Corrosion, Failure Analysis and Materials Selection Specialists

 

 

 

 

 

Corrosion Testing / Failure Analysis

 

 

 

Corrosion Testing

Failure Analysis

Field Investigations

Litigation

Metallography

Technical Papers

CTL Profile

Pricing & Policies

Contact CTL

Quality Assurance

Return to Failure Analysis Case Histories

Dezincification of Brass Wax Actuator

ENVIRONMENT:

Residential

EQUIPMENT:

Water Supply System

MATERIAL:

Commerical Bronze
SERVICE TIME: 5 months

FAILURE MODE:

Dezincification

 

Background

The wax actuator was part of a residential plumbing fixture. The actuator was exposed to well water at 100°F for about 10 hours per day, 70°F for about 13.5 hours per day and 140°F for about 0.5 hours per day for approximately 5 months.

 

Findings

Visual Examination

The large end of the failed wax actuator was covered with a layer of green deposits. Underneath the green layer was a layer of reddish-brown deposits. These deposits had the appearance typically associated with dezincification of brass, Figure 1.

 

 

Figure 1. Wax Actuator submitted for analysis.

 

SEM/EDS Analysis

A small sample of each of the two different colored deposits was removed, mounted on a carbon stub, and placed onto the goniometer stage of a scanning electron microscope (SEM) equipped with Energy Dispersive x-ray Spectroscopy  (EDS). The EDS spectra of each were obtained. Both types of deposits were primarily copper and oxygen.  

The new actuator was placed into the SEM and EDS spectra was obtained of the metal surface at the large end of the actuator. A quantitative analysis indicated the material to be a copper-zinc alloy commonly known as commercial bronze (91% Cu, 9% Zn).  

Metallographic Analysis

The failed end of the actuator and the corresponding end of a new actuator were mounted in epoxy and polished following the guidelines of ASTM E 3. The structures of the deposits on the failed actuator are consistent with layer-type dezincification of brass, Figures 2 and 3.  When compared to a new actuator the failed actuator had lost from 0.5 to 3 mils of wall thickness due to dezincification. 

 

 

Figure 2. Polished cross-section of actuator showing wall loss due to corrosion, as-polished, 25X magnification. Figure 3.  Close-up of de-alloyed surface, as-polished, 750X.
 

Water Analysis

The water sample was analyzed for pH, dissolved oxygen and carbon dioxide, and by ion chromatography with the following results (Table 1). 

 

Table 1.

Water Analysis

CO2

27.0 mg/L

O2

8.6 mg/L

pH

6.50

Chloride

3.88 ppm

Nitrate

6.98 ppm

Sulfate

2.41 ppm

Bromide

0.56 ppm

 

Discussion

The analysis indicates that the wax actuator failed due to corrosion. The mode of corrosion is referred to as de-alloying.  In the case of copper-zinc alloys, it is dezincification. As the alloy is attacked the zinc goes into solution and the copper is redeposited onto the metal as a spongy, porous deposit. This deposit of pure copper usually oxidizes to copper oxide (reddish brown deposits) and/or may further react with the corrodent to form a layer of copper corrosion products on top (green deposits). In this case, the green deposits are characteristic of copper carbonate.  

Dezincification of brasses is generally limited to alloys that contain less than 85 wt% of copper. Commercial bronze (91 wt% Cu) is considered resistant but not immune to this type of corrosion.  

“Dezincification is the usual form of corrosion for uninhibited brasses in prolonged contact with waters high in oxygen and carbon dioxide” (ASM Metals Handbook Volume 13, Corrosion, pg. 614). Water that has been exposed to the atmosphere, as most city waters are, generally contains less than 1 mg/L of carbon dioxide. However, well water may contain substantially higher amounts of carbon dioxide, which is picked up when rainwater percolates through carbon dioxide rich areas in the soil.  

Site Index

Site Copyright © 1995 - 2007, All Rights Reserved,

Corrosion Testing Laboratories, Inc.

60 Blue Hen Drive

Newark, Delaware USA 19713

Phone: 1-302-454-8200

Fax: 1-302-454-8204

web@corrosionlab.com