Silver deposition on stainless steel container surfaces in contact with disinfectant silver aqueous solutions

Highlights

• Silver is one of the biocides of water consumed in the International Space Station.

• Ionic silver is depleted from potable water when in contact with stainless steel (SS).

• SEM and XPS analysis reveal a uniform silver deposition over the SS surface.

• Silver deposits in its metallic form, in line with a galvanic deposition mechanism.

• Evidence is provided that Cr and/ or Ni oxide builds-up on SS surfaces.

Abstract

Silver is the preservative used on the Russian segment of the International Space Station (ISS) to prevent microbial proliferation within potable water supplies. Yet, in the frame of the European Automated Transfer Vehicle (ATV) missions to ISS, silver depletion from water has been detected during ground transportation of this water to launch site, thereby indicating a degradation of water quality. This study investigates the silver loss from water when in contact with stainless steel surfaces. Experiments are conducted with several types of stainless steel surfaces being exposed to water containing 10 or 0.5 mg/L silver ions. Results show that silver deposits on stainless steel surfaces even when a passivation layer protects the metallic surface. The highest protection to silver deposition is offered by acid passivated and electropolished SS 316L. SEM and XPS experiments were carried out at several locations of the sample area that was in contact with the Ag solution and found similar morphological (SEM) and compositional (sputter-etch XPS) results. The results reveal that silver deposits uniformly across the wetted surface to a thickness larger than 3 nm. Moreover, evidence is provided that silver deposits in its metallic form on all stainless steel surfaces, in line with a galvanic deposition mechanism. Combination of ICP-MS and XPS results suggests a mechanism for Ag deposition/reduction with simultaneous substrate oxidation resulting in oxide growth at the exposed stainless steel surface.

Introduction

The disinfection capacity of ionic silver has been acknowledged against numerous types of microbes, bacteria, viruses, protozoa, algae etc. [1]. In addition, silver may enhance processes, e.g. improve the photocatalytic activity of TiO₂ [2]. Biocide properties of ionic silver and silver nanoparticles rely on several mechanisms and modes of action including reactive oxygen species (ROS) formation, DNA multiplication disruption and cell membrane malfunction [3]. These mechanisms are provoked considerably by ionic silver that will be eventually present even in the case of silver nanoparticles dispersions [4], [5].

Ionic silver is used as a preservative for water used aboard the Russian segment of the ISS. NASA plans also to use silver as a preservative for potable water in their future manned space missions. The availability of potable water is essential both in terms of quality and quantity. Currently, potable water according to either Russian or American quality standards is prepared on Earth and is regularly re-supplied to ISS. ATV has been designed by the European Space Agency (ESA) to provide potable water instead of payloads, consumables, etc. [6]. For each ATV flight water tank to be loaded, three water tanks are prepared: one tank that contains water with elevated silver content (10 mg/L) for the pre-conditioning of the ATV flight tank and two other tanks that contain potable water of 0.5 mg/L silver concentration for flushing and loading of ATV flight tank, respectively. During ATV launch campaigns, a fluctuation of silver was observed by performing water quality analyses at different steps of the ATV water process [7].

Although there is extensive literature about silver coatings on other metals for decorative, anticorrosion, antimicrobial, electronic and optical applications [8], [9], [10], [11], a very limited number of publications associated with silver deposition on water container walls exist. The few relevant studies found in literature show that silver depletion from bulk water might occur depending on various factors, such as the wetted surface area to water volume ratio (S/V), wetted surface type, etc. [12], [13], [14].

Apart from tracing silver loss in bulk water, there has been also SEM microscopic evidence for Ag species deposition [12], [13]. It was suggested that the deposited silver was in its metallic form, based on EDS spectroscopic evidence, without giving details of how Ag, Ag₂O and AgO were distinguished, given the difficulty of the technique in providing other than elemental information. Moreover, it was suggested that the mechanism of Ag deposition on these materials is that of galvanic corrosion (which should be termed more accurately as galvanic deposition) e.g. the reduction of Ag⁺ on the metal surface, coupled with the oxidation (or further oxidation) of the latter; detection of Ni ions in the water in contact with non-passivated Inconel [14] supported the notion of galvanic corrosion.

