1. Introduction

Metallic materials in home construction are subjectto corrosive attack by the normal internal atmosphere

of homes as well as the other environments theyencounter during service in the home. The corrosionresistance of the metals in these environments is amajor factor in determining the normal deterioration of

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[J. Res. Natl. Inst. Stand. Technol. 115, 201-208 (2010)]

Interim Report on the Examination ofCorrosion Damage in Homes Constructed WithImported Wallboard: Examination of Samples

Received September 28, 2009

Volume 115 Number 3 May-June 2010

D. J. Pitchure, R. E. Ricker,M. E. Williams, and S. A.Claggett

Metallurgy Division,Materials Science andEngineering Laboratory,National Institute of Standardsand Technology,Gaithersburg, MD 20899-8553

david.pitchure@nist.govrichard.ricker@nist.govmaureen.williams@nist.govsandra.claggett@nist.gov

Since many household systems arefabricated out of metallic materials,changes to the household environment thataccelerate corrosion rates will increase thefrequency of failures in these systems.Recently, it has been reported that homesconstructed with imported wallboard haveincreased failure rates in appliances, airconditioner heat exchanger coils, andvisible corrosion on electrical wiring andother metal components. At the request ofthe Consumer Product Safety Commission(CPSC), the National Institute ofStandards and Technology (NIST) becameinvolved through the InteragencyAgreement CPSC-1-09-0023 to perform

metallurgical analyses on samples andcorrosion products removed from homesconstructed using imported wallboard.This document reports on the analysis ofthe first group of samples received byNIST from CPSC.

The samples received by NIST onSeptember 28, 2009 consisted of coppertubing for supplying natural gas and twoair conditioner heat exchanger coils. Theexaminations performed by NISTconsisted of photography, metallurgicalcross-sectioning, optical microscopy,scanning electron microscopy (SEM), andx-ray diffraction (XRD). Leak tests werealso performed on the air conditioner heatexchanger coils. The objective of theseexaminations was to determine extent andnature of the corrosive attack, the chemicalcomposition of the corrosion product,and the potential chemical reactions orenvironmental species responsible foraccelerated corrosion.

A thin black corrosion product wasfound on samples of the copper tubing.The XRD analysis of this layer indicatedthat this corrosion product was a coppersulfide phase and the diffraction peakscorresponded with those for the mineraldigenite (Cu9S5). Corrosion products werealso observed on other types of metalsin the air conditioner coils where conden-sation would frequently wet the metals.The thickness of the corrosion productlayer on a copper natural gas supply pipewith a wall thickness of 1.2 mm ± 0.2 mmwas between 5 μm and 10 μm.

These results indicate that a chemicalcompound that contains reduced sulfur,such as hydrogen sulfide (H2S), is presentin the environment to which these sampleswere exposed. The literature indicates thatthese species strongly influence corrosionrates of most metals and alloys even atlow concentrations. None of the samplesexamined were failed components, and noevidence of imminent failure was found onany of the samples examined.

All of the corrosion damage observed todate is consistent with a general attackform of corrosion that will progress in auniform and relatively predictable manner.No evidence of localized attack was found,but these forms of attack typically requirean incubation period before they initiate.Therefore, the number of samplesexamined to date is too small to draw aconclusion on the relative probability ofthese forms of corrosion being able tocause or not cause failure. Samples fromfailed systems or from laboratory testsconducted over a wide range of metallurgi-cal and environmental conditions will berequired to assess the probability of theseother forms of corrosion causing failure.

Key words: atmospheric corrosion;copper; copper sulfide; householdappliances; hydrogen sulfide; indooratmosphere; sulfide; wallboard.

Accepted: February 24, 2010

Available online: http://www.nist.gov/jres

Editor’s Note: This paper is based on NIST IR 7652, Interim Report on the Examination of Corrosion Damage in Homes Constructed WithImported Wallboard: Examination of Samples Received September 28, 2009. Most of the content remains the same.

the performance of household appliances, switches, safe-ty systems, and water, electrical, and gas supply systems.When these systems deteriorate more rapidly thannormal, there is a high probability that accelerated corro-sion is the cause. Identification of the factors responsiblefor accelerated corrosion can be done through threeways: (i) identification of the unique characteristics ofthe homes with accelerated deterioration, (ii) identifica-tion of the differences in the environments encounteredin the homes with accelerated deterioration, or (iii) iden-tification of failure modes, corrosion product composi-tions, and responsible chemical reactions.

