.010
|
–
|
| Molybdenum |
0.004
|
0.004
|
–
|
| Aluminum |
0.013
|
0.012
|
–
|
The mounting plates meet the specification for AISI/SAE 1005 low carbon steel.
CONCLUSIONS:
The oil cooler core failed due to pitting corrosion initiated on the interior (oil side) at the edges of the fins. The orientation of the leaks was consistent with corrosion due to drop out of condensed moisture or sludge at the bottom edges of the fins.
The cooler is manufactured from 0.014-in. thick ferritic chromium stainless steel fins assembled by copper brazing to a low carbon steel mounting plate. No manufacturing defects were noted.
IMAGES:
Figure 1: A side view of oil cooler core “B” as received for analysis. The two stacks on the core are oriented horizontally (as shown) when installed on the engine. (Photo PB0791)
Figure 2: An oblique view of oil cooler core “B” as received for analysis. Note two stacks of hollow cooling fins attached to a mounting plate (arrow). Each core has an inlet and an outlet hole. Oil circulates inside the fins and the coolant circulates outside. (Photo PB0793)
Figure 3: A bottom view of one stack (“B2”) of fins from oil cooler core “B”. Leak testing revealed the leak was located in the boxed area, and stereomicroscope examination revealed multiple pinhole leaks at edges of the fins. One leak was examined using the SEM and another was cross-sectioned for metallographic examination. (Photo PB0841)
Figure 4: A side view of a typical pinhole leak location (arrow) on stack “B2”. The leak is located at a bottom edge where the fin overlaps at a braze joint. (Photo PB0842)
Figure 5: A scanning electron microscope (SEM) side view of a pinhole leak on the core reveals irregular edges on the hole. Two boxed areas are shown at higher magnification in Figures 6 and 7. (SEM Photo 2S6467, Mag: 100X)
Figure 6: An increased magnification SEM view of the lower edge of the hole reveals that the metal has an extremely thin edge (arrow) consistent with corrosion. (SEM Photo 2S6468, Mag: 300X)
Figure 7: An increased magnification SEM view of the upper edge of the hole reveals the irregular contour of the leak. (SEM Photo 2S6471, Mag: 300X)
Figure 8: A close-up interior view of the pinhole leak (after sectioning to expose the internals) reveals a corrosion pit (arrow) that has exposed the edge of a braze joint. Several reinforcing ribs also are noted (arrows). (Photo PB0865)
Figure 9: An oblique SEM interior view of the pinhole leak more clearly reveals the corrosion pit and the reinforcing ribs. (SEM Photo 2S6482, Mag: 30X)
Figure 10: An SEM interior view of the pinhole leak reveals the corrosion pit and the reinforcing ribs. Two boxed areas are shown at higher magnifications in Figures 11 and 12. (SEM Photo 2S6475, Mag: 30X)
Figure 11: An increased magnification SEM view of the fin and braze material at the pit bottom shows that etching from corrosion has revealed the grain boundaries in both materials. Two locations where EDS analyses were subsequently taken are indicated. (SEM Photo 2S6476, Mag: 100X)
Figure 12: A high magnification SEM view of the outer edge of the pit reveals grain boundary facets. (SEM Photo 2S6481, Mag: 250X)
Figure 13: EDS spectrum of the fin material on the marine oil cooler. Large iron peaks, and a smaller chromium peak are detected, along with small amount of manganese, silicon, and copper, indicating the fin material is a chromium stainless steel. (2SPT1363)
Figure 14: EDS spectrum of the braze material on the marine oil cooler fin. Large copper peaks and smaller iron and chromium peaks are detected, indicating a copper based braze alloy was used for assembly. (2SPT1362)
Figure 15:A low magnification optical microscope view of a cross-section through a comparison non-leaking fin on the oil cooler core. Note the copper braze joints (arrows) and steel ribs (arrows). The leak location is also noted. (Photo D1379, Mag: 15X, Vilella’s etch)
Figure 16: A low magnification optical microscope view of a cross-section through a pinhole leak on the oil cooler core. Note thinning of the stainless steel (arrows) due to internal pitting corrosion. The leak is shown at increased magnification in Figures 17 and 18. (Photo D1380, Mag: 15X, Vilella’s etch)
Figure 17: A higher magnification optical microscope view of a cross-section through a pinhole leak on the oil cooler core. Note thinning (arrows) of the 0.014-in. stainless steel fin due to internal pitting corrosion. (Photo D1384, Mag: 50X, Vilella’s etch)
Figure 18: A higher magnification optical microscope view of a cross-section through the overlap area adjacent to the pinhole leak. Note thinning (arrows) of the stainless steel due to internal pitting corrosion. (Photo D1381, Mag: 50X, Vilella’s etch)
CONFIDENTIALITY:
The information contained in this report is confidential property of our client and is intended solely for the use of the named recipient(s). The information and statements in this report are derived from and related only to the material tested. This report shall not be reproduced except in full, without written approval of Metallurgical Technologies, Inc.
