induced damage to materials in marine environments - Part ...Petrologia, Institute Superior Tecnico (Technical University of Lisbon), Av. Rovisco Pais 7,1096 Lisboa Codex, Portugal;

A two-case study on the environmentally-

induced damage to materials in marine

environments - Part I: Metallic materials

A.M.G. Pacheco* & A. Mauricio*

Dept. Engenharia Quimica and *Lab. Mineralogia e

Petrologia, Institute Superior Tecnico (Technical Universityof Lisbon), Av. Rovisco Pais 7,1096 Lisboa Codex, Portugal;EMail: [email protected]

Abstract

This two-part paper addresses the specific hazards that most materials are facedwith in coastal areas, particularly in their atmosphere. Pretty common featureslike high humidity and airborne salts of marine origin, which are inherent in suchan environment, may turn into a nightmare for conservationists, architects andmaterials scientists, that is for everyone involved with old (historic) or newinfrastructure. Two cases are presented and discussed herein. Neither of themwas designed or singled out specially for the occasion: both were taken fromextended programs of metal-corrosion and stone-decay monitoring in the open.The first case (Part I) deals with the implication of saline contamination for thetime of wetness (TOW) of a metallic surface. The results show that standardprocedures for assessing TOW from weather data can severely underestimate theduration of surface wetness and, in the final analysis, lead to somemisclassification of atmospheric corrosivity. The second case (Part n) followsthe evolution of salt efflorescences at an ancient building as a function of local(microclimatic) conditions, in order to get the time probability associated withdeliquescence- crystallisation transitions at a given location. By doing this, it waspossible to identify more-or-less risky areas in the stone monument, which couldthen be subjected to differential surveillance and/or care. Both studies seempertinent to illustrating the need for establishing risk thresholds for materialsselection and infrastructure maintenance that can really hold in marineenvironments.

Transactions on Ecology and the Environment vol 18, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541

78 Environmental Coastal Regions

1 Introduction

In the living world, environmental assets and liabilities are often aquestion of perspective. Some few examples of that could be easily given.Stratospheric ozone, for instance, is absolutely necessary for shielding theearth from the outer-space radiation and yet, should such a reactive

species pervade the lower troposphere, its presence would be a seriousconcern for the world as we know it. A similar situation may occur whenan environmental component is pushed away from its place and/or extent,or as long as a negative synergy arises between components even if they

are quite harmless when kept apart from each other and sensitive items.

Probably due to the global concern about the spreading of man-made

pollution all over the four ecosystems - atmosphere, hydrosphere,lithosphere and biosphere - comparatively little attention has been paid tothe fate and extent of natural inputs, and to their consequences ineveryday life. There is no reason for such an intellectual apartheid.Hazardous substances can as well occur naturally and pose some major

threats on life-supporting systems and infrastructures. Besides, once an

anthropogenic substance is released into the environment, natural forcestake over its interphase transfer and intraphase variability. Ultimately,both natural and man-made ("unnatural") substances are moved acrossphase boundaries by natural driving forces and conditioned within phaseboundaries by natural factors of variability. Significantly enough, in amodern textbook like Thibodeaux's [1], chemical is used in a broad sensethat goes from water and oxygen to DDT...

A good example of the aforesaid, and most germane to this paper, isairborne salinity. The basic mechanism responsible for the production ofspray is the bursting of air bubbles at the sea surface [2-4]. It is generallyaccepted that some 3 to 4 percent of that surface is covered with bubblesat any moment [5], which means that the yearly production of salt liesbetween l(f and 10*° tonnes [5-13]. About 10% of such an amount fallsoff the ocean [6,12,14-16], turning inland saltfall into an air pollutionissue. Needless to say that high dampness as well as airborne salinity areinherent in marine environments and thus essential for the equilibrium ofany coastal ecosystem. Notwithstanding, it is by no means less true thatthey invariably pose a serious threat to infrastructure, cultural artefactsand life-supporting facilities, not to mention vegetation and soil/waterresources. Such an issue is definitely not liable to an abatement strategywhatsoever. In the realm of materials, for instance, only careful selection,protective measures and close surveillance can prevent extended damagein the open, in salt-laden environments.


Environmental Coastal Regions 79

The present paper (Parts I and II) deals with the nasty environmental

(electro)chemistry set by common atmospheric moisture and regular

airborne salts upon the surface of a plain metal (I) or into the bulk of a

composite geomaterial (II). Both studies seem pertinent to illustrating thespecific hazards that most materials are faced with in coastal areas andthe need for establishing risk thresholds for materials selection andinfrastructure maintenance that can really hold in marine environments.