Of immediate concern is the accelerated deteriora-tion of systems in homes constructed with importedwallboard. Due to the very large number of complaintsfrom the owners of homes constructed with importedwallboard, the Consumer Product Safety Commission(CPSC) has become concerned that this product con-tains chemical species that alter the internal atmosphereof the home and accelerate corrosion and the deteriora-tion of systems that rely on metals to perform criticalfunctions. As a result, the CPSC has instituted a largemultitasked program to look at this product and how itinfluences the health of occupants, the performance ofcritical household systems, and the prevalence of thisproduct in homes in the US. As part of this program, theMaterials Science and Engineering Laboratory (MSEL)of the National Institute of Standards and Technology(NIST) became involved through the InteragencyAgreement CPSC-1-09-0023 to examine metallicsamples removed from service in homes constructedusing imported wallboard. The objective of theseexaminations is to identify potential corrosion relatedfailure modes, corrosion product chemistry, and poten-tial chemical reactions. This document reports on thefindings of the examinations performed on thefirst group of samples provided to NIST by CPSC.All findings reported here should be consideredpreliminary, as more examinations are planned. Noneof the samples provided to NIST in the first group werefailed components. Therefore, none of the conclusionsdrawn here can address the mechanisms of the reportedfailures.

2. Examinations

A. Sample Selection and Delivery

The samples were selected by CPSC staff and trans-ported and hand delivered to NIST. The samples were

received on September 28, 2009 and immediately cata-loged for recording and tracking the examinations. Allof the samples received were identified with a numberassigned to the sample by CPSC. These numbers con-sisted of eleven digits, starting with: 09-302-####-##.NIST used the last six unique digits of the samples totrack them during the examinations. The samplesreceived consisted of a number of lengths of coppertubing used as natural gas supply lines and two air con-ditioner heat exchanger coils.

B. Visual Examination and Photography

The lengths of copper tubing were discoloredto varying degrees. This discoloration, which isillustrated in Fig. 1, was found to be due to the presenceof a thin adherent layer of a black substance. This isclearly illustrated in Fig. 2 where the sample numberhas been scribed into the sample exposing freshbare copper. The contrast in this figure is obvious.While this film adhered to the surface, physical contactwould result in the transfer of a small quantity of afine black powder from the film. In some cases, thediscoloration was found to be very uniform, but inother cases it was irregular. The extent of discolorationvaried with the thickness of the layer.

The air conditioner evaporator coils were also exam-ined, Fig. 3. From the two coils in this figure itcan be seen that all exposed copper surfaces haveturned dark. These coils are constructed out of parallellines of copper tubing placed through thin sheetsof aluminum that serve as the heat transfer fins. Thecopper tubing is linked together by soldered U-bendsto the ends of adjacent lengths of copper tubing. Theend plates that hold this assembly in place are madeout of galvanized steel. Copper manifolds and valvescomplete the assembly. Figure 4 is a close-up ofthe end plates and the U-bend region. In this figure,brown and orange corrosion products can be seenon the galvanized end plates, while black and greencorrosion products can be seen on the copper tubing.The normal patina that forms on copper in the cyclical-ly wet and dry environment of an air conditionerevaporator coil would be expected to contain copper (I)or copper (II) ions in oxides (black to brownish-red),carbonates (green to blue), sulfates (bright blue),and chlorides (greenish white to blue-green)[1,2]. Again, the black color was found to be dueto the presence of a thin layer of a loose blackpowder.

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Fig. 1. Photographs of two representative samples of copper natural gas supply tubing.

Fig. 2. Scribe marks illustrating the contrast between the color of bare copper and a sample of copper naturalgas line tubing.

Fig. 3. Photograph of the two air conditioner evaporator coils showing the black discoloration of the copperU-bends and corrosion of the end plate.

C. Identification of the Black Corrosion Productby X-Ray Diffraction

The black substance observed on the surfaces of thecopper components was identified by x-ray diffraction.This was accomplished by scraping the black substancefrom the surface of one of the sample of copper naturalgas line tubing using a razor blade. The powderremoved in this manner was accumulated on doublestick tape and attached to a glass slide as shown inFig. 5. A powder diffraction pattern was then obtainedfrom this sample of scrapings using Siemens D500x-ray diffractometer with a Cu Ka radiation sourceoperated at 40 kV and 30 mA using a step size of0.03 degrees, and a dwell time of 5 s.1

Figure 6 shows the results of these measurements inred along with the peak locations in black for coppermetal (Fig. 6a), copper (I) oxide (Fig. 6b) and

copper (I) sulfide (Fig. 6c) as listed in the InternationalCrystal Structure Database (ICSD). This figure showsthat peaks corresponding to all three of these phases arepresent in the sample of scrapings from the surfaceof the copper natural gas supply line. Given that thesample was collected by scraping the surface of coppermetal with a sharp razor blade, the peaks for copper andcopper oxide were expected since copper is a soft metalcompared to the steel of the razor blade and willnormally form copper oxide when exposed to air. Thethird phase, copper sulfide (Cu2S) is not a typicalatmospheric corrosion product as the concentration ofthe reduced sulfur species, such as H2S, required toform this product is usually very low under normalatmospheric conditions [1, 3-5]. Figure 7 is a summaryof the XRD results showing that all but two smallpeaks, indicated by question marks, are accounted forby these three phases.

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Fig. 4. Close-up photograph of the U-tubes and end plate of an air conditioner evaporator coil showing thatthere appears to be 5 colors associated with the corrosion products: white, brown, orange, green, and black.

1 Certain commercial equipment, instruments, or materials are iden-tified in this paper to foster understanding. Such identification doesnot imply recommendation or endorsement by the National Instituteof Standards and Technology, nor does it imply that the materials orequipment identified are necessarily the best available for thepurpose.

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Fig. 5. Photographs of the scraped copper natural gas supply line and the slide holding the sample for x-raydiffraction.

Fig. 6. Powder diffraction patterns for the sample of corrosion products scraped off a copper natural gassupply line (red) with, identified in black, the peaks for (a) copper, (b) copper (I) oxide (ICSD #38233),and (c) copper (I) sulfide (ICSD #95395).

D. Air Conditioner Evaporator Coil Leak Testing

Leak testing was performed on the air conditionerheat exchange coils by attaching air tight fittings to theends of the copper lines and connecting these to ahelium gas detector. The inside of the coil was evacuat-ed and helium gas passed over different regions of thecoils. No leaks were found by this method. Then, thecoils were pressurized with helium and a solution ofsoapy water was squirted onto different regions of thecoils. This method also failed to find any leaks in thesecoils.

E. Air Conditioner Evaporator Coil Sectioning

To evaluate the possibility of corrosion damageoccurring between the aluminum heat transfer fins andthe copper tubing inside them, the heat exchanger unit

was sliced with a band saw as shown in Fig. 8 to exposethese internal surfaces for examination of the copper. No evidence of localized attack was found, andthe black corrosion product layer was not detected byvisual examination.

F. Measurement of Corrosion Product LayerThickness

The thickness of the black corrosion product wasmeasured using optical microscopy of samplesprepared in cross section. For this analysis, samples ofnatural gas supply lines were cut, mounted in epoxy,and polished using standard metallurgical practice,exercising care to maintain the edge of the sample withthe corrosion products in place. Figure 9 shows opticalmicrographs of this sample where the corrosion productlayer varied from 5 μm to 10 μm thick.

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Fig. 7. Summary of peaks found in the corrosion product sample and their identification for showing that onlytwo small peaks were unidentified by the three phases.

3. Discussion and Conclusions

The copper metal surfaces of the samples providedfor examination were covered with a black film thatappears to be a corrosion product. The x-ray powderdiffraction results unambiguously identified this sub-stance as a copper sulfide phase. Copper sulfides(CuxSy) can have a wide range of stoichiometry rangingfrom CuS2 to Cu2S with 9 distinctly different phaseshaving been identified over this range depending on thestoichiometry. The powder diffraction pattern wasconsistent with the digenite phase of copper sulfide thathas a stoichiometry of Cu9S5 or Cu1.8S. The presence

of a copper sulfide phase on the surface of the copperindicates that reduced sulfur compounds, such ashydrogen sulfide (H2S), sulfur (S8), or mercaptans(thiols), are present in the indoor atmosphere of thehomes from which these samples were removed.

