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Fundamentals of Corrosion
7.4.7 Early rusting
When a latex paint is applied to a cold steel substrate under high moisture
conditions, a measles-like appearance may develop immediately when the
coating is touch-dry. This corrosion takes place when the following conditions exist:
1. The air humidity is high.
2. The substrate temperature is low.
3. A thin (up to 40 μm) latex coating is applied.
7.4.8 Flash rusting
Flash rusting refers to the appearance of brown stains on a blasted steel
surface immediately after applying a water-based primer. Contaminants
remaining on the metal surface after blast cleaning are responsible for this
corrosion. The grit on the surface provides crevices or local galvanic cells
that activate the corrosion process as soon as the surface is wetted by the
water-based primer.
7.5 Stages of Corrosion
To prevent excessive corrosion, good inspection procedures and preventative
maintenance practices are required. Proper design considerations are also
necessary, as well as selection of the proper coating system. Regular inspections of coatings should be conducted. Because corrosion of substrates under
coatings takes place in stages, early detection will permit correction of the
problem, thereby preventing ultimate failure.
7.5.1 First Stages of Corrosion
The first stages of corrosion are indicated by rust spotting or the appearance
of a few small blisters. Rust spotting is the very earliest stage of corrosion
and in many cases is left unattended. Standards have been established for
evaluating the degree of rust spotting and these can be found in ASTM 610–
68 or Steel Structures Painting Council Vis-2. One rust spot in 1 square foot
may provide a 9+ rating but three or four rust spots drop the rating to 8. If the
rust spots go unattended, a mechanism for further corrosion is provided.
Blistering is another form of early corrosion. Frequently, blistering
occurs without any external evidence of rusting or corrosion. The mechanism of blistering is attributed to osmotic attack or a dilution of the coating film at the interface with the steel under the influence of moisture.
Fundamentals of Corrosion
7.4.7 Early rusting
When a latex paint is applied to a cold steel substrate under high moisture
conditions, a measles-like appearance may develop immediately when the
coating is touch-dry. This corrosion takes place when the following conditions exist:
1. The air humidity is high.
2. The substrate temperature is low.
3. A thin (up to 40 μm) latex coating is applied.
7.4.8 Flash rusting
Flash rusting refers to the appearance of brown stains on a blasted steel
surface immediately after applying a water-based primer. Contaminants
remaining on the metal surface after blast cleaning are responsible for this
corrosion. The grit on the surface provides crevices or local galvanic cells
that activate the corrosion process as soon as the surface is wetted by the
water-based primer.
7.5 Stages of Corrosion
To prevent excessive corrosion, good inspection procedures and preventative
maintenance practices are required. Proper design considerations are also
necessary, as well as selection of the proper coating system. Regular inspections of coatings should be conducted. Because corrosion of substrates under
coatings takes place in stages, early detection will permit correction of the
problem, thereby preventing ultimate failure.
7.5.1 First Stages of Corrosion
The first stages of corrosion are indicated by rust spotting or the appearance
of a few small blisters. Rust spotting is the very earliest stage of corrosion
and in many cases is left unattended. Standards have been established for
evaluating the degree of rust spotting and these can be found in ASTM 610–
68 or Steel Structures Painting Council Vis-2. One rust spot in 1 square foot
may provide a 9+ rating but three or four rust spots drop the rating to 8. If the
rust spots go unattended, a mechanism for further corrosion is provided.
Blistering is another form of early corrosion. Frequently, blistering
occurs without any external evidence of rusting or corrosion. The mechanism of blistering is attributed to osmotic attack or a dilution of the coating film at the interface with the steel under the influence of moisture.