Let’s Talk About Surface Preparation of Steel by Laser Ablation

KTA’s Certified Coating Inspector Forum Volume 3, Issue No. 10 – October 2024

William Corbett, COO

AMPP Senior Certified Coating Inspector & Certified Protective Coating Specialist

KTA’s Certified Coating Inspector Forum is designed to provide professional development/continuing education on standards, inspection practices, new instruments, and other topics to help keep certified AMPP and FROSIO coating inspectors current. It represents the views of the author and KTA-Tator, Inc. It may or may not represent the views of AMPP: The Association for Materials Protection & Performance, even though SSPC, NACE, and AMPP standards are frequently referenced in the content.

Introduction

In August 2024 AMPP published two new documents focused on preparation of steel surfaces (coated and uncoated) using pulsed laser ablation, or PLA. The first of two documents is a Technical Guide (AMPP Guide 21611[1]) intended to help owners and specifiers decide whether PLA is suitable for their project. The second document is a Standard Practice (AMPP SP21511-1[2]) that can be invoked by a project specification when it is determined that PLA is an appropriate method of surface preparation. Both documents are available from the AMPP website (www.ampp.org). Given that this Forum is primarily written for coating inspectors, we will focus on the content of the Standard Practice and highlight a few interesting tables that are presented in the Technical Guide. If you prepare coating specifications and want to consider implementing pulsed laser ablation, I encourage you to obtain a copy of the Guide to learn about pulsed laser system feasibility, benefits, and limitations (relative to more traditional methods of surface preparation), to make an informed decision on whether to implement this technology on your project.

The two images in this article are credited to Matt Binsfield – NAVSEA. They were included in his AMPP 2024 presentation, “Implementation of Pulsed Laser Ablation Technology in Surface Preparation Processes,” which was approved for public release with unlimited distribution.

Background

Laser (Light Amplification by Stimulated Emission of Radiation) ablation of protective coatings is not new technology. In fact, lasers have been used to remove coatings and clean surfaces for at least 10 years. However, until recently there has been limited effort to formalize these processes and quality requirements into an industry standard.

PLA (Pulsed Laser Ablation) employs portable or stationary pulsed, Q-switched, Class 4 laser technology. It does not use any media (like abrasive blast cleaning or power tool cleaning) to remove coatings, oxides, and contaminants, but rather a high intensity, pulsed, focused laser beam. It is frequently referred to as Laser Ablation Coating Removal, or LACR. It not only removes materials from the surface but can also be used to prepare uncoated steel substrates. One drawback is that it will not productively remove intact mill scale. If mill scale needs to be removed, LACR will need to be used in conjunction with traditional methods of surface preparation. While the scope of this Standard Practice is the preparation of ferrous metals, a separate Standard Practice for the preparation non-ferrous metals by PLA (SP21511-2) is planned, as are Standard Practices for composite substrates and all metals (SP21511-3 and SP21511-4, respectively) using another technology known as Continuous Wave (CW) lasers.

Overview of AMPP SP21511-1

AMPP SP21511-1 contains six sections and one appendix composed of explanatory notes. The content of the Appendix is nonmandatory unless invoked by a project specification.

Section 1 contains the scope of the Standard Practice and Section 2 contains a list of 24 definitions of terms used throughout the document.

Section 3 of the Standard Practice contains the Technical Requirements, including a table listing five levels of surface preparation that are summarized below. Note that the table in the Standard Practice provides more detail than this summary. The end condition is based on the degree of ablation specified.

Obtaining Clarification of Cleanliness Levels

Recognizing that AMPP SP21511-1 is a new standard and that visual guides do not yet exist for PLA, it is important to gain clarification on the specific requirements and expectations of the specifier prior to starting work. Fortunately, the standard does require preparation of a Job Reference Standard (JRS) that may help clarify some potential ambiguities, but the written definition takes precedence over the JRS. Here are a few examples of some questions that may arise, depending on the degree of preparation required.

