photo: MASKALIN / getty images

Corrosion and its impact on ferrous structures has been known for centuries. The effects of corrosion are manifested in decreased asset availability due to increased maintenance and preservation activities, an increase to total ownership cost and an increased risk to the safety to personnel and systems. In fact, the average annual cost of corrosion to Navy ships between 2004 and 2019 was $3.64 billion and $16.9 billion for the whole of the U.S. Department of Defense,¹ and the U.K. Institute of Corrosion estimates the financial cost on the global economy to be more than 3% of global GDP.² As such, commitment to developing strategies to evaluate new technologies for the identification and mitigation of corrosion are essential to reducing the cost of corrosion on Naval assets and infrastructure.

While technological advances in corrosion monitoring, inspection, coatings, cathodic protection and materials science have enabled improvements in corrosion control affecting infrastructure, the effective mitigation of corrosion is ultimately dependent upon our ability to apply protective coatings. Surface preparation for coating application and reapplication due to failed or damaged coatings has become the focus of new technologies that will enable the maintenance worker to do their job more effectively.

Pulsed laser ablation (PLA) is a process that may be used for both pre-cleaning and cleaning the surface of aged or damaged coatings. While laser ablation does not typically create a surface profile, extensive testing indicates it can remove all coating without yielding any measurable differences between PLA and the original underlying abrasively blasted surface.  However, various forms of microscopy have revealed a less tortuous surface after PLA compared to the original blasted surface. This may attribute to coating adhesion results using PLA surface preparation on parity or better than that achieved with abrasive blasted surfaces where profiles are statistically similar.³ The use of other tools to produce a profile, where required, is essential to surface preparation activities. In either case, whichever tools are used to clean and to roughen the substrate the inspector must view the results as two separate acceptance criteria as required by job specifications.

The challenge to industry and large organizations responsible for mitigating corrosion damage—such as the U.S. Navy—is how to qualify and transition new technologies into the hands of maintenance contractors. This is a multistep process that requires leadership and a commitment to innovation backed-up by funding, along with collaboration among the stakeholders to break conventional paradigms and reduce duplicate efforts.

Testing, qualification, acceptance and use of new technologies to support surface preparation in the marine and transportation industries are often slowed due to uncoordinated efforts and the perpetuation of myths. This article will benchmark the progress made to advance the use of PLA as a new tool to modernize corrosion control management and increase the surface preparation technologies available—including efforts to implement PLA methods on Navy ship structures, and the development and acceptance of a new AMPP laser ablation standard.

PLA Process

The PLA method of coatings and corrosion removal imparts a localized heat cycle (or cycles) to the underlying substrate. The amount of heat build-up in the substrate depends on many factors. These include the substrate material itself, the laser ablation tool options/variables (laser power, wavelength, spot size, pulse peak/duration, impact angle, stand-off distance, etc.), the removal process/procedure (translation rate, number of passes, etc.), and the coating system (or corrosion) to be removed. 

Ideally, the laser energy is completely absorbed by the target coating resulting in rapid vaporization and ejection from the substrate in a way that limits heat input to the base metal.  As a focused, high-energy input, a well set-up laser can create a very small heat affected zone, within a few microns of the surface, without significantly heating the bulk material. Some lasers can be set up for both cutting and welding as well as ablation and cleaning, so as with any technology, PLA must be used responsibly by trained personnel.

Therefore, prior to using PLA to treat sensitive substrates, it’s important to consider the specific performance requirements of the base metal and the possible adverse impact to sensitive material properties by the laser’s near surface heat-treatment. The risk of not doing so could be equivalent to selecting a less than optimal material in an engineering design process.

To mitigate the risk of deleterious impacts to the substrate it is necessary to adjust the process parameters of the laser ablation system. Laser ablation pro­cedures and process qualifications have
been developed with due consideration for controlling substrate temperatures and impacts to surface morphology.