In the case of non-passivated metal surfaces in acidic solutions, galvanic deposition is expected indeed to be a case of galvanic corrosion, also known as galvanic replacement or transmetallation [15]. In particular, when a non-noble, oxide free, metal, M (M: Fe, Cr, Ni, Al, Mo, Mn, Co, etc.), is immersed in an aqueous solution containing ions of a noble metal, M_nobleⁿ⁺ (M_noble: Ag, Pt, Au, Pd, Ag, Ru, Ir, Rh, Os etc.) then, due to a difference in the electrochemical potentials of the two metals (E⁰_noble − E⁰ > 0), the following reaction is thermodynamically favored and can take place spontaneously:

M_nobleⁿ⁺ + n/m M → M_noble + n/m M^m+

As an example relevant to this work, e.g. the contact of stainless steels with Ag⁺ solutions, one could consider the galvanic replacement of metallic Fe, Ni or Cr (main components of most SS) by Ag, in acidic environments, according to the following reactions. E⁰ values taken from literature [16]:

2Ag⁺ + Fe → 2Ag + Fe⁺⁺

(E⁰_noble = E⁰_Ag⁺_/Ag = +0.7991 V and E⁰ = E⁰_Fe⁺⁺_/Fe = −0.440 V vs. SHE)

3Ag⁺ + Fe → 3Ag + Fe⁺⁺⁺

(E⁰_noble = E⁰_Ag⁺_/Ag = +0.7991 V and E⁰ = E⁰_Fe⁺⁺⁺_/Fe = −0.037 V vs. SHE)

2Ag⁺ + Ni → 2Ag + Ni⁺⁺

(E⁰_noble = E⁰_Ag⁺_/Ag = +0.7991 V and E⁰ = E⁰_Ni⁺⁺_/Ni = −0.257 V vs. SHE)

3Ag⁺ + Cr → 3Ag + Cr⁺⁺⁺

(E⁰_noble = E⁰_Ag⁺_/Ag = +0.7991 V and E⁰ = E⁰_Cr⁺⁺⁺_/Cr = −0.740 V vs. SHE)

The galvanic replacement process has been used for the preparation of Pt-, Pd-, Au-, Ru- etc. based poly-metallic catalysts, mainly on Cu, Ni and Co substrates [15], [17], [18], [19]. In this case, at near neutral pH values or/and passivated SS materials all three major metallic components of SS (Fe, Cr, Ni) exposed to the solution are expected to be in an oxide/hydroxide form. In more detail, according to the respective Pourbaix diagrams [20], [21], [22] at the pH value of ca. 8 of the drinking water of this application, Fe should be present as Fe₂O₃, and Cr as Cr₂O₃ if contacted by an aerated solution. A native surface layer of Ni(OH)₂ is also known to form on Ni under non-acidic conditions [23], [24]. The same Fe and Cr oxides are formed on samples that have been pre-passivated, whereas Ni should be transformed to Ni₂O₃ or Ni(OH)₃. Hence, reactions (2)–(5) could only proceed through oxide/hydroxide defects or else reactions involving some of these oxides and soluble species of higher metal element oxidation states should be considered if galvanic deposition proceeds as a galvanic replacement-corrosion-transmetallation process. Alternatively, mechanisms for substrate oxide growth may be considered if Ag galvanic deposition proceeds with no metal substrate dissolution.

This work is part of a project supported by ESA (European Space Agency) to examine the phenomena responsible for biocide concentration fluctuations in water systems for crew usage. ESA interest stems from the need to provide the ISS crew with potable water of long term chemical stability, in order to exclude any potential threat in crew health related to water quality degradation. The scope of this work is to investigate the decrease of biocidal Ag⁺ concentration in water exposed to different types of stainless steel and study the mechanism of Ag deposition on these materials. On this account, specific objectives of the work are: (i) to follow the decrease of Ag⁺ concentration over specific periods of time, (ii) to analyse the water composition in contact with the stainless steel specimens, (iii) to quantify Ag deposited on the specimens, thus closing the silver mass balance and (iv) to identify the chemical state of Ag deposits.