Steels erosion rate

If the amount of metal removal by erosion is significant the surface will probably be continually active. Metal loss will be the additive effect of erosion and active corrosion. Sometimes the erosion rate is higher than that of active corrosion. The material selection judgment can then disregard coirosion and proceed on the basis of erosion resistance provided the corrosion rates of aetive surfaces of the alloys considered are not much different. As an example of magnitudes, a good high-chromium iron may lose metal from erosion only a tenth as fast as do the usual stainless steels. 

In the depth-profiling analysis of steel by GD-MS, initial degassing caused serious interference problems independent of whether fast or slow erosion rates were adopted. Thermal degassing under vacuum conditions in the ion source before igniting the discharge has been shown to be helpful. 

The AO exposure of 0, 3.5, 7.0, 8.75 wt % Si8On MC-POSS-PI (POSS R group is cyclopentyl) films was carried out in a simulated LEO environment where films of these materials were etched by exposure to a hyperthermal O-atom beam [2]. The difference in etch depth between the eroded and stainless steel screen-protected areas of the samples, made it possible to calculate an AO reaction efficiency (Re) or erosion rate of the material for a given AO flux. 

The GRP generally showed higher cavitation erosion rates than 6 Mo-steel, titanium or high-density polyethylene. Damage incubation times for GRP ranged from a few minutes to almost 2h. No detectable erosion loss was recorded for the other three materials after more than 20 h. However, the author cautions that there are no scaling methods available to relate the high intensity accelerated cavitation test results to in-service performance. 

Figure 15.4 Mapping electrochemical material loss against mechanical erosion rates for a nonpassivating surface carbon steel (AISI1020) along with two potentially passivating surfaces of nickel aluminum bronze (NAB) one that has been thermally sprayed by high-velocity oxy-fuel deposition as a coating on carbon steel ( j and another which has been cast (A.). These results were obtained from jet impingement erosion-corrosion tests. Reprinted from Ref. [7]. Copyright (2007) with permission from Elsevier.

Storch and Pojar (1970) performed an interesting and rare study of erosive wear within a variety of steel cylinder-on-cone cyclones with tangential inlets. Erosion rates in each cyclone s cylinder and cone sections were measured as a function of gas inlet velocity and solids concentration for two different types of abrasive dust. The experimental results are illustrated in Fig. 12.1.1, frames a through d. The wear reported is that after exposing the cyclone to a fixed total quantity of dust. 

The morphology of the oxide scale that forms during erosion plays an important role, as noted by the extensive work of Levy and co-workers [47, 49,53,58,59]. Segmented scales have better erosion resistance than thick, continuous and dense scale, since in the former case the spalled area is confined to crystalline oxide only. This observation is validated by adding Si to the steel. Addition of Si to low chromium steel results in the formation of a segmented scale even at high impact velocity and thereby reduces the erosion rate substantially as compared to the same steel without Si [59]. 

Fig. 17-2 Protection current densities of uncoated steel as a function of flow rate in flowing seawater, F with undisturbed film formation F2 with disturbed film formation due to erosion.

Erosion and Corrosion combined require special consideration. Most of the stainless steels and related corrosion-resistant alloys ow e their surface stability and low rate of corrosion to passive films that develop on the surface either prior to or during exposure to reactive fluids. If conditions change from passive to active, or if the passive film is removed and not promptly reinstated, much higher rates of corrosion may be expected. 

Basically there are two approaches to predicting the occurrence of erosion corrosion. Practical or experience based methods typified by Keller s approach for carbon steels in wet steam. Keller developed an equation that related the erosion corrosion rate as a function of temperature, steam quality, velocity and geometric factor. In recent years this approach has... 

The corrosion rate of carbon steel increases with increase in velocity until a critical velocity is reached. This behavior is different from that of the carbon steel in fresh water where the corrosion rate decreases beyond a critical velocity due to the formation of a passive him. In seawater passive films are not formed because of the presence of high concentrations of chloride. The erosion corrosion occurs after critical velocity 20 m/s is reached. The maximum corrosion rate of 1,0/mm/yr is reached at velocities up to 4 m/s. 

Galvanic effect. The galvanic cell between two different metals can have serious effects in a flowing system. For example, the galvanic cell was not present between stainless steel type 316 and lead in 10% sulfuric acid under static conditions, but when the flow rate increased to 11.89 m s the rate of erosion-corrosion increased enormously... 

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