Metalizing should be considered for all steel bridges, large and small, overpasses, and signature bridges. Why metalize? Bridges should be metalized because a sprayed-metal coating will provide the very long service life not available by painting.
Owners should not default to a standard paint system without first comparing the service life and life-cycle costs of metalizing versus painting. And don’t be sidetracked by paint manufacturers’ claims or overpromises about paint performance; promises of performance based on lab tests are not enough. Metalizing has earned its reputation for long service-life the hard way – by results in the field, not from a lab.
It almost goes without saying that contractor selection is the most important decision when awarding a work contract. Experience counts and contractor expertise will be reflected in the quality of the work performed on every metalizing job.
In this section, we address these and other questions
life to first maintenance?
In BS 5493 life to first maintenance is defined as
. . . the life requirement of the initial protective coating is based on the time which can elapse before major or general maintenance of the coating becomes necessary. That time is known as the ‘life to first maintenance’ and its values, related to types of environment and coating systems, are given in table 3. . . .70
The Standard’s Table 3 is in ten parts with each part headed by the description of its environment. These environments range from the benign,
Exterior exposed non-polluted inland to the unforgiving,
Sea water, immersed. Coatings are ranked by typical lives to first maintenance ranging from less than 5 years to 20 years or more.
In Table 3’s ten service environments, a dozen or more coatings including metalizing, galvanizing, and various paints are grouped according to their typical lives to first maintenance; very long (20 years or more), long (10 to 20 years), medium (5 to 10 years), and short (less than 5 year). It is only the sealed metalized coatings that are listed in the very long category for all ten environments including in Parts 9 and 10,
Sea water splash zone, or frequent salt spray, and
Salt water, immersed.
AWS C2.14, the 19-Year Report, covers AWS’ evaluation of flame-sprayed zinc, and aluminum in various locations across the United States over a nineteen year period. The primary finding was that metalized zinc and aluminum coatings gave complete base metal protection in seawater for 19 years.71
Canadian Standard G189 provides two “Life Expectancy Tables,” one for zinc and one for aluminum. Each is a matrix cross referencing type of exposure, coating thickness, and service life. This specification proposes that metalized zinc and aluminum coatings can protect steel for
over 40 years.
Unsealed sprayed-metal coatings are usually designed to protect the structure for its required life. Maintenance operations for such coatings are usually more elaborate than for sealed coatings.72
Depending on the condition of the existing coating and the steel, the more elaborate maintenance can range from blast cleaning and reapplication of the sprayed-metal, to a lesser surface prep, perhaps brush blasting of the steel and existing sprayed-metal, followed by sealing or painting.73 Maintenance of the sprayed-metal coating, with or without sealer, and with or without topcoat is spelled out in Standard 5493 at Table 10, which is quoted below.
On this subject, Standard 5493 can be summarized by this sentence
Sealing is particularly desirable when it is desired to retain the sprayed coating when the surface is eventually to be maintained, and such maintenance requires only the renewal of the sealer. (Emphasis added.)74
The maintenance of paint topcoats was mentioned earlier. The Standard addresses it under maintenance interval (Notes to Table 10).
Maintenance intervals for metal coatings which are to be painted. When bare galvanized surfaces or sealed sprayed-metal surfaces are maintained by the use of paint, the future maintenance intervals will be those of the metal-plus-paint system, which is often less than that for the bare metal coating but longer than for a similar paint system applied directly to steel.75
The British Standard points out that thick coats of paint applied to sprayed-metal will fail, will become an aesthetic problem, and will draw the public’s attention to the paint failure. Keep in mind that in spite of a paint topcoat failure, the underlying sealed sprayed-metal coating will continue to provide the necessary corrosion control even when the failed paint topcoat becomes an eyesore.
Coating service life should be the first thing to consider in the coating selection process, and not the last step in the painting process - as part of the forensic examination of a paint failure.
Table 10 is a flowchart for assessing the present condition of, and recommending on-site repairs to metalized coatings.
Column 1 divides the table between
Bare metal coating and
Sealed or painted metal coating.
Column 2 is an assessment of the “present condition” of the coating.
