In Support of Phosphoric Acid Wash for Pipe Coating
End-users and coaters alike should demand the use of phosphoric acid in all FBE coating applications and that acid providers supply only the highest quality product and provide proven application methods and criteria specific to pipe coating.
As with most industries today, the oil and gas pipeline segment is undergoing major changes with regards to safety and liability. Governmental regulatory mandates are continually updating in response to pipeline accidents -- and, specifically, injuries and deaths resulting from these accidents.
Coating manufacturers and applicators continually make efforts to increase the reliability, durability, and service life of applied fusion bonded epoxy (FBE) coatings, however the substrate, or pipe surface, condition is more often than not out of their control.
Factors that can cause forms of contamination on the pipe surface include:
- Foreign countries of origin requiring ocean transport.
- Surface transportation across regions known for the use of road salt.
- Storage conditions and proximity to salt sources.
- Length of storage.
- Rust grades.
- Human handling.
If not treated properly, each of these factors can lead to premature failure of even the best-applied FBE coating.
The Pipe Cleaning Process
Fusion bonded epoxy coatings are used for all new constructions. To obtain a pipe surface that is as clean as possible, pipes can be cleaned subsequently after blasting. In the phosphoric acid wash cleaning process, a carefully premixed acid solution is sprayed on the pipe where the idea is to remove possible contaminations such as salts (chlorides and sulphates), light oxidation, oil traces, surface dust and/or carbon residues.
The advantages of this include enhanced adhesion, good process control and efficient use of consumables.
The pipe cleaning process consists of the acid application as first step, followed by a dwell time interval -- which is required for the the acid solution do its work -- and, finally, a high pressure water rinsing. The water used for the final step must be free from contamination so as to avoid pollution of the pipe surface . Therefore, this water should be preferably demineralized or from a reversed osmosis water supply. The acid solution is usually prepared on a skid-mounted equipment module with designated process tanks for mix preparation and ready mix. These process tanks are normally fitted with level indicators, stirrers and the required electrics and instrumentation hardware.
The proper process control is critical to ensure the consistent life, which is mainly dependent upon the raw material as well as the reliable process control. (For more about process control, read: Increasing Coating Performance through Application Process Control.)
The State of Phosphoric Acid as Pipe Coating
For decades, line pipe coating applicators have used phosphoric acid after blast cleaning as an efficient and cost-conscious way to remove harmful chlorides from the pipe surface prior to fusion bond epoxy coating. According to the National Association of Pipeline Coating Applicators, even trace amounts of chlorides can cause osmotic blistering and potential disbondment and failure of the applied FBE coating.
As such, commonly, many end-user coating specifications require testing for the presence of chlorides and the use of phosphoric acid in the event a positive indication. A growing number now recommend using phosphoric acid wash regardless of test results -- positive or negative -- as a precaution, considering the extremely limited typical testing area in comparison with the cumulative surface area to be coated. (For more information on pipeline coating specifications, read: Problems Caused by Obscurity in Coating Specifications.)
For example, a coating project of 1000 linear feet of 16-inch outside diameter (OD) pipe has nearly 4200 square feet of surface area to be coated. Yet, even with aggressive chloride testing, per industry standards, less than half of a square foot will be verified. That leaves a lot of uncertainty -- especially for pipe that is known to have been potentially exposed to chloride environments.
Benefits of Phosphoric Acid Wash in Pipe Coating Applications
Chloride removal isn't the only benefit of applying a phosphoric acid wash to pipe coatings, however. Often overlooked is the additional benefit of applying a phosphoric acid wash, which is that it provides superior pipe surface conditioning and enhancement that helps remove other possible detrimental contaminants from blast-cleaned steel pipe.
In the coating application process, blast cleaning can impress other contaminants -- such as mill lacquer, dirt, and even trace non-detectable chlorides -- from contaminated blast media into the anchor profile, leaving the finished coating susceptible to spot failures and disbondment that could have otherwise been isolated or prevented with a phosphoric acid wash.
Additionally, as evidenced in laboratory testing, coating adhesion and performance is noticeably increased with the use of phosphoric acid wash prior to coating. In a case study by Womble Company, Inc. (formerly LaBarge Coating) and Crest Chemicals, using NACE SP0394 24-hour test procedures and given identical coating substrate conditions, cathodic disbondment test results when acid wash is applied are typically nearly three times better than when not using acid wash and results in Hot Water Soak testing are consistently a "1" rating. (For more on laboratory testing for cathodic disbondment, read: Corrosion Assessment: 8 Corrosion Tests That Help Engineers Mitigate Corrosion.)
