Critical humidity

Critical Humidity—the relative humidity (RH) at and above which the atmospheric corrosion rate of a metal increases significantly. 

Critical relative humidity The primary value of the critical relative humidity denotes that humidity below which no corrosion of the metal in question takes place. However, it is important to know whether this refers to a clean metal surface or one covered with corrosion products. In the latter case a secondary critical humidity is usually found at which the rate of corrosion increases markedly. This is attributed to the hygroscopic nature of the corrosion product (see later). In the case of iron and steel it appears that there may even be a tertiary critical humidity . Thus at about 60% r.h. rusting commences at a very slow rate (primary value) at 75-80% r.h. there is a sharp increase in corrosion rate probably attributable to capillary condensation of moisture within the rust . At 90% r.h. there is a further increase in rusting rate corresponding to the vapour pressure of saturated ferrous sulphate solution , ferrous sulphate being identifiable in rust as crystalline agglomerates. The primary critical r.h. for uncorroded metal surfaces seems to be virtually the same for all metals, but the secondary values vary quite widely. 

This concept may be invoked to account for electrolyte formation in microcracks in a metal surface or in the re-entrant angle formed by a dust particle and the metal surface. More importantly, it can also explain electrolyte formation in the pores of corrosion product and hence the secondary critical humidity discussed earlier. Ferric oxide gel is known to exhibit capillary condensation characteristic and pore sizes deduced from measurements of its adsorptive capacity are of the right order of magnitude to explain a secondary critical relative humidity as70 7o for rusted steel . 

Moisture the critical humidity Moisture can reach a steel surface directly in liquid form as a result of precipitation processes, i.e. rain and dew, but... 

This important fact was first demonstrated by Vernon in a series of classical experiments, some of which are summarised graphically in Fig. 3.1. He showed that rusting is minimal in pure air of less than 100 l o relative humidity but that in the presence of minute concentrations of impurities, such as sulphur dioxide, serious rusting can occur without visible precipitation of moisture once the relative humidity of the air rises above a critical and comparatively low value. This value depends to some extent upon the nature of the atmospheric pollution, but, when sulphur dioxide is present, it is in the region of 70-80%. Below the critical humidity, rusting is inappreciable, even in polluted air. 

The presence of moisture on steel above the critical humidity but below the saturation point may be caused by an adsorption mechanism or by the presence of particles of deliquescent salts on the surface. Once rusting has begun, the composition of the rust already formed will influence the relative humidity at which further rusting will occur, because rusts formed in polluted atmospheres contain hygroscopic salts. The method by which moisture reaches the surface is probably less important, however, than the length... 

FIG. 32 Top Semilog plot of the time constant t for ionic motion as a function of RH for KF. Bottom Simultaneously measured contact potential. At a critical humidity A, there is a break or a change in slope in these two surface properties. Below A, water solvates preferentially cations at the step edges. Above A, the rates of dissolution (solvation) of anions and cations are similar and water uni-... 

There also appears to exist a critical water concentration within the adhesive below which water-induced damage of the joint will not occur. This also infers that there is a critical humidity for deterioration. For an epoxy system, it is estimated that the critical water concentration is about 1.35 to 1.45 percent and that the critical humidity is 50 to 65 percent.39,40 Any loss in joint strength by the absorbed water can be restored upon drying if the equilibrium moisture uptake is below the critical water concentration. 

If corrosion rate is plotted against humidity, then a curve such as that in Fig.3 would be obtained. Here, corrosion rate is low until, over a narrow range of humidity, the rate suddenly begins to increase. This point is termed the critical humidity and its value will depend upon the metal and nature of any dissolved species in the water film. For example, iron in a sulphur dioxide polluted atmosphere will have a critical relative humidity of above 75%, whereas a copper surface polluted with iodide will reach a critical relative humidity of about 35%. 

AG Free energy change AGt Free energy of activation ff Critical humidity Ji Chilton-Colburn mass transfer factor... 

Fig. 1 is a sorption isotherm constructed for a hypothetical solid with a molecular weight of 180, which is capable of forming a monohydrate. In this case, the solubility of the substance in water is assumed to be 50 wt% and the critical humidity for the transition from anhydrous to monohydrate is assumed to be 60% RH. It is also assumed that a solution of the solid in water behaves ideally. 

Moisture content of the polymer phase and gas flow humidity are the main parameters determining adsorption of gases by ion exchange materials. The presence of moisture in the polymer phase of gel-type resins is necessary for a chemical reaction to proceed between an active gas and a functional group. As a rule, for a majority of ion exchanger/gas systems, a critical humidity exists below which the adsorpticxn of a gas does not occur [ 8 ]. 

C. Lebreton, Z.Z. Wang, Critical humidity for removal of atoms from the gold surface with scanning tunneling microscopy. J. Vac. Sci. Technol. B14, 1356-1359, 1996. 

Time-of-wetness as previously defined was the time exceeding some critical relative humidity ( ), In this paper time-of-wetness is the time a critical relative humidity is exceeded and the dew point is greater than 0 C, plus any time the critical humidity is not exceeded and it is raining. For that reason a regression relating relative humidity to dew points above 0 C and temperature was used to calculate relative humidity for each hour. This relationship is ... 

A number of studies on the use of these gas flow cells in calorimeters are now being published (e.g., 30-32). These include studies that are designed to characterize the surface (30), and also those that show critical humidities at which samples start to deliquesce (29). 

Dynamic moisture sorption, in particular, provides an excellent opportunity to study solid form conversion Fig. 18.6 depicts a typical sorption curve of an antiarrhythmic compound that shows the conversion of an anhy-drate to a monohydrate. Moisture uptake by the anhydrous form is very small on the moisture uptake curve until a critical humidity of about 70% is achieved. At this point, rapid moisture uptake occurs and a hydrate form containing 10%moisture is generated. Subsequent reduction in the humidity (desorption) shows the hydrate to remain until approximately 5% RH, when it spontaneously converts to the anhydrous form. It is important to recognize, however, that conversion between solid forms is very time dependent. The relative humidities at which conversion was seen in Fig. 18.6 are significantly dependent on the length of time the solid material was equilibrated. For the material shown in Fig. 18.6, conversion from the anhydrous to the hydrate "at equilibrium" will occur somewhere between 10 and 70% RH. More precise determination of the critical humidity at which conversion occurs may be determined as described in Section 2.3.1. 

At low relative humidities, moisture produces a layer of adsorbed vapour on the surface of particles. Above a critical humidity, typically in the range 65-80 percent, it will form liquid bridges between particles. The attractive force due to the adsorbed layer may be about 50 times the van der Waals force for smooth surfaces, but surface roughness will reduce the effect. Where a liquid bridge forms, it will give rise to an attractive force between the particles due to surface tension or capillary forces. 

For sufficiently rough particles the value of may be equal to or greater than the critical humidity He and thus no adsorbed layer interaction will occur for such powders. 

The critical humidity, below which it is no longer possible to have stable liquid bridges, can be obtained by elimination between equations 4 and 7 and gives ... 

The critical humidity is a function of the temperature, pressure and the nature of the solid and liquid. As would be expected, decreases as P increases, as increasing the pressure favours condensation. 

Industrially, it is important to be able to either calculate or determine experimentally the value of the critical humidity as it marks an incremental change in the structural strength of the powder and if crossed could well lead to serious production problems. Experimentally, the critical humidity may be determined by bulk density- or electrical measurement. ... 

Figure 5.5 A typical curve showing the effect of the B.E.T. constant, p, on the critical humidity value, H. ...

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