With the advent of solid-state lasers, the technology has progressed to a very mature state. Solid-state lasers require minimal to no periodic alignments and the lasing modules are very small relative their early predecessors. What is still a difficult design challenge is the beam delivery package, consisting of laser optics, or end effectors, and fiber optic cables. The laser optic allows the laser ablation process to be used hand-held, by deflecting and rastering the laser beam energy into a desired pattern at the target surface (Fig. 1).

Fig. 1: Scanned laser ablation coating removal.

PLA removal rates rely primarily upon the laser system’s rated power output and operating parameters. Parameters include scan frequency, scan width (depicted by the arrows in Figure 1), and pulse repetition rate. In general, the fiber optic beam delivery and laser end effectors, are the limiting factors for increasing laser power. At higher laser powers, fiber optics, end effectors and many internal components must be able to withstand an additional thermal load without damage. From an engineering standpoint this is typically handled by increasing the size of system and its components. As such, the engineering dilemma to increase removal rates is designing a higher power tool without making it unwieldy.

Technology Transition Requirements

Requirements identified for laser ablation processes to transition to Navy preservation organizations were developed by cognizant Navy entities within Naval Sea Systems Command (NAVSEA) (Fig. 2). These included technical evaluation and qualification, training, environmental safety, process management and human safety.

Fig. 2: Laser ablation implantation roadmap for U.S. DoD, Navy.

The primary concerns to the surface preparation and preservation industry include deleterious metallurgical impacts on fatigue, structural strength and adhesion of subsequently applied protective coatings. Areas evaluated for technical qualification included weldability, coating adhesion, metallic substrate tensile strength, substrate stress corrosion cracking and high cycle fatigue. Previous work completed by the Virginia Transportation Research Council and the University of Virginia found no detrimental effects on the mechanical properties of ASTM A36 steel and that fatigue debit (i.e., loss of strength) was on parity with expected values. However, testing of HY-80 steel—the primary structural steels in areas of the greatest stress once the ship is underway—identified fatigue debit.⁴ Completion of initial transition requirements and consideration of the fatigue testing resulted in limited uses of PLA aboard surface ships.

These transitional requirements and limitations were outlined in a NAVSEA approval letter, but did not wholly enable implementation at various maintenance activities. Activity specific requirements (i.e., Naval shipyards) differ between activity as well as parent organization (i.e., NAVSEA 04 Industrial Maintenance Operations and NAVSEA 21 Commander Naval Regional Maintenance Centers). These differences were manifested in the different business models for planning and completion of depot and intermediate level maintenance. They also consisted of training support organization, issuance of activity process instructions, tailoring of operating procedures, training and implementation within engineering and maintenance planning groups, appropriate safety program instantiation, and operating permits. Due to the function of advanced laser systems and the new stricter DoD cybersecurity requirements, most, if not all, laser ablation units will require platform-specific approval for use.

Assessment of the laser systems available and how to address the primary concern of fatigue debit requires research and detailed testing. The trade-off is between what is sufficient coating removal, how this correlates to fatigue debit observed in some steels, and with laser beam dwell time on the base metal (strip rate). The correlation between energy imparted into the substrate as a function of laser dwell time (number of passes and energy per pulse) and fatigue debit indicates that for HY-80 steel, and by inference other HY and HSLA steels with similar constituent concentrations, minimization of energy input through reduction of the number of laser passes is desired. Adhesion testing was then conducted to evaluate re-coating adhesion properties of partially ablated surfaces. The anticipated result would be reduced fatigue debit by a decreased amount of laser dwell time, thereby reducing labor while maintaining coating adhesion.

While PLA technology is another tool in the surface preparation toolbox, it is not a direct replacement for every job previously accomplished by abrasive blasting. Evaluating surface preparation challenges, and the capabilities and limitations of the tools available for a specific application, is the first step to consider the potential process improvements PLA can offer and how it may be implemented.

