Coating Failure 3

[dik]

We are looking at a quality coating system over hot dipped galvanizing. The total coating system consists of hot dipped galvanised steel frames. These are coated with a suitable epoxy primer. The primer has an epoxy intermediate coat over followed by a polyurethane top coat.

The anticipated life of the frames is 20 years. The environmental exposure of the frame is that it will be submerged for several months at a time in, generally, non-saline river water. In addition, the frames will likely be subject to impact by water borne debris which may damage the coating system. It is also possible that there may be a slight salinity of the river water caused by road salt ‘run-off’.

The frames will be well maintained.

For a small breach in the coating, it is my understanding that the HDG will sacrificially prevent/reduce corrosion.

Is it possible that for a larger amount of damage, where there is a significant 'gap' in the coating, that corrosion will increase due to the insulating characteristics of the coating? I think this is generally referred to as anodic corrosion.

Dik

 

[CoryPad]

Local effects like those you describe can initiate corrosion by multiple mechanisms, such as crevice, differential oxygen cell, etc. I think your design and planned maintenance can enable you to meet your desired life.

 

[blacksmith37]

As long as you are coating anyway; why not replace the hot dip gal with a zinc rich primer ? As I remember , the inorganic zinc rich provided anodic protection in salt spray tests. The Zn rich could be the primer for the epoxy.

 

[Metaljon]

You need to get a coating specification and durability assessment for the environment prepared by a consultant. The assessment would include an assessment of the corrosivity of the water which will be used to predict the "time for first coating maintenance". They will give you a full coating specification which will work for the environment.

For water immersion condition for HDG steel we normally use two coats of high build epoxy with dry film 150 microns for each layer. I would not use polyurethane top coat for immersion condition.

http://nz.linkedin.com/pub/jonathan-smith/39/25/589

 

[dik]

I'm looking at the last three coating systems, that have been 'worked' out in discussion with the manufacturers and have successfully been used on past projects. The application of the polyurethane top coat is basically cosmetic.

Dry film thickness is stipulated elsewhere and as recommended by the manufacturer.

There will be a program of inspection of the coating and maintenance of same.

These are from my General Notes... a compilation of 30+ years of 'mistakes'... <G>.

SYSTEM 05: HOT DIPPED GALVANISING
BASE METAL - STEEL
CLEANING - TO SSPC 1 AND SSPC 6
HDG - TO CSA C164, THICKNESS TO Z275

SYSTEM 07: QUALITY PRIMER WITH TOPCOAT (INTERIOR)
BASE METAL - STEEL
CLEANING - TO SSPC 1 AND SSPC 6
PRIMER - DEVOE 'DEVGUARD 4160 - MULTIPURPOSE TANK AND STRUCTURAL PRIMER'
TOPCOAT - DEVOE 'SPEEDENAMEL 4318 - QD GLOSS ENAMEL'

SYSTEM 08: QUALITY PRIMER WITH TOPCOAT (EXTERIOR)
BASE METAL - STEEL
CLEANING - TO SSPC 1 AND SSPC 6
PRIMER - DEVOE 'DEVGUARD 4160 - MULTIPURPOSE TANK AND STRUCTURAL PRIMER'
TOPCOAT - DEVOE 'DTM URETHANE-ALKYD GLOSS ENAMEL'

SYSTEM 09: EPOXY WITH ZINC RICH PRIMER
BASE METAL - STEEL
CLEANING - TO SSPC 1 AND SSPC 10
PRIMER - DEVOE 'CATHACOAT 302HB - REINF INORGANIC ZINC PRIMER
TOPCOAT - DEVOE 'BARRUST 235 - MULTIPURPOSE EPOXY COATING'

SYSTEM 10: EPOXY COATING OVER HDG
BASE METAL - HDG STEEL
CLEANING - TO SSPC 1
WASH - POLYCLUTCH 97-687 BY PPG OR APPD EQ
PRIMER - AMERLOCK 400 TWO COMPONENT EPOXY BY PPG OR APPD EQ
TOPCOAT - AMERCOAT 450H TWO COMPONENT ALIPHATIC POLYURETHANE TOPCOAT BY PPG

SYSTEM 11: EPOXY COATING OVER HDG (ALTERNATIVE)
BASE METAL - HDG STEEL
CLEANING - TO SSPC 1
PRIMER - DEVOE DEVRAN 201 EPOXY
TOPCOAT - DEVOE DEVTHANE 379 URETHANE

SYSTEM 12: THREE PART EPOXY COATING OVER HDG
BASE METAL - HDG STEEL
CLEANING - TO SSPC 1
PRIMER - DEVOE DEVRAN 201 EPOXY
SUBCOAT - DEVOE 'BARRUST 235 - MULTIPURPOSE EPOXY COATING'
TOPCOAT - DEVOE 'DEVTHANE 379H - ALIPHATIC URETHANE GLOSS ENAMEL'

My concern is that a 'wide' breach in the coating system may contribute to 'increased' anodic corrosion, because of the 'concentration' of exposed metal.

Dik

 

[dik]

I should have added that System 09 can be used for slip critical connections, too...

Dik

 

[SJones]

If you lose more organic paint, of course there will be an increase in the loss of zinc coating but that doesn't mean more corrosion of the steel substrate. Why didn't you just use thermal spray aluminium? The labour cost in the set up you have is excessive.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[dik]

Thanks, Steve... I had considered metal spray, but had discarded it due to abrasion, impact and a long term history. I wasn't aware that it was used in the late 1960's.

Dik