ASHRAE Journal - April 2009 - 28
Dissimilar Metals In Heating and AC Piping Systems By Walter J. Sperko, P .E. M ost engineers understand that the presence of different metals in a heating or air-conditioning piping system may cause accelerated local corrosion, and that one should avoid using dissimilar metals wherever possible. This is good thinking, but sometimes availability or cost of materials make it necessary to join dissimilar metals together to build engineered piping systems. Using different metals in a heating or air-conditioning piping system does not mean that galvanic corrosion will cause a failure, nor does it mean that dielectric fittings must be used at transitions between materials. This article presents guidance on what to do when different metals are used in heating and air-conditioning piping systems. Basics of Aqueous Corrosion Four conditions must be present for corrosion to occur in aqueous solutions: • An anode; • A cathode; • A metallic current path between the anode and cathode; and • An ionic conducting medium. Corrosion can be reduced by blocking current access to the anode or cathode or both, or by increasing resistance to 28 ASHRAE Journal current flow in either the current path or the liquid medium. Galvanic Couples depends on the environment; when dissimilar metals are electrically connected to each other, each interacts with the environment, and the more reactive metal takes the brunt of the reaction as shown in the iron/zinc couple in Figure 1. While corrosion in a couple always occurs at the anode, there is normally little if any corrosion at the cathode. The potential of metals for galvanically driven corrosion is ranked using the galvanic series, and the most widely published of these is for metals in seawater. At the top of this series1 (i.e., the most cathodic or corrosion-resistant) is platinum followed by titanium, Hastelloys, 18-8 and other high-chromium stainless steels, Inconels, copper-based alloys, tin, lead, cast iron and steel, aluminum and zinc. Magnesium is last and is the most anodic metal on the list. The further apart two metals About the Author Walter J. Sperko, P is president of Sperko .E., Engineering Services, Inc. and Brazing Dimpler Corporation. He is past chairman of the ASME B31.9, Building Services Piping Committee and a current member of various other ASME Pressure Technology committees. He is also past-chairman of the AWS D10 committee on Pipe and Tube. The reason that joining two metals together may cause accelerated corrosion is that some metals are more easily corroded than others. This goes back to basic chemistry. All engineering metals react with their environment, and some are more reactive than others. The rate of reaction ashrae.org April 2009
ASHRAE Journal - April 2009
Table of Contents for the Digital Edition of ASHRAE Journal - April 2009
ASHRAE Journal - April 2009
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Ventilating Façades
Dissimilar Metals in Heating and AC Piping Systems
Anniversary Feature: Central Chilled Water Distribution Systems Design
Total Cost of Ownership for Air-Cooled and Water-Cooled Chiller Systems
Controlling Air Leakage in Tall Buildings
Building Sciences
Emerging Technologies
Washington Report
Special Products
People
Products
Classified Advertising
Advertising Index
ASHRAE Journal - April 2009 - ASHRAE Journal - April 2009
ASHRAE Journal - April 2009 - Cover2
ASHRAE Journal - April 2009 - 1
ASHRAE Journal - April 2009 - 2
ASHRAE Journal - April 2009 - Contents
ASHRAE Journal - April 2009 - 4
ASHRAE Journal - April 2009 - Commentary
ASHRAE Journal - April 2009 - 6
ASHRAE Journal - April 2009 - Industry News
ASHRAE Journal - April 2009 - 8
ASHRAE Journal - April 2009 - 9
ASHRAE Journal - April 2009 - 10
ASHRAE Journal - April 2009 - 11
ASHRAE Journal - April 2009 - 12
ASHRAE Journal - April 2009 - Letters
ASHRAE Journal - April 2009 - Meetings and Shows
ASHRAE Journal - April 2009 - 15
ASHRAE Journal - April 2009 - Ventilating Façades
ASHRAE Journal - April 2009 - 17
ASHRAE Journal - April 2009 - 18
ASHRAE Journal - April 2009 - 19
ASHRAE Journal - April 2009 - 20
ASHRAE Journal - April 2009 - 21
ASHRAE Journal - April 2009 - 22
ASHRAE Journal - April 2009 - 23
ASHRAE Journal - April 2009 - 24
ASHRAE Journal - April 2009 - 25
ASHRAE Journal - April 2009 - 26
ASHRAE Journal - April 2009 - 27
ASHRAE Journal - April 2009 - Dissimilar Metals in Heating and AC Piping Systems
ASHRAE Journal - April 2009 - 29
ASHRAE Journal - April 2009 - 30
ASHRAE Journal - April 2009 - 31
ASHRAE Journal - April 2009 - 32
ASHRAE Journal - April 2009 - 32A
ASHRAE Journal - April 2009 - 32B
ASHRAE Journal - April 2009 - 32C
ASHRAE Journal - April 2009 - 32D
ASHRAE Journal - April 2009 - Anniversary Feature: Central Chilled Water Distribution Systems Design
ASHRAE Journal - April 2009 - 34
ASHRAE Journal - April 2009 - 35
ASHRAE Journal - April 2009 - 36
ASHRAE Journal - April 2009 - 37
ASHRAE Journal - April 2009 - 38
ASHRAE Journal - April 2009 - 39
ASHRAE Journal - April 2009 - 40
ASHRAE Journal - April 2009 - 41
ASHRAE Journal - April 2009 - Total Cost of Ownership for Air-Cooled and Water-Cooled Chiller Systems
ASHRAE Journal - April 2009 - 43
ASHRAE Journal - April 2009 - 44
ASHRAE Journal - April 2009 - 45
ASHRAE Journal - April 2009 - 46
ASHRAE Journal - April 2009 - 47
ASHRAE Journal - April 2009 - 48
ASHRAE Journal - April 2009 - 48A
ASHRAE Journal - April 2009 - 48B
ASHRAE Journal - April 2009 - 49
ASHRAE Journal - April 2009 - Controlling Air Leakage in Tall Buildings
ASHRAE Journal - April 2009 - 51
ASHRAE Journal - April 2009 - 52
ASHRAE Journal - April 2009 - 53
ASHRAE Journal - April 2009 - 54
ASHRAE Journal - April 2009 - 55
ASHRAE Journal - April 2009 - 56
ASHRAE Journal - April 2009 - 57
ASHRAE Journal - April 2009 - 58
ASHRAE Journal - April 2009 - 59
ASHRAE Journal - April 2009 - 60
ASHRAE Journal - April 2009 - 61
ASHRAE Journal - April 2009 - Building Sciences
ASHRAE Journal - April 2009 - 63
ASHRAE Journal - April 2009 - 64
ASHRAE Journal - April 2009 - 65
ASHRAE Journal - April 2009 - 66
ASHRAE Journal - April 2009 - 67
ASHRAE Journal - April 2009 - Emerging Technologies
ASHRAE Journal - April 2009 - 69
ASHRAE Journal - April 2009 - 70
ASHRAE Journal - April 2009 - Washington Report
ASHRAE Journal - April 2009 - Special Products
ASHRAE Journal - April 2009 - 73
ASHRAE Journal - April 2009 - People
ASHRAE Journal - April 2009 - Products
ASHRAE Journal - April 2009 - 76
ASHRAE Journal - April 2009 - Classified Advertising
ASHRAE Journal - April 2009 - 78
ASHRAE Journal - April 2009 - 79
ASHRAE Journal - April 2009 - Advertising Index
ASHRAE Journal - April 2009 - Cover3
ASHRAE Journal - April 2009 - Cover4
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