1. If the specification simply requires that the surface be prepared according to SP21511-1-2024, it is incomplete.  The specific degree of cleaning required must also be specified (e.g., PLA-TA, PLA-PA, etc.). 
2. If PLA-TA (Through Ablation) is invoked by the specification, all tightly adhering foreign matter must be removed, which may infer that loosely adhering foreign matter can remain. The intent is that all foreign matter (loosely and tightly adhering) be removed, but it is important to make sure all parties understand the intent. Also, the definition of PLA-TA does not address removal of mill scale other than allowing stains of mill scale to remain. This implies that the mill scale has been removed, however as stated later in the standard lasers won’t remove tight mill scale. While the Appendix states that PLA can remove loose mill scale, Section 4 states that poorly adhered mill scale should be removed prior to PLA using other methods. When intact mill scale must be removed, PLA will need to be supplemented by mechanical means of cleaning such as abrasive blast cleaning or power tool cleaning. PLA alone will not productively remove it.
3. If PLA-PA (Partial Ablation) is invoked by the specification, it is important to gain clarification on what is intended by “exposed substrate.” For example, the definition states that up to 75% of the substrate can be exposed, which suggests that a contractor can perform PLA-PA and not expose any substrate, but that’s likely not the intent. The definition also states that the exposed substrate can have randomly dispersed stains of rust, tightly adhered thin coating, or other tightly adhered matter. If material other than stains are present, then should the substrate in those areas be exempt from the maximum 75% restriction?
4. Excerpts a-e, below show variations in the types of visible material that must be removed from the surface for each of the degrees of cleaning.  The differences between each could potentially lead to some confusion that should be clarified. For example, the removal of visible salts is only mentioned in PLA-AC. Does this mean that visible salts can remain for the other methods? Similarly, removal of chalking is required in PLA-SSA and PLA-SA. Since chalking is not specifically addressed in PLA-PA or PLA-AC, can it remain? Because of these and other differences, clarification on expectations for removal of visible material should be obtained in advance.
   • PLA-TA “…free of all visible oil, grease, dirt, rust, coating, and other foreign matter.”
   • PLA-PA “…free of all visible oil, grease, dirt, and rust.”
   • PLA-SSA “…retained coating without chalking or residual corrosion staining.”
   • PLA-SA “…free of all visible oil, grease, dirt, dust, chalking, or residual corrosion staining.”
   • PLA-AC “…free of all visible oil, grease, dirt, dust, salts, and other foreign matter.”

A final comment regarding salts. Salts are often non-visible, so if salts are of concern, verify that the project specification includes requirements for remediation, surface soluble salt testing, and corresponding acceptance thresholds.

Section 4 of the Standard Practice itemizes the requirements before laser ablation is conducted, and includes:

• Environmental Permitting.
• DFT survey of the existing coating where a certain amount will remain. The data is used to help determine the thickness to be removed by the laser.
• Laser Safety (operator and supervisor training, fire protection, worker and adjacent personnel safety, use of fume extractors as an engineering control, etc.).
• Precleaning, including removing heavy deposits of surface contaminants, pack rust, rust scale, and mill scale. Appendix A6 states that rust scale must be removed before PLA using mechanical methods. Appendix A7 states that poorly adhered mill scale can be removed by PLA, but tightly adhered mill scale must be removed by mechanical cleaning.
• Preparation of a Job Reference Standard (JRS) illustrating the required level of cleanliness and roughness (preserved for the duration of the project). I can’t overemphasize the importance of the JRS. The JRS will visibly demonstrate the expectations of the specified degree of cleaning.

Section 5 of the Standard Practice lists the various operating requirements including:

• Procedures for use of hand-held PLA systems, including use of an appropriate angle of incidence to maintain efficiency without reflecting the beam to other operators or adjacent personnel that may be unprotected.
• Use of clean, dry compressed air as tested according to practice ASTM D4285[3].
• A work area illuminated to a minimum of approximately 50 foot-candles.

Section 5 also lists potential applications of PLA, including:

• Removal of all coatings, