Pool Ceiling Collapse

Recently a "drop-in ceiling" literally "dropped in!" The ceiling tiles, above a swimming pool, in a retirement community periodically  started falling into the pool. The drop-in ceiling aluminum grid framework was held up by stainless steel wires. Samples of the failed stainless steel aluminum grid attachment wires are shown in Photograph A.

Photograph A: Overall view of failed stainless
 steel ceiling tile grid work attachment wires.

The ends of the stainless steel wire were fractured with very little evidence of corrosion. These wires held up the aluminum grid network which supported the ceiling tiles. A profile view of a failed support wire is shown in Photograph B.

Photograph B: Close-up profile view of failed
 stainless steel grid wire.

An end view of one of the failed ceiling grid support wires is shown in Photograph C.

Photograph C: End view of failed stainless
 steel ceiling grid wire.

A scanning electron microscopic (SEM) examination revealed a brittle fracture, typical of stainless steel which has failed by stress corrosion cracking. An SEM view of one of the failed ceiling grid support wires is shown in Photograph D.

Photograph D: Scanning electron micrograph
 (mag 30x) of failed stainless steel ceiling
 grid wire.

X-Ray energy dispersive spectroscopy (EDS) allows the investigator to conduct a localized chemical analysis on the sample being examined in the scanning electron microscope (SEM). The corroded area on the surface of one of the failed ceiling support grid wires was examined using EDS. The area examined is shown in Photograph E

Photograph E: Area on subject wire surface where
 energy dispersive spectroscopy (EDS) was conducted.

The EDS spectra from the area shown in Photograph E is displayed in Figure 1.

Figure 1: Energy dispersive spectra (EDS) and semi-quantitative analysis of chemical species detected.

The semi-quantitative analysis of the EDS spectra above indicates a concentration of chlorine (Cl) of 9.6 wt. %. The chlorine is of course the result of evaporation of the warm swimming pool water and the resulting high humidity in the swimming pool room.

An X-Ray energy dispersive analysis (EDS) only tells the investigator what chemical elements are present within the area examined, i.e., on the wire surface shown in Photograph E.

An elemental X-Ray map gives the investigator a visual display or map showing the location of each element in the EDS spectra and the area or areas in which that element is concentrated. An X-Ray map of elements found in the EDS spectra (Figure 1) is shown in Photograph F.

Photograph F: Energy dispersive x-ray maps of various chemical species found on subject wire surface.

An EDS map for chlorine (Cl), along with an SEM image of the area examined, is shown in Photograph G.

Photograph G: X-ray map of chlorine (Cl) and SEM image of surface area examined.

Discussion

Putting chlorine (Cl) in contact with stainless steel can result in very rapid failure of the stainless steel item. The failure will exhibit corrosion product, sometimes to a very limited extent, on the stainless steel.

The failure of the stainless steel in the presence of chlorine (from chlorine being added to the pool water) is known as chloride stress corrosion cracking. The fracture is the result of the synergistic effect of chloride AND stress. Stainless steel, without the presence of chlorine (Cl) in most operating environments will perform quite adequately. Stainless steel will perform quite adequately in dilute solutions of chlorine (Cl) without the presence of stress. However, when both stress and chlorine (Cl) are present, catastrophic failure can result.

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