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Technical Memorandum 005-9 - Joining Dissimilar Metals With A Mechanical Connector
Technical Memorandum 005-9 - Joining Dissimilar Metals With A Mechanical Connector Imprimir
Domingo, 13 de Noviembre de 2011 22:28

Introduction

Most high-capacity water wells are constructed of casing and well screen manufactured from a single type of steel such as low-carbon steel, copper-bearing steel, high-strength low-alloy steel, or stainless steel (Types 304 and 316L). However, there are exceptions such as when well screen manufactured from a noble metal (stainless steel) is welded to casing manufactured from a less noble metal. This direct coupling of two highly dissimilar metals typically promotes the occurrence of galvanic corrosion unless some suitable control measure, such as a mechanical connector, is employed. This memorandum provides a brief overview of galvanic corrosion and describes the design and use of a mechanical connector that is supplied by Roscoe Moss Company (RMC).

Galvanic Corrosion

Galvanic corrosion is a common form of corrosion that is induced when two dissimilar metals are in direct contact with one another under water. When a galvanic couple of this type is formed, the more positive (noble) metal functions as a cathode and will corrode slower than it would by itself. The less noble of the metals functions as an anode and will corrode faster than it would by itself. Two significant effects of galvanic corrosion that affect the operation of water wells are: 1) an increase in sand production caused by the widening of the screen openings which allow aquifer material and even gravel pack to enter the well; and 2) structural failure of the casing or well screen caused by a loss of collapse strength for the affected casing and/or screen.

Galvanic corrosion is preventable, controllable, and predictable. One way to qualitatively assess the relative tendency of various metals to corrode if connected to one another is to refer to each metal’s position in the galvanic series (See the reference list for sources of information on this topic). If two metals are close together on the table, then they have a greater potential to corrode if they are coupled. Unfortunately, however, the galvanic series does not forecast the rate of corrosion; nevertheless, it is a reliable qualitative guide.

ENVIADO POR RAÚL CAMPILLO U., HIDROGEÓLOGO.

 

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