Applied at a dry film thickness of 75 microns on SA 2.5 abrasive blast-cleaned steel, both water-borne and
solvent-based zinc silicates show excellent performance with excellent scribe protection for periods of
10,000 hours.

Zinc epoxies, applied at the same dry film thickness on SA 2.5 abrasive blast-cleaned steel show coating failure at saltspray exposure periods of 1000 hours. Depending on their composition, blistering and face corrosion began to occur.

R. Ramsey of the Florida DOT compared the relative performance of zinc rich formulations with different organic binders. The increasing order of performance was phenoxy chlorinated rubber and epoxy.

He also reported the results on the performance of un-topcoated zinc rich coatings on a Florida bridge in maritime exposure:

- excellent 18 years performance of the zinc silicate coating
- poor 18 months performance of a zinc rich phenoxy primer
- general superiority of inorganic zinc primers compared to organic zinc primers

It was concluded by him, that many of the organic zinc rich products tested, appear to protect primarily as a barrier rather than cathodically.

The excellent performance of un-topcoated zinc silicates reported above, is in line with the experience we have achieved with large projects, where a single coat of 75 microns of a zinc silicate has been used:

�� Portal Crane, Danish shipyard. 80 Meter high portal crane with a span of 140 metres has been protected with one single coat of a zinc silicate coating at 75 microns for a period of 18 years. Although after this period the coating was in
excellent condition, a topcoat was applied for cosmetic reasons.

�� Chemical plant, Botlek, The Netherlands. Excellent corrosion protection has been obtained with a single coat of a zinc silicate coating applied at 75 microns dry film thickness in the heavy chemical industrial Botlek-area near Rotterdam.
For cosmetic reasons, topcoats have been applied on the various parts of the plant, 7 to 10 years after the plant was put in operation.

�� Decks 98000 T crude oil carrier (La Rabida). Single coat of zinc silicate applied at 75 microns on decks provided excellent protection for a period of 13 years.

A comparative test program was made with the exposure of test panels of 1 m2 coated with three and four coat systems, consisting of a zinc silicate prime coat and respectively two and three epoxy coatings. After exposure for 10 years in an industrial marine atmosphere in the Botlek/Rozenburg area in the Netherlands, following results have been reported (see also pictures 1 and 2):

Coating system dry film thickness Surface corrosion Scribe corrosion
zinc silicate/epoxy intermediate/epoxy topcoat 300 my none none
zinc epoxy/epoxy intermediate/epoxy topcoat 300 my none 4 cm
zinc epoxy/2 epoxy intermediates/epoxy topcoat 375 my none 2,5 cm

SSPC-SP6
Sa2
NACE 3

Commercial Blast Cleaning - Removal of mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels, to the degree specified. A Commercial Blast Cleaned Surface Finish is defined as one from which all oil, grease, dirt, rust scale and foreign matter have been completely removed from the surface and all rust, mill scale and old paint have been completely removed except for slight shadows, streaks, or discolorations caused by rust stain, mill scale oxides or slight, tight residues of paint or coating that may remain; if the surface is pitted, slight residues of rust or paint may by found in the bottom of pits; at least two-thirds of each square inch of surface area shall be free of all visible residues and the remainder shall be limited to the light discoloration, slight staining or tight residues mentioned above.

dcfrush (Petroleum) 18 Aug 09 21:58
For corrosion to occur you need an anode, a cathode, and an electrolytic solution; remove any one of these from the equation and you will eliminate the corrosion mechanism. The anode will experience an oxidation reaction and the cathode will experience a reduction reaction, or more simply the anode will lose electrons and the cathode will gain electrons. Whether a metal is anodic or cathodic is relative to the metal it's in contact with and their distance from each other in the galvanic series. Zinc is anodic to carbon steel, so the zinc layer will give up its electrons before the carbon steel will (sacrificial anode). This is also why a lot of coatings systems will call for an inorganic zinc primer.

Hope this helps!

unclesyd (Materials) 10 Sep 09 22:47
A Zinc rich primer will lay dormant until it damaged, like a scratch, which exposes the Zinc. At this point the Zinc will do it's thing.
I have some ZRC+epoxy+urethane that have been in service at the fish house since the eighties. There are a few cases where ZRC+chlorinated rubber have lasted even longer.

structuresguy (Structural) 12 Aug 08 13:12
NASA and the Air Force use Inorganic Zinc coatings for all their exposed structural steel at Kennedy Space Center and Cape Canaveral Air Force Station. The steel framed launch towers are right on the beach, fully exposed to salt spray from the ocean.

I have personally done work on these towers. Some are quite old and still in very good condition. Some areas, which may have been more prone to standing water or poor drying, have shown some corrosion damage, though proper periodic maintenance could have helped these areas.

IO Zinc can be field applied, and touched up in the field. The critical step is proper sand blasting and time after blasting until painting. The steel needs to be blasted near white, and then painted immediately. This can be tricky for large areas in the field.

If I could not HDG steel, I would use IO zinc.

4.5.8 Salt spray resistance. Salt spray resistance shall be determined in
accordance with ASTM B 117, except that three 10.6 by 15.24 by 0.32 cm (4 by
6 by 1/8 inches) mild steel panels in accordance with MIL-S-22698 shall be
used for this test. Surfaces shall be blasted with a medium grit (pass
through 0.071-cm (710-micrometer) and retain on 0.042-cm (420-micrometer))
sieve conforming to RR-S-366 prior to coating. A 1.27-cm (0.5-inch) wide
strip shall be left uncoated along the longitudinal center of test face of
each panel to observe degree of cathodic protection provided by the coating.
Two coats of paint shall be brush-applied uniformly to all remaining surfaces
of each panel to a minimum thickness of 0.0508 millimeter(2 mils). A drying
time of 24 hours between coats and 48 hours for the second coat shall be
observed. The panels shall be exposed in the salt spray apparatus for 288
hours, then evaluated for percent rust on the coated and uncoated areas. The
uncoated area of the test panel shall not be completely rusted. The coated
area shall show less than 5 percent rust. Rust streaking shall be
disregarded in determining these percentages. The average of the three
panels shall be determined for each area. Film failure of coated areas shall
be noted and recorded.