Corrosion, galvanic action and service life · Lesson 2 of 4

Corrosion, galvanic action and service life: materials, standards and design constraints

Atmospheric corrosivity categories and their zinc rates; coating thickness and linear zinc loss; why batch-galvanizing charts do not transfer to sheet; aluminum-zinc coatings; warranty distances and exclusions; and UFGS 07 60 00 as a specification model.

You will be able to: Use the ISO 9223 categories and zinc rates, relate coating thickness to life without misusing batch-galvanizing charts, compare how makers define coastal, and read warranties and a guide specification as design checklists.

14 min · 12 slides · 10 questions

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Slide 1 of 12

Atmospheric corrosivity: ISO 9223

ISO 9223:2012 sets up a classification of how corrosive atmospheric environments are.

The ISO committee page names three key factors: the temperature-humidity complex (time of wetness), sulfur dioxide pollution and airborne salinity.

The 2012 edition was confirmed in 2022. The full text is a paid standard and is not reproduced in this course.

Sources

ISO 9223:2012 Corrosion of metals and alloys - Corrosivity of atmospheres - Classification, determination and estimation (committee page; scope only, full text paid) - ISO (ISO/TC 156)

Glossary

ISO 9223 category: One of six atmospheric corrosivity classes, C1 very low to CX extreme, that ISO 9223 uses; AGA ties each to a first-year zinc corrosion rate in micrometres per year.
Linear zinc loss: The typical pattern of a galvanized coating losing zinc at a steady rate, so twice the thickness means roughly twice the life in the same environment.
G90: A galvanized sheet coating designation of 0.9 oz/ft2 of sheet in total on both sides, about 0.76 mil per side, which AGA prints as 18 micrometres.
Time to first maintenance: AGA's term for the time to 5% rusting of the steel surface on batch hot-dip galvanized steel; it is not a service life and does not apply to thin sheet coatings.
Parabolic loss: A corrosion rate that flattens with time, as GalvInfo describes for 55% aluminum-zinc coating, in contrast to the straight-line loss of galvanized zinc.
Warranty exclusion: A condition a maker's warranty does not cover, such as runoff from dissimilar metals, rust at panel edges or installation within a stated distance of salt water.