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Failure Analysis, Corrosion Investigation, Metallurgical Testing and Engineering • Located in Mooresville, NC

ANALYSIS OF DEGRADED BRASS PLUMBING FITTINGS

 

 

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SUMMARY:

Two, one-inch, 90° elbow brass plumbing fittings were received for analysis of extensive deterioration and leakage.  Results indicated the fittings were severely degraded, due to “plug-type” dezincification corrosion.  The dezincification was due to the high concentration of internal deposits, hot water temperature, water pH and chloride content, possible stagnant or low flow conditions early in the fitting life, and the use of a brass material susceptible to this form of corrosion (copper alloy 36000, leaded brass).

 

Zinc was found to be totally leached from the brass alloy (normally approximately 34% zinc) leaving a porous matrix through the entire thickness of the fitting tube at isolated locations.  Leakage was imminent at all locations with similar fittings in the hot water line where similar flow velocities were found.

 

Heavy internal deposits resulted in localized corrosion cells where dezincification initiated.  Energy dispersive x-ray spectrographic (EDS) micro-analysis identified the composition of the corrosion products to be zinc oxide.  The attacked regions exhibited essentially pure metallic copper (with only traces of zinc remaining).  The heavy internal deposits were mainly zinc oxide (due to the corrosion) and aluminum oxide.  The source of aluminum oxide, without significant levels of calcium carbonate or silicon oxide, was unusual and may indicate corrosion of an aluminum component in the system.

 

All similar fittings should be replaced with dezincification resistant brass alloy fittings.  The water should be checked for pH and chlorine content.  Reduction of the maximum hot water temperature would also help reduce corrosion.  Any aluminum components in the system should be inspected for corrosion damage.

 

ANALYSIS:

Two brass plumbing elbow fittings were received for analysis.  The components (Figure 1) were reported to have been installed in an occupied residential home.  The construction was reported to have taken two years to complete.  Excessive corrosion deposits, stains and discoloration were observed on the OD surface of the fittings.  A close-up view of the ID of the fittings appears in Figure 2.  Heavy internal deposits were observed. 

 

Both fittings exhibited similar features.  One fitting was selected for an in-depth analysis.  The fitting was cross-sectioned to observe the internal deposits, as viewed in Figure 3.  The heavy deposit morphology was consistent with low flow velocity or a stagnant condition over an extended period of time.

 

EDS micro-analysis in general accordance to ASTM E1508-98 was performed to determine the composition of the internal deposits, with the results presented in Figure 4.  Results indicated zinc corrosion product and aluminum oxide.  No significant calcium (calcium carbonate, typical of hard water deposits) or silicon (silicon oxide, typical silt or sand deposits) were detected.  Smaller amounts of sulfur and chlorine were also detected in the deposits.  Chlorine in alkaline water (pH above about 8.3) promotes dezincification.  Mildly acidic water (pH below 7) without chlorine can also contribute to dezincification.  The water pH level was unknown and should be checked.

 

The ID surface was mechanically and ultrasonically cleaned of surface deposits and analyzed at various magnifications using a scanning electron microscope (SEM).  Figure 5 provides a SEM view of the cleaned ID surface.  Fine patches of corrosion were observed.  An increased magnification SEM view of the corrosion patches revealed evidence of localized corrosion cells, as shown in Figure 6

 

A high magnification SEM view of one of the cell sites is shown in Figure 7EDS analysis of the corrosion cell site identified nearly 100% copper with only a trace of zinc (Figure 8).  (The fitting alloy was subsequently verified to be leaded brass, containing approximately 34% zinc in non-corroded areas.)  The zinc has leached from the fitting alloy at the concentration cell sites.  This form of “de-alloying corrosion” in brass is known as dezincification.

 

A transverse cross-section was taken through one of the tube nipples extending from the elbow. The resulting sample was prepared for metallographic analysis in accordance with ASTM E3-01.  The sample was examined using an optical microscope in accordance with ASTM E883-02.

 

Figure 9 provides an over-exposed optical microscopic view of the mounted and polished fitting nipple cross-section, which revealed a thick ID surface deposit.  A properly exposed image of the cross-section is detailed in Figure 10.  The normal non-corroded brass metal exhibited a light yellow color.  Dezincification resulted in a copper color and fine voids in the affected areas.  Dezincification attack had penetrated 100% through the tube wall in one localized area.  The morphology was typical of plug type dezincification rather than uniform attack. 

 

The photo montage of Figure 11 shows the dezincification of the brass fitting cross-section at a second location.  High magnification optical microscopic views of attack revealed the voids in the microstructure as the zinc was leached out near the fitting ID surface and at mid wall in Figures 12 and 13, respectively.

 

Rockwell hardness testing was performed in accordance with ASTM E18-02.  Four separate readings were taken at random locations on a cross-section of fitting A, for an average hardness value of 63 HRBW.  The value indicated the part was in the H01 temper condition, typical of leaded brass alloy C36000 plumbing fittings.