2 Case study

2.1 First case: Salt-induced corrosion of an atmospheric cell

2.1.1 Background

Other than its economic impact, which makes it accountable for more

than half the whole corrosion bill [17], atmospheric corrosion bears othertwo distinctive features: transient nature and thin-layer electrochemistry.The (cumulative) period during which a metallic surface is covered byadsorptive and/or phase films of electrolyte that are capable of causing orsustaining corrosion activity in the open, is the time of wetness (TOW) by

ISO 9223 [18]. Along with sulphur dioxide and airborne chloride levels,TOW is a primary variable in categorising atmospheric environments asto their aggressiveness, according to that Standard. The classification ofatmospheres provides a basis for the selection and protection of materials,

so there should be little room left for questioning its technical relevance.The wetting of surfaces is quite a complex phenomenon and the

TOW itself is an event, location and time-dependent variable [19]. Forpractical purposes, however, TOW is usually derived from general dataand defined as the length of time when both relative humidity (RH) andtemperature (T) remain above 80 % and 0 °C, respectively [18]. It hasbeen argued that such an approach should prove accurate enough to beused in corrosivity assessment - see [20], for example. This could be anexception rather than the rule, though, especially in marine atmospheres.That standard procedure is likely to yield very conservative estimates (tosay the least...) and lead to a most significant divergence between thewetness actually experienced by a decaying surface and its nominalduration from weather records, as we shall see below.

2.1.2 Experimentation, results and discussion

For several months, an exposure program was carried out in order toevaluate printed-circuit cells (in brief, iron lines on an epoxy substrate) asatmospheric corrosion monitors (ACMs). Details on the cell construction



and preparation prior to work as well as on the experimental set-up were

given elsewhere [21,22]. The rationale behind the experiments is quite

straightforward. Each cell consists of a multilinear array of 40 iron strips

(30.0010.10 mm x 0.50±0.05 mm; line spacing: 0.20 mm) alternatelyshorted by an external track, resulting in an interdigitated, two-electrodeconfiguration. There is no contact between an iron strip and its immediateneighbours (they are parallel to each other), and every cell is exposedoutdoors in a completely dry condition at an angle of 45° to the horizon,roughly facing south, under a shield for rain and direct sunlight.

Once enough moisture builds up to form an electrolyte layer,

corrosion starts or resumes at the surface and an electric signal can berecorded from a cell. Should the dampness fade away and the surface get

dry, the thin-layer electrochemistry comes to a halt and ceases to drive acurrent through the data-acquisition system. There is neither an appliedelectromotive force (emf) nor any other kind of external polarisation:

these cells are self-driven devices and their output relates to nothing butwhat really happens on the corroding surface.

The present results refer to a 96-h term during a mild winter season.Temperature went from a low of 8 °C to a high of 18 °C, with an averageof 13 °C, whereas relative humidity stayed between 48 % and 84 % forthe whole term (mean RH: 63 %). The galvanic output (short-circuitcurrent, Isc) was measured at zero external impedance from a NaCl-contaminated cell with no applied emf, under a simulated chloride rate of100 mg m^ day* (mmd CO. This figure falls within the 82 category ofairborne salinity, according to ISO [18]: in practical terms, it is likely tostand for an average saltfall rate at a distance of at least 250 m downwindfrom the surf [23]. The entire exposure program was factorially designedfor assessing the influence of primary atmospheric stimuli (T and RH)plus Cl~ levels on the corrosion of iron ACMs. In the particular series

which this experiment was taken from, an average rate of Cl" presentationwas fixed (simulated) in order to ensure that cells were responding tochanges in T and RH only, and to prevent any major kinetic ("ohmic")control during the periods of corrosion activity [24].

For zero-resistance ammetry (ZRA), the cell signal was put through alow-noise (0.01 pA Hz ) , chopper-stabilised operational amplifier, andread as an ohmic drop across virtually-null resistors by a high-impedance(> 10 ohm) voltmeter. Continuous records on cell output as well as onsite relative humidity and air temperature were kept throughout the run(onset: 22.00 GMT). An overview of the full test interval in terms of RHand Isc is given in Figure 1; logarithmic (right) scale is just a plottingdevice to accommodate current data over several orders of magnitude.



50

40

10000

1000

.100

j iff ATMOSPHERE — 100 mmd CELL j

Figure 1. Short-circuit current (Isc) at zero external impedance vs relativehumidity (RH), from an iron cell under 100 mg m* day"* (mmd) of Cl".

Other than an overall impression of close agreement, the quality ofassociation between corrosion activity on the cell and relative humidity inthe air was checked by means of nonparametric measures of correlation.Why nonparametrics? First, rank-order statistics imply fewer and/or lessstringent assumptions about data (if at all...) than parametric measures ofcorrelation usually do. Second, even though all observations attain aninterval scale of measurement, data structure seems much more importantan asset here than numerical magnitude itself, so the assignment of ranksto raw scores does not result in any waste of information [25]. Third,ranks are less sensitive to the experimental error inherent in translatingdata from analogical records.