The decision at Column 3 is whether or not to apply more sprayed-metal which dictates the surface preparation for the repair, i.e. blast cleaning prior to remetalizing [Column 4], or a lesser surface prep before the replenishment of sealer or application of a paint, Column 5.
|Initial Condition||Present Condition||Surface Preparation||Replacement of metal where applicable||Paint treatment over Sprayed-metal|
|Bare metal coating||Areas of corrosion and/or some rusting of substrate*||If metal is to be replaced, blast-clean||Spray metal to appropriate specification||Not normally necessary. If over-coating is required, see appropriate specification|
|If metal not to be replaced, clean corroded areas by best means available||Not applicable||Build-up cleaned area with suitable paint system and, preferably, apply chemical-resistant finish overall|
|Areas with some white corrosion products||If decoration required wash to remove salts, using stiff brush if necessary. Remove loose material with non-metallic brush||Not applicable||Apply sealing coat and chemical-resistant finish for maximum life|
|If decoration not required, no action is necessary||Not applicable||Not applicable|
|Areas in sound condition||If decoration required wash to remove salts, using stiff brush if necessary. Remove loose material with non-metallic brush||Not applicable||Apply suitable paint which should be chemical-resistant for maximum life|
|If decoration is not required, no action is necessary||Not applicable||Not applicable|
|Sealed or painted metal coating||Areas of corrosion or some rusting of substrate||If metal is to be replaced, blast-clean||Spray metal to appropriate specification||Refer to appropriate specification and see notes below, consider one or two coats overall, preferably chemical-resistant|
|If metal is not to be replaced, remove corrosion product by best method available||Not applicable||Build-up cleaned areas with suitable paint. Apply one or two coats overall, preferably chemical-resistant|
|Areas with some degradation of paint, dissipation of sealer, or loss of adhesion of either||Remove loose material with non-metallic wire brush||Not applicable||Apply further coats of paint or sealer, preferably chemical resistant|
|Areas in sound condition||If decoration is required, dust-down||Not applicable||As above|
|If decoration is not required, no action is necessary||Not applicable||Not applicable|
Maintenance of sprayed-metal is covered in AASHTO S8.2/SSPC-PA 18 at Section 11 and in Appendix A.
There are two practical comments in Section 11:
For cosmetic repairs (nonconforming coating that has unacceptable appearance but is expected to perform adequately for corrosion protection) consider
how the aesthetics of the repair will look compared to the original coating. Cosmetic repairs may not need remediation if they will not be exposed.
Where a repair procedure requires complete removal of the TSC, the impact of the repair on the performance of the coating should be considered. For example,
attempting to remove all TSC from the profile could result in excess profile depth. . . .
76 [Emphasis added.]
In AASHTO S8.2 table Appendix A, the condition to be repaired is stated in Column 1; the likely cause in Column 2 and a recommended corrective action or remedy is in Column 3.
All six problems/conditions are applicator/operator related.
|Excessive TSC||Improper application technique||Conduct adhesion testing on a companion coupon to determine whether the excess thickness is detrimental. If the companion coupon fails the adhesion test, proceed in accordance with 11.2. See Joint Standard Section 9.3; also see 9.5 for requirements for faying surfaces.|
|Insufficient TSC||Improper application technique||Apply additional coating to specified film thickness. See Joint Standard Section 9.3.|
|Contaminants IN the TSC||Poor application conditions||Remove contaminated TSC by blast-cleaning to the required surface conditions and reapply TSC|
|Contaminants ON the TSC (could include rust-contaminated dust from nearby blasting) Rust under the TSC (may be discovered through adhesion testing) Rust in the TSC||Poor storage conditions||Clean surface using solvent, hand tools, or other methods.|
|Poor surface preparation||Clean surface using solvent, hand tools, or other methods.|
|Improper application technique or storage, or insufficient TSC thickness||Brush-blast the area with nonmetallic grit to remove surface oxides and restore TSC to required thickness. See Joint Standard Section 8.5.|
Where to go from here?
Current practice is that when a steel bridge is painted, the paint coating will remain in place until it fails; there will be no periodic or partial maintenance of that coating.
Once again we ask our friends in the departments of transportation, are you open to the idea of substituting sprayed metallic zinc for your department’s standard paint coating to achieve a longer coating service life, less traffic disruption and lower life-cycle cost?