Quantifying indirect costs associated with setup and cleanup of conventional surface preparation technologies and the current direct costs of de-paint activities is essential to building PLA use cases to support the business model of preservation activities. The business models may then be constructed, and individual use cases evaluated, to determine the most cost-effective applications for PLA technology. Variables to be consider include:

• If ultra-high-solids content coatings or  high-solids coatings are being removed (directly relates to strip rates);
• The location/application on the substrate;
• The criticality of the coating and nature of the substrate; and
• Whether the structure is owner-operated or contractor serviced and what that means to policy.

Roadblocks

Two significant roadblocks to completing the transition became evident upon issuance of limited approval. The first was the need for inclusion of PLA in policy and guidance. Without guidance, there is always a reluctance to implement new technology along with the level of effort to work through waivers or departures from specification precluded integration of PLA into preservation planning activities.

Additionally, the absence of an inspection standard for PLA presents a roadblock. Without an inspection standard, coatings inspectors relied on understanding of the definition of the Near-White Metal Blast Cleaning standard (SSPC-SP 10/NACE No. 2). The paradigm of applying current practices that don’t fit new technologies, such as applying SP 10 descriptions to PLA-treated surfaces, or the prescription of a “95% clean” surface, has prevented an understanding how PLA may be applied and what the desired end state or description of the PLA treated surface should look like.

Implementation of new technologies requires the modification of policy and application of the new policy to job planning. Current standards and practices for surface preparation of naval ships are defined by the Naval Sea Systems Command Standard Items (NSI) section 009-32 and the Naval Ship’s Technical Manual (NSTM) Chapter 631. Different processes require different views of how PLA is to be used while addressing structural and coating adhesion concerns. Standards associated with abrasive blasting cannot be used to assess an acceptable end state resulting from the use of laser ablation.

Therefore, review of applicable policy and guidance documents to determine extent of change or creation of new guidance required to support that change is needed. A plan to determine responsibility, change management requirements, and how the changes are to be implemented is also necessary. These plans must include allocation of resources to support non-governmental organizations such as AMPP to develop standards and guides.

Solution: New PLA Standard, Guides

Development of an AMPP standard was seen as critical to removing roadblocks to operational use of PLA for surface preparation and preservation. In fact, an SSPC Technology Update was underway for several non-mechanical surface preparation technologies even prior to the NACE/SSPC merger and the creation of AMPP.

Working with AMPP and industry partners outside of DoD control to prepare a new standard was initiated in 2021. The new standard, AMPP SP 21511, “Laser Ablation for Surface Preparation of Ferrous Metals, Pulsed Laser,” approved in August 2024, “is intended to support specifiers and end users achieve a defined level of surface cleanliness specifically using pulsed laser ablation (PLA), also referred to as laser ablation coating removal (LACR), hereafter PLA, for coated or uncoated ferrous metal substrates prior to the application of a protective coating, lining and other suitable surface preparation applications. For the purpose of this standard, PLA consists of pulsed, Q-switched laser technology.”

Specifically, the “standard provides guidance to specifiers and operators of laser ablation equipment for the purpose of surface preparation as well as inspectors on the general appearance of laser cleaned surfaces.” The initial path forward in drafting and taking the new standard to vote required the need to educate the audience on the new technology. Because of this need, too much
of the initial draft was anecdotal.

There was also an intentional effort to use terms familiar to coatings and preservation professionals, but this kept them operating within the paradigm of SP 10.  It was therefore determined that the creation of a technical guide was necessary.  The content of the resultant standard was then focused on the application of PLA. Some of the terminology was the same, but there was a purposeful intent to move toward independent descriptions of the desired end state.

In addition to the AMPP standard, AMPP Guide 21611, “Pulsed Laser Ablation Technical Guide for Ferrous Metal Substrates,” was initiated in January 2023 with the intent to provide explanation of the descriptions of the desired PLA end states and to provide guidance for use and was also approved in August 2024. Major sections of the guide include descriptions, definitions, considerations for use, workplace safety, byproducts and waste control, material testing and qualification, explanatory notes, and technical consideration and feasibility for use of laser ablation, environmental considerations and equipment and operating parameters.

