Surface recession

Fig. 2. A simplified material thermal performance analysis for a reentry vehicle thermal protection system where = density x surface recession thickness = total aerodynamic heat/heat of ablation ...

The practical motivation for understanding the microscopic details of char reaction stem from questions such as How does the variability in reactivity from particle to particle and with extent of reaction affect overall carbon conversion What is the interdependence of mineral matter evolution and char reactivity, which arises from the catalytic effect of mineral matter on carbon gasification and the effects of carbon surface recession, pitting, and fragmentation on ash distribution How are sulfur capture by alkaline earth additives, nitric oxide formation from organically bound nitrogen, vaporization of mineral constituents, and carbon monoxide oxidation influenced by the localized surface and gas chemistry within pores ... 

A statistically significant linear relation has been found between carbonate-rock surface-recession rate and hydrogen ion loading to the rock surface. 

Analytical results for sulfate ions are for the net change in rainfall composition, after the rainfall flows over the surface of the exposed stone (Tables I and II). The net concentration, resulting from the interaction of rain with the stone surface, is calculated by subtracting the blank-sample concentration value from the value measured for the stone runoff. For very small rain amounts (typically 2 to 4 millimeters), blank volumes were too small to allow complete chemical analysis. Sulfate ion net concentration, in excess of that possible from sulfate loading, was calculated to identify the presence of soluble sulfate-containing minerals on the rock surface. Loss of rock mass, measured by the calcium concentration in the runoff, is expressed as surface recession, in micrometers of rock surface lost per rainstorm. The net calcium concentration in the runoff solution is converted to an equivalent volume of rock removed this result divided by the surface area of the exposed rock, yields a measurement of surface recession. ... 

Relation between environmental variables and surface recession. Carbonate-rock surface recession and the environmental variables measured in this program (i.e., rain amount, rain pH, hydrogen ion loading to the rock surface, and antecedent sulfur dioxide) exhibited a range of values at the North Carolina site during the last half of 1984. To examine qualitatively the relation between... 

Table II. Rainfall quantity and pH, antecedent sulfur dioxide, net sulfate, surface recession for a limestone reference rock at Research Triangl June to October 1984 hydrogen ion loading, and e Park, North Carolina, ...

U.S. Geological Survey Sample no. Rain- fall (milli- meters) pH Antecedent sulfur dioxide (parts per billion times days) Net sulfate ion in runoff (milligrams per liter) Hydrogen ion loading (milli-equivalents per meter squared) Surface recession (micro- meters)... 

The situation differs for large rain amounts. For large rain amounts, excess sulfate values for limestone exceed excess sulfate values for small rain amounts. Large-excess sulfate values for a large-rain-amount for limestone are associated with twice the surface recession of a marble sample for the same event. Some of this surface loss may be the result of the removal of a calcium sulfate mineral. 

Table III. Incident rainfall, rain pH, hydrogen ion loading, and surface recession for limestone and marble at Research Triangle Park, North Carolina, June to October 1984...

Acid-Rain-Induced Calcium Carbonate Surface Recession. Calcium carbonate surface recession may be correlated with rainfall characteristics, using standard statistical techniques. The relation may be written as ... 

Marble-surface recession at the North Carolina site was correlated with rainfall amount. The correlation coefficent was significant at greater than the 99-percent confidence level. Marble-surface recession also was correlated with hydrogen ion deposition to the rock. The linear relation between surface recession and hydrogen ion deposition was found to be ... 

Although the results reported below are not yet sufficient in themselves to produce a materials damage function (i.e., an equation relating damage to one or more environmental factors), they can be used to help evaluate and perhaps modify functions that have been proposed on other bases. This paper tentatively concludes that stone material is lost from skyward-facing surfaces at a rate equivalent to about 15 micrometers of surface recession per year at the field test sites and that an approximate agreement with the results of Reddy s runoff chemistry is found where comparison is now possible, i.e., at sites having low ambient concentrations of sulfur dioxide. 

Surface Roughening/Recession. Two approaches to measurement of surface recession were explored. One method applied a classical form of optical interferometry to optically smooth samples of stone in an attempt to measure results after brief exposure of the surfaces to rain this approach was only partially successful and will be only... 

Table I. Stone Surface Recession and Roughness after First Year of Exposure in North Carolina ...

Table IV. Marble Surface Recession and Roughness after Two Months of Exposure in District of Columbia ...

Embossing means above the surface (raised). Debossing means below the surface (recessed). Debossing or embossing may be achieved in a moulding operation by the use of male/female dies with or without heat. Embossing rollers or dies may also... 

Fig. 12. Predicted oxidation lifetime for various intermetallic compounds as a function of specimen thickness using the time to produce a surface recession equivalent to 10 % of the original specimen thickness [45].

An additional aspect of the oxidation of Ti3Al alloys is dissolution of oxygen into the alloy at the scale/alloy interface. The embrittlement associated with this phenomenon can be more damaging to the mechanical properties than the surface recession caused by scale formation in the temperature range where Ti-,A1 will likely be used (< 700°C) [58],This subject will be discussed in a separate section. 

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