Rusting, rate

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. 

When low-alloy steels are exposed outdoors, the rust formed on them is generally darker in colour and much finer in grain than that formed on ordinary steel. Moreover, the slowing down in rusting rate with time (cf. Section 3.1, p. 3 13) seems to be more marked for low-alloy steels than for ordinary steels. This can be illustrated by the BISRA figures given in Table 3.8. 

Basis steel Alloying elements. % and rusting rates ... 

B210/GBL system showed no rust (an ASTM rust rating of 10) after 24 hours exposure. However, approximately 50% surface rust (an ASTM rust rating of 1) was observed for the B210/NVP system. 

Exposure of the B210/GBL post-thermaIly treated system to the salt fog environment for 24 hours gave no rusting of any of the panels. All panels had a rust rating of 10. When the open circuit potentials were plotted (Figure 3), minor changes in potential were observed which paralleled the B210/NVP system. 

Previous work has shown that thermally cured high nitrile polymer coatings have good thermal resistance (20). Thus, the B210/NVP photocured panels were then heated in an attempt to improve their performance. The 24 hour rust rating of these thermally treated test panels varied depending on the temperature of the treatment (Figure 2). 

Figure 2. Changes in open circuit potential (OPC) and rust rating (RR) with temperature for post-thermally treated Bonderite 40 coated steel (B40) panels and Barex 210/N-vinylpyrrolidone (B210/NVP) photocured coatings on B40 panels.

Based on the literature, the correlation between open circuit potential rust rating and temperature was unexpected. To rule out the fact that the open circuit potential measurements did not simply represent changes in the phosphated substrate due to temperature,... 

In Region III, the maximum in the rust rating is observed. The maximum is possible due to the combined changes in the organic coating matrix and the B40 panel. Polyvinylpyrrolidone becomes water insoluble when heated to 150°C due to cross I inking (19). 

Rust Rating - Open Circuit Potential - Temperature Correlation... 

The open circuit potential data for the B210/NVP system mirrors the behavior of the rust ratings over the temperature range examined. A plausible explanation of the change of the open circuit potential is as follows. As temperature is increased, the composition of the various oxides and hydroxides which make up the zinc phosphate conversion layer and the base iron oxide layer undergo changes. 

The NACE corrosion scale is a visual rating of surface rusting from 0% to 100%. Most refiners, product pipeline companies, and marketers of fuel require an NACE surface rust rating of 5% or less. The NACE corrosion rating scale is outlined in TABLE 7-1. 

BRT ASTM D6557 Rust rating Average grey value 100 min... 

The different levels of oxygen access in the different water zones may, in extensive structures, for example sheet pilings or mooring posts, lead to the formation of aeration elements. Measurements have shown that the cathodic areas are usually on or just above the waterline [53]. The result is then increased corrosion in the tidal zone, which may be much larger than would be expected based on the different rusting rates in samples exposed separately in the tidal and immersion zones. 

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