Reduced sulfur compounds generally accelerate thecorrosion of engineering metals, as sulfur forms stablecompounds with most of the parent and alloying ele-ments in these alloys [3]. However, copper and silverare reported to have the greatest sensitivity to this envi-ronmental contaminant [2,6]. Rice et al. [2,6] conduct-ed studies on the sensitivity of indoor corrosionrates of different metals to environmental contaminants

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Fig. 8. Air conditioner evaporator coil after slicing to allow for examination for corrosion damage between thealuminum sheets and the copper tubing.

Fig. 9. Optical micrographs of the wall of a copper natural gas supply line and the corrosion product layer at twomagnifications (wall ≈ 1.2 mm thick).

including hydrogen sulfide. These authors fit the rate ofmass increase (dm /dt) to the Eq. (1)

(1)

where A is a constant, [H2S] is the concentration oractivity of hydrogen sulfide in the atmosphere and n isa measure of the sensitivity of the corrosion rate tochanges in the hydrogen sulfide concentration. Thisparameter, n, is also the slope of a log-log plot as

(2)

Table 1 shows the results of Rice et al. [2,6]. Thistable clearly shows that copper and silver are themost sensitive of these metals to the concentration ofhydrogen sulfide.

While the results clearly indicate that a reducedsulfur species is present in the indoor atmosphere of thehomes that these samples came from, the corrosionproduct layer observed on these samples was less than10 μm thick. This layer makes the copper look dark dueto the black color and opaque nature of this corrosionproduct. All of the corrosion damage observed to dateis consistent with a general attack form of corrosionthat will progress in a uniform and relativelypredictable manner. No evidence of any type of local-ized attack was found, but these forms of attack typical-ly require an incubation period before they initiate.Therefore, the number of samples examined to dateis too small to draw a conclusion on these forms of

corrosion being able to cause or not cause failure.Samples from failed systems or from laboratory testsconducted over a wide range of metallurgical andenvironmental conditions will be required to assess theprobability of these other forms of corrosion causingfailure.

4. References

[1] N. de Zoubov, C. Vanleugenhaghe, and M. Pourbaix, Copper,Atlas of Electrochemical Equilibria in Aqueous Solutions,M. Pourbaix ed., NACE International, Houston, TX, 1974, p.384-92.

[2] D. W. Rice, R. J. Cappell, P. B. P. Phipps, and P. Peterson,Indoor Atmospheric Corrosion of Copper, Silver, Nickel,Cobalt, and Iron, Atmospheric Corrosion, W. H. Ailor ed.,J. Wiley & Sons, New York, 1982, p. 651-66.

[3] K. Barton, Protection Against Atmospheric Corrosion—Theories and Methods, J. Wiley & Sons, New York, 1976.

[4] G. Valensi, J. VanMuylder, and M. Pourbaix, Sulphur, Atlas ofElectrochemical Equilibria in Aqueous Solutions, M. Pourbaixed., NACE International, Houston, TX, 1974, p. 545-553.

[5] T. E. Graedel and N. Schwartz, Air Quality Reference Data forCorrosion Assessment, Mater. Perf. 16(8), 1977, p. 17-25.

[6] D. W. Rice, R. J. Cappell, W. Kinsolving, and J. J. Laskowski,Indoor Corrosion of Metals, Journal of the ElectrochemicalSociety, 127(4), 1980, p. 891-901.

About the authors: David J. Pitchure is a mechanicalengineer, Richard E. Ricker is a metallurgist, MaureenE. Williams is a materials scientist, and SandraClaggett is a senior metallographer and all are with theMetallurgy Division of the NIST Materials Science andEngineering Laboratory. The National Institute ofStandards and Technology is an agency of the U.S.Department of Commerce.

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2[ ] ndm A H Sdt

⎛ ⎞ =⎜ ⎟⎝ ⎠

2log log log[ ] .dm A n H Sdt

⎛ ⎞ = +⎜ ⎟⎝ ⎠

Table 1. The corrosion rate parameter, n, in equations (1) and (2) fordifferent metals exposed to hydrogen sulfide at different concentra-tions in air specifically formulated to represent a typical indoor airenvironment and air that is purified to remove all contaminants otherthan hydrogen sulfide.[Rice, 1982 #4]

Metal Cu Ag Co Ni Fe

Representative 0.30 0.55 0.02 0.03 0.05Indoor Air

Purified 70 % 0.93 0.14 --- --- ---RH Air