The correlation between Isc and RH was found significant at anylevel by either nonparametric measure - Spearman, Kendall and gammastatistics - which means that the cell acts as a reliable moisture sensor:coefficients are 0.897, 0.763 and 0.812, respectively (0-96 h; raw data =

97 cases). Censoring data for tied-at-zero observations did not lead to animprovement in any correlation. Nonparametrics seem robust enough to



account for a few ties in the ranks and the risk of randomness does not

exhibit an obvious trend along the exposure. Now, the total time duringwhich an electric signal could be recorded from the cell amounts to 76 hfor an evaluation interval of 96 h: in other words, the cell was wet for

about 80 % of the experiment. An appraisal of the time-of-wetness basedon temperature-humidity data (ISO 9223 [18]) would give just 3 h, a mostconservative estimate to say the least. Needless to comment further onthis issue: Figure 2 speaks for itself as showing the cumulative build-upof TOW by either approach.

The experiment was designed to exclude visible water inputs, which

makes it possible to locate an average threshold for current flow around58 % RH, for the entire period of evaluation. Variate by variate, corrosion

activity can be traced down to 54 % RH. These results are in excellentagreement with early studies on mild-steel surfaces inoculated with NaCl

nuclei [26,27], which indicate that corrosion becomes appreciable slightly

below 60 % RH, actually at about 58 % RH.

Figure 2. Time-of-wetness conforming to ISO 9223 (calculated TOW)and corresponding to actual corrosion activity (measured TOW).



On the other hand, the same results cast a serious doubt on thevalidity of TOW estimates, as defined by ISO 9223, at least in whatconcerns marine atmospheres. It is true that the Standard itself includesseveral disclaimers and waives exclusiveness in assessing TOW forclassification purposes. However, it is by no means less true that most

surface-wetness assessment, whether for atmospheric classification ornot, still follows the predetermined levels of T and RH in accordancewith that Standard, despite the sound evidence coming from a few othersources - see [28] and references therein, for instance.

Finally, it should be emphasized that the case reported herein is justone in many proprietary experiments pointing to an underestimation of

TOW to a fairly large extent, whenever the ISO-standard approach is

used under saline conditions. This is neither an odd, special case, nor

should the simulation be viewed as an accelerated, unrealistic procedure.In fact, the present rate of Cl" deposition fits into the lower half of boththe 82 category by ISO [18] and the reference range by Mattsson [29], soit hardly qualifies as an extreme-pollution event (83 category goes up to1500 mmd CF and, beyond that, there are still splash and spray episodes).Besides, the situation could only be worse in the presence of real sea-salt,that is in an actual marine environment. The critical humidity for rustingin the presence of sea salt is somewhat lower due to its composite nature.

The existence of highly hygroscopic components in sea water, especially

MgCL MgSO4 and CaCL may trigger corrosion at about 40 % RH [30],whereas little corrosion should be expected below about 60 % RH in thepresence of pure NaCl, according to Evans and Taylor [31] and our ownexperience.

3 Conclusions

As far as corrosion resistance is concerned, and apart from user-definedproducts, most materials for outdoor service are selected and protectedunder some previous classification of the service location in terms of timeof wetness and air contamination, that is on an ab initio basis. However,the ability of an environment to induce and/or sustain an atmospheric

corrosion process can lie way far from what might be anticipated throughstandard procedures for corrosivity assessment and classification. This isan almost inevitable issue when dealing with salt-laden environments,though it seems that major Standards - like ISO's, for instance - failed toaccommodate such an issue as yet.

The present results show that, even in mild conditions, a significantdivergence was found between standardized (estimated) and experimental(measured) time of wetness, an increase by a 25-fold factor to be precise.



The situation could only be worse in some harsher conditions of exposureand/or in an actual marine environment, due to the enhancedhygroscopicity of real sea salt when compared to pure sodium chloride.

Conservative estimates of TOW to a fairly large extent are most likely tooccur in coastal environments and thus lead to an ill classification ofatmospheric aggressiveness for service locations down there. In the finalanalysis, such an underrating would undermine the basis of selection

criteria and protection systems, that is the whole front trench in the fightagainst corrosion in the open.

This first case-study (Part I) is an example of how some peculiaritiesof marine environments should be accounted for before damage is done.An approach to an after situation will be dealt with in the second study(Part II of this paper), which is going to focus on the salt-induced stress toan ancient, stone building.

(to be continued in Part II)

Acknowledgements

Research contracts PBIC/C/QUI/2381/95 and PBICT/C/CTA/2127/95(JNICT - Portugal) assisted in meeting the production cost of this paper(Parts I and II).

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induced damage to materials in marine environments - Part ...Petrologia, Institute Superior Tecnico (Technical University of Lisbon), Av. Rovisco Pais 7,1096 Lisboa Codex, Portugal;