Finally, the visual guide, AMPP Guide 21711, “Guide and Reference Photographs for Steel Surface Non-Mechanical Cleaning by Pulsed Laser Ablation,” also provides a visual depiction of the resultant surface prepared by PLA for follow on preservation. The original thought of how the desired end states would be depicted related to the correlation between the extent of PLA application and the resultant fatigue debit, i.e. beginning of substrate exposure, then 25%, 50%, 75%, and 95% of substrate exposed. However, variability in laser power and strip rates as a function of coating type change how the desired end state was defined. After initial sample preparation and application of lasers of various powers it was determined that prescriptive definitions for application would be difficult to attain based on coating type, condition, and the efficacy of the laser.

These subsequent end state conditions considered the following: abrasion (roughening of the coating surface), coating preparation for removal of aged or damaged coatings, bulk coating removal, and percent of substrate exposed. Base conditions and desired end-states were then redefined to support descriptions of the operational use.

The visual guide provides acceptable examples of surfaces laser ablated to the conditions described in Table 1. The technical guide provides unacceptable examples of the application of PLA. Prescription of these end states is dependent upon survey of coating conditions prior to developing maintenance and preservation scope. The visual guide has been prepared and has been referred to an AMPP committee for review and approval. Some photographic examples of PLA treated surfaces from the draft visual guide are provided in Figures 3, 4 and 5.

Fig. 3 (top): Aged intact coating prepared to PLA-SA. Fig. 4 (middle): Aged intact coating prepared to PLA-PA. Fig. 5 (bottom): Aged intact coating prepared to PLA-TA.

Conclusions, Next Steps

The next steps for use of PLA are to ensure remaining policy and guidance is updated to support decisions in procurement, program instantiation, and training for both operators and inspectors on application of the proposed standard definitions. Obtaining funding and a commitment to use PLA by organizational management is critical to building the toolbox to optimize critical resources in efforts to preserve our ships and infrastructure. Disparity in organization policy to enable use of maintenance and operation’s funds and caps on procurement amounts has prevented the purchasing of equipment by the organizations that need it the most. Leadership commitment to changing the status quo and supporting line management and planning activities in using the best tool for the job is critical to success. Continued barriers to internal acceptance of change include changing policy that allows use of the tool without departures and waivers and the training of engineers, inspectors, and planners on the technology. Misconceptions and understanding of PLA are perpetuated due to a lack of centralized capability management and communication across different components of the organization. The paradigm of conforming to the status quo represented by prescribed surface preparation requirements surrounding abrasive blasting has to change through leadership and a commitment to improve. Publication of the technical guide and the visual guide with training for operators and inspectors to recognize their individual part in the process of developing the Job Reference Standard will support proper execution of work documents as prescribed by engineers and planners.  Operational use of the laser ablation systems to determine the appropriate business cases that minimize indirect work and optimize laser firing time based on coating type will assist preservation planning activities in assigning the best tool for the job.

References

1. Corrosion Prevention and Control: A Program Management Guide for Selecting Materials, Spiral 2 (2nd Edition)
2. Institute of Corrosion, https://www.icorr.org.
3. Shamsujjoha, Md., Agnew, S.R., Melia, M.A,,  Brooks, J.M., Tyler, T.J., and Fitzgerald, J.M. “Effects of laser ablation coating removal (LACR) on a steel substrate: Part 1: Surface profile, microstructure, hardness, and adhesion.” Surface & Coatings Technology 281 (2015) 193–205.
4. Virginia Transportation Research Council, “Innovative Coating Removal Techniques for Coated Bridge Steel.” https://www.virginiadot.org/vtrc/main/online_reports/pdf/20-r1.pdf.

Acknowledgements

The authors would like to thank Adapt Laser Systems for their technical support in qualification and training requirement, Puget Sound Naval Shipyard for materials preparation, and Elzly Technology Corporation/KTA-Tator, Inc., for technical support.