It should also be noted that, in sample preparation of coating porosity and backside contamination, the temperature required for a clean break (i.e., the complete separation of the coating from the substrate) is much lower when acid wash has been applied than when not. At the standard break temperature of -40°F, the acid-washed sample will frequently result in coating shear, with the actual coating separating in layers and leaving a distinct base layer of coating still firmly attached to the substrate. A temperature of -60°F to -65°F using dry ice or another capable method is highly recommended for accurate testing as opposed to common laboratory freezer capabilities, which typically limited to between -30°F and -40°F.
Issues Regarding The Application of Phosphoric Acid Wash to Pipe Coatings
While the use and benefit of a phosphoric acid wash is obvious, the application process for phosphoric acid in line pipe FBE coating has been largely undefined at best, leaving end-user specification engineers to define the use and criteria for the application of a product with which they are often not entirely familiar -- or leaving the applicator themselves to define a method.
Many times, requirements for phosphoric acid washes are simply carried over from existing specifications, using the "copy-and-paste" method and ignoring current data. Alternatively, in some cases, the "more-is-better" approach can severely inhibit a coater's ability to run efficiently, if they are able to do so at all.
Multiple changes in the acid wash application to meet the numerous variables from spec to spec is time-consuming and costly for the coating plant. As a result, it is quite common for a coater to simply take exception to the specified process, citing adherence to the acid manufacturer's recommendations. The problem was finding those recommendations, especially specific to pipe coating. Acid manufacturers have been of limited help, primarily due to their inexperience with actual coating application parameters and restrictions. The vagueness in the information acid manufacturers provide is often seen as reactive to a coater’s needs and lacking any substantial credibility regarding effectiveness.
Additionally, while perhaps not intentionally, many phosphoric acid suppliers are provide subpar products -- without any specificity to pipe coating and offering almost no guidance as to proper usage and application in the coating process. And, unfortunately, many coating applicators tend to weigh minute cost-savings over quality in purchasing these products.
Suggestions for Improving the Use of Phosphoric Acid Wash in Pipe Coatings
As a long-time coater and user of phosphoric acid, I, along with other industry experts, recognize and wholly support the demand for public safety in the oil and gas pipeline industry as essential for the future and growth of our nation. Just as importantly, I believe it is an obligation of manufacturers and suppliers of both raw materials and finished goods to provide the highest quality products in every application and industry they serve.
Combining these initiatives, it is essential for material suppliers to provide a superior, yet cost-effective, phosphoric acid wash that is both:
- Specific to the pipe coating industry.
- Intended for universal application.
One example of this is Crest Chemicals' AC10B, which uses only raw materials tested, is certified by an independent laboratory and verifies that each batch meets strict standards and specifications -- both industry and military. Its contents are defined with an identifiable certificate of compliance, issued with each order, which documents quality criteria such as:
- Minimum phosphoric content.
- Specific gravity.
- Maximum levels of lead and arsenic.
Considering the benefits and pipeline safety implications, this methodology should be the minimum standard for all acid manufacturers and an absolute requirement for all end-users and coating applicators alike. (For more on making coating specifications safer, read: Writing Safety Into Your Coating Specification.)
Equally important is the lack of documented procedures for plant-applied acid wash. This is scarce, and very few acid suppliers have taken the initiative to change that.
That could be because documenting such procedures would require years of in-plant coating experience to create a comprehensive recommended application procedure with defined and measurable parameters and criteria for both chloride removal and preventive and surface preparation enhancement application -- and it is difficult for acid suppliers to provide that experience.
Clearly, however, there is enormous benefit to offering an effective and proven procedure exclusively intended for phosphoric acid cleaner for pipe coatings: It reduces the possibility of incorrect use and provides a precise application guide, allowing both the coater and end-user to review and maintain a tangible approved procedure direct from the manufacturer.
Additionally, technical staffing with hands-on pipe coating experience that understands how individual coating plant capabilities and layouts vary can also help modify the process and/or product concentrations to suit each plant, while still ensuring the most effective application of phosphoric acid to meet the intended results consistently and efficiently.
It is unfortunate that so few acid suppliers can offer this service, but hopefully this will become the norm sooner rather than later. (For more on the current state of the pipe coating industry, read: The Growing Disconnect in the Pipe Coating Industry.)
Regardless of the source, providing the best possible FBE coating and reducing failures must be the goal, and the correct application of a phosphoric acid wash must be included in the coating process to achieve those results.
As safety standards tighten, end-users and coaters alike should demand the use of phosphoric acid in all FBE coating applications, and that acid providers supply only the highest quality product and proven application methods and criteria specific to pipe coatings.