ABSTRACT Flow induced localized corrosion (FILC), i.e. erosion corrosion, is initiated and developed beyond critical flow intensities. However, it appears difficult to quantify such critical flow intensities because none of the terms tried so far (e.g. ,,critical flow velocities" given in m/s, ,,critical Reynolds Numbers" (dimensionless), or ,,critical wall shear stresses" given in Pa) represent the real hydrodynamic forces responsible for the mechanical destruction of protective scales as a precondition for FILC initiation. In recent years critical wall shear stresses have been measured for a number of technical corrosion systems. However, it appears that the values obtained are several orders of magnitudes too small to meet fracture stresses of corrosion product scales. A new approach is presented in this paper which allows for the first time to quantify the maximum interaction forces between flowing media and the wall which appear to be well in the order of fracture stresses of protective scales. With a new electrochemical tool the boundary conditions can now be defined more accurately and more straightforward under which certain additives (e.g. inhibitors, flow improvers / drag reducers) can really inhibit the onset of FILC. Examples are given for different flow regimes.
INTRODUCTION Localized materials attack occurs when hydrodynamic forces locally destruct protective scales and create local flow-shaped shallow pitting. It is known that functional chemicals (e.g. corrosion inhibitors, emulsifiers, surfactants, etc) which belong to the group of amphipathic substances and exhibt potential for micelle formation, can exert significant drag reducing effects 1-4 which contribute significantly to the mitigation of the initiation of flow induced localized corrosion (FILC). Thus, it was found that critical wall shear stresses for FILC initiation can be significantly increased (as opposed to the uninhibited system) by addition of corrosion inhibitors with pronounced drag reducing properties.
Starting from the assumption that FILC is always initiated if the flow-immanent fluid dynamic forces are large enough to destruct existing protective layers, a correlation (Equation 1) was established between the mechanical fracture stress OB [Pa] of the protective layer and a critical wall shear stress Xw, kat [Pa], serving as an easy-to-measure parameter characterizing the flow intensity beyond which FILC is initiated..
While the fracture stresses and adhesion forces of corrosion product layers (carbonates, sulfides, oxides) range in the order of Megapascals n-s the magnitude of critical wall shear stresses encoutered in real flow systems are only in the Pascal range. Thus, wall shear stresses are orders of magnitudes too small to be directly responsible for the destruction of protective scales. Therefore, the assumption was made that near-wall turbulence elements in the hydrodynamic boundary layer exchange momentum with the scale-covered materials surface and cause fatiguing of the protective scale with cracking and spalling if the turbulence elements are critical with respect to its local energy densities. Kturb in Equationl thus characterizes the fluid dynamic momentum exchange intensity necessary to become critical for scale destruction.