 

CHEMICAL ANALYSIS:

Spectrographic chemical analysis was performed on the non-corroded fitting metal using an optical emission spectrometer in accordance with MTI Procedure No. MTI-0241, Rev. 0.  Results indicated the brass fitting meets elemental composition for copper alloy C36000 a common plumbing fitting alloy.  This alloy is susceptible to dezincification corrosion.  Table 1 provides the elemental analysis of the fitting.

 

TABLE 1

Brass Fitting Composition Analysis Results

(w%)

Element

Corroded Fitting

(wgt. %)

Copper Alloy C36000 Specification

(wgt. %)

Copper

62.8

60.0 – 63.0

Lead

2.61

2.5 – 3.7

Iron

0.19

0.35 (max)

Other

0.44

0.5 (max)

Zinc

34.0

Balance

 

CONCLUSIONS:

The alloy C36000 leaded brass fittings were severely compromised, due to dezincification of the brass matrix.  Patches of corrosion, due to localized cells under surface deposits, resulted in corrosive attack across the entire fitting nipple wall thickness.  All of the zinc had leached from the matrix, resulting in a porous fitting at these localized regions.  The dezincification will continue and all similar fittings in the hot water line are susceptible to imminent leakage. 

 

All similar fittings should be replaced with dezincification resistant brass alloy fittings.  The water should be checked for pH and chlorine content.  Reduction of the maximum hot water temperature would also help reduce corrosion.  Any aluminum components in the system should be inspected for corrosion damage.

IMAGES:

An overview of the two brass elbow plumbing fittings as received for analysis. Significant deposits and various stains were observed on the OD surface of the fittings. 


Figure 1:
An overview of the two brass elbow plumbing fittings as received for analysis.  Significant deposits and various stains were observed on the OD surface of the fittings. 

A close-up view of the ID surface of the fittings. Heavy internal deposits were observed. 

 

Figure 2: A close-up view of the ID surface of the fittings.  Heavy internal deposits were observed. 

A cross-section through one of the fittings revealed heavy orange and white internal deposits. The deposit morphology indicated low flow velocity or a stagnant condition over an extended period of time.


Figure 3:
A cross-section through one of the fittings revealed heavy orange and white internal deposits.  The deposit morphology indicated low flow velocity or a stagnant condition over an extended period of time.

EDS micro-analysis of the internal deposits indicated they were mainly comprised of zinc corrosion product and aluminum oxide. Sulfur and chlorine were also noted.

 

Figure 4: EDS micro-analysis of the internal deposits indicated they were mainly comprised of zinc corrosion product and aluminum oxide.  Sulfur and chlorine were also noted.

A SEM view of the cleaned ID surface. Localized patches of corrosion were observed. 


Figure 5:
A SEM view of the cleaned ID surface.  Localized patches of corrosion were observed. 

An increased magnification SEM view of the fitting ID surface showed the effects from localized corrosion cells.


Figure 6:
An increased magnification SEM view of the fitting ID surface showed the effects from localized corrosion cells.

A high magnification SEM view of a corrosion cell location on the fitting’s internal surface. EDS analysis of the attacked area is displayed in Figure 8.


Figure 7:
A high magnification SEM view of a corrosion cell location on the fitting’s internal surface.  EDS analysis of the attacked area is displayed in Figure 8.

EDS analysis of the localized corrosion site shown in Figure 7 identified nearly 100% copper with only a trace of zinc. Evidence of dezincification was revealed, as zinc had leached from the matrix.


Figure 8:
EDS analysis of the localized corrosion site shown in Figure 7 identified nearly 100% copper with only a trace of zinc.  Evidence of dezincification was revealed, as zinc had leached from the matrix. 

An over-exposed optical microscopic view of the cross-section fitting nipple tube, which revealed a thick ID surface deposit. 


Figure 9:
An over-exposed optical microscopic view of the cross-section fitting nipple tube, which revealed a thick ID surface deposit. 

An optical microscopic view of a cross-section through the fitting nipple tube. The unattacked brass is light yellow. The dezincification areas are copper colored. Dezincification attack had penetrated 100% through the tube wall in the encircled area. 


Figure 10:
An optical microscopic view of a cross-section through the fitting nipple tube.  The unattacked brass is light yellow.  The dezincification areas are copper colored.  Dezincification attack had penetrated 100% through the tube wall in the encircled area. 

This photo montage reveals the plug type (localized) dezincification penetration at a second location in the brass fitting nipple cross-section.


Figure 11:
This photo montage reveals the plug type (localized) dezincification penetration at a second location in the brass fitting nipple cross-section.

A high magnification optical microscopic view of attack of the zinc phase along the ID surface of the fitting cross-section. The unattacked brass is light yellow. The attacked area is copper colored and filled with voids where the zinc had leached out.


Figure 12:
A high magnification optical microscopic view of attack of the zinc phase along the ID surface of the fitting cross-section.  The unattacked brass is light yellow.  The attacked area is copper colored and filled with voids where the zinc had leached out.

A high magnification optical microscopic view of attack of the zinc phase at the fitting mid-wall.


Figure 13:
A high magnification optical microscopic view of attack of the zinc phase at the fitting mid-wall.