Immersion Corrosion

ABSTRACT
The basic concepts of probability theory in structural reliability analysis are overviewed and its effect on structural deterioration is noted. For material loss representation due to corrosion, accurate models are required. Therefore, many variables involved in natural environments are considered. In our paper, phenomenological models for marine immersion corrosion are derived. As known, the diffusion of oxygen through the corrosion products to the corroding surface controls the rate of corrosion at least during some period in the corrosion–time behaviour relationship. It was usually assumed that the derived relationship applied from initial immersion and only very limited laboratory support for the derived relationships exists. The

Aim- Mathematical modelling of the diffusion controlled phase in marine immersion corrosion of mild steel.

2.2. Problem formulation/ Case Study :
The general corrosion which occur under near-surface immersion conditions at sea but how this varies with time still it is not well-founded theoretically and practically. Previous efforts to develop mathematical models of immersion marine corrosion are either empirical (e.g. [2,3]) or start from diffusion considerations. The overall corrosion process and its evolution with time is more complex. Therefore we focus only on oxygen transport as limited by diffusion.

2.3. System Description:
a. Open System : A open system has consirered.
b. Dynamic and Discrete system: Corrosion is a dynamic process but a considerable amount of material loss occurs only after a long period.
c. Variables: These are the following variable which effect the object in the system- 1. Time 2. Depth of the corrosion product 3. Distance from the corroding surface
d. Parameters: These are the parameters of the system- 1. Oxygen mass flow 2. Oxygen concentration in the

+ 2OH- → (α-FeOOH)rust + etc. (3)
The process is controlled by the activation energy in the initial stage of reaction and soon controlled by oxygen transport kinetics. Metal ion kinetics including local diffusion mechanics appear not to control the rate of the process [5,6]. The rate of the reactions (1)–(3) is a function of the “surface roughness” of the specimen and the “dissolved oxygen content” in the seawater. As indicated schematically in Fig. 1, the overall rate of the reactions (1)–(3) tend to be constant in time.This phenomenological approach does not reflect the formation of localized corrosion cells and non-linear behaviour at first immersion but is considered as close approximation for modelling purposes.
In quiescent conditions, It is assumed that all the iron consumed are converted into corrosion product. Therefore, from mass balance: γcpy = αγc (4)
Where, γcp is the density of the corrosion product, y is the depth of the corrosion