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An alloying element is a chemical element deliberately included in an alloy to control its properties or processing behaviour. In steel, carbon changes the phases and transformations available during heat treatment; other additions can modify corrosion, hardenability or high-temperature behaviour.

The effect depends on how an element is incorporated. Atoms dissolved in a solid solution can hinder dislocation motion, while an element concentrated in a secondary phase can affect strength through the size and distribution of particles. Thus a bulk composition does not uniquely determine mechanical strength: processing controls the microstructure through which the composition acts.^[ref]

A secondary phase is a distinguishable material phase present alongside a principal matrix phase. It can differ in chemical composition, crystal structure or both. Particles of a secondary phase within an alloy are therefore different from atoms simply dissolved in a solid solution.

Their effects depend on size, distribution and interface properties. Fine particles can obstruct dislocation motion and increase mechanical strength, while coarse or brittle regions can assist crack initiation. Heat treatment can change a secondary phase's amount and morphology, explaining why overall composition alone does not determine an alloy's behaviour.

A single-pass flow-through leach test measures the dissolution of a solid while fresh solution continuously enters a reaction cell and the outlet solution is collected for analysis. Unlike a closed vessel, the cell continually exports dissolved constituents. A reported implementation uses a pump, a solution reservoir, a reaction cell and controlled temperature.^[ref]

At steady state, an elemental mass balance gives the normalized release ratewhere $Q$ is the volumetric volumetric flow rate, $c_e$ is the mass concentration of element $e$, $f_e$ is its initial mass fraction in the solid, and $S$ is the exposed surface area. For example, using litres per day, grams per litre and square metres gives grams of equivalent solid per square metre per day. This expression follows by dividing the exported elemental mass per day by $f_{e}S$.

Changing $Q/S$ changes the accumulation of dissolved material and therefore the chemical affinity for dissolution. A rate independent of further increases in $Q/S$ can help identify a dilute-solution regime. Different elemental release rates can also reveal incongruent dissolution or retention in secondary phases, so interpreting a release rate as whole-solid loss requires an explicit material model.