Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For corrosion
Reexamination Certificate
1998-07-15
2001-07-10
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For corrosion
Reexamination Certificate
active
06258252
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of effecting a computerized analysis for predicting corrosion and corrosion prevention of metals, and more particularly to a method of analyzing macro cell corrosion such as bimetallic corrosion (also referred to as “galvanic corrosion”) and differential aeration corrosion and cathodic corrosion prevention, among various metal corrosion and corrosion prevention phenomena. The present invention is also concerned with an analytical method applicable to a system such a plating system, a battery, an electrolytic tank, etc. where a macro anode and a macro cathode exist across an electrolyte, developing a field of electric potential.
2. Description of the Prior Art
In solutions having high electric conductivity, such as seawater, metals are susceptible to macro cell corrosion such as bimetallic corrosion caused when different metals are used together or differential flow velocity corrosion, i.e., differential aeration corrosion, due to flow velocity distribution irregularities. It has been desired to predict those corrosions accurately in advance so that appropriate preventive measures can be taken. Cathodic corrosion prevention based on the positive use of a corrosion inhibiting phenomenon at a cathode in a macro cell is finding wide use as the most basic corrosion prevention process. There has been a demand for the prediction of a range of corrosion prevention and the rate of consumption of a sacrificial anode depending on the material of the anode, the position of installation of the anode, the shape and materials of devices to be protected against corrosion, and solution conditions including electric conductivity, flow velocity, etc.
Experimental approaches to the precision analysis of macro cell corrosion suffer limitations because the configuration of the field has a large effect on the behavior of macro cells. Specifically, when an experiment is conducted on bimetallic corrosion to inspect in detail the effect of various factors including the ratio of areas, the combination of materials, and the electric conductivity of the solution, the experimental result applies only to the three-dimensional shape of a region occupied by the solution in the experiment. Since actual devices and structures are quite complex in shape, the liquid junction resistance in a macro cell cannot accurately be estimated, and the experimental result cannot apply directly to the actual situation. It is practically impossible to carry out an experiment on the particular shape of a device to be protected against corrosion each time the shape of the device is changed. For these reasons, it has heretofore customary to predict macro cell corrosion and cathodic corrosion prevention for actual structures mostly according to empirical rules.
Many attempts have been made to achieve a more accurate and quantitative analysis of macro cell corrosion and cathodic corrosion prevention for actual structures. One effort has been to solve purely mathematically a Laplace's equation governing a potential distribution for determining a potential distribution and a current density distribution. Objects to be analyzed by this process are limited to relatively simple systems in the form of flat plates, cylinders, etc. Processes long known in the art for analyzing electric field problems including a conformal mapping process and a process using electrically conductive paper. These processes, however, handle two-dimensional fields only.
With the development in recent years of the computer technology, various efforts have been made to apply numerical analyses using a difference method, a finite element method, and a boundary element method. The difference method and the finite element method are disadvantageous in that the time required for calculations is very long because an object to be handled needs to be divided into elements. According to the boundary element method, since only the surface of an object to be handled needs to be divided into elements, it is possible to greatly reduce the time required to divide the object into elements and the time required for calculations. Based on the belief that the boundary element method is most suitable for analyzing corrosion problems where physical quantities including a potential and a current density on a surface are important, the inventors have developed an analytical technique based on the boundary element method for the prediction of macro cell corrosion and cathodic corrosion prevention problems.
Basic Equations and Boundary Conditions
The corrosion of a metal in an aqueous solution develops due to electrochemical reactions which comprise a pair of anodic and cathodic reactions. For example, the reactions which corrode iron in an aqueous solution of neutral salt, such as seawater, proceed according to the following equations (1) and (2):
Fe→Fe
2+
+2e
−
(anodic reaction) (1)
½·O
2
+H
2
O+2e
−
→2OH
−
(cathodic reaction) (2)
On a surface of metal, an area where an anodic reaction occurs is referred to as anode, and an area where a cathodic reaction occurs is referred to as cathode. With respect to the corrosion of iron in seawater, anodes and cathodes are usually very small and mixed together, and their positions are not fixed. Therefore, the corrosion progresses substantially uniformly over the entire surface while producing some surface irregularities. If the material, the surface state, and the environment are not uniform, then anodes and cathodes are localized, allowing corrosion to concentrate in certain regions (anodes). The former type of corrosion is referred to as micro cell corrosion, and the latter type of corrosion as macro cell corrosion. The type of corrosion which is often responsible for extensive damage to seawater pumps is the macro cell corrosion which includes bimetallic corrosion and differential aeration corrosion. The cathode in a macro cell is inhibited from corroding because only a cathode current flows in the cathode. Cathodic corrosion prevention is a corrosion prevention process which positively uses such a corrosion inhibiting phenomenon.
Each of systems of macro cell corrosion and cathodic corrosion prevention may be considered as a cell comprising an anode and a cathode disposed across an electrolyte. A potential (&phgr;) distribution in the electrolyte is governed by the following Laplace's equation (3):
∇
2
&phgr;=0 (3)
It is assumed that, as shown in
FIG. 1
of the accompanying drawings, an electrolyte is surrounded by boundaries &Ggr;
1
, &Ggr;
2
, &Ggr;
3a
, and &Ggr;
3c
. The boundary &Ggr;
1
is a boundary where the value of a potential &phgr; is set to &phgr;
0
, i.e., a boundary where the potential is constant. The boundary &Ggr;
2
is a boundary where the value of a current density q is set to q
0
, i.e., a boundary where the current density is constant. The boundaries &Ggr;
3a
and &Ggr;
3c
are the surface of an anode and the surface of a cathode, respectively. Boundary conditions in the respective boundaries are given by the following equations (4)-(7):
On &Ggr;
1
:&phgr;=&phgr;
0
(4)
On &Ggr;
2
:q{≡&kgr;∂&phgr;/∂n}=q
0
(5)
On &Ggr;
3a
&phgr;=−f
a
(
q
) (6)
On &Ggr;
3c
:&phgr;=−f
c
(
q
) (7)
where &kgr; represents the electric conductivity of the electrolyte, ∂/∂n a differential in the direction of an outward normal line, and f
a
(q) and f
c
(q) nonlinear functions indicative of polarization characteristics of the anode and the cathode, respectively, the nonlinear functions being determined by way of experimentation. By solving the equation(3) under the boundary conditions (4)-(7), it is possible to determine a potential distribution and a current density distribution near the surface. The potential &phgr; and an actually measured electrode potential E are related to each other by &phgr;=−E.
Amaya Kenji
Aoki Shigeru
Miyasaka Matsuho
Takayama Hirokazu
Bell Bruce F.
Ebara Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
LandOfFree
Method of analyzing corrosion and corrosion prevention does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of analyzing corrosion and corrosion prevention, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of analyzing corrosion and corrosion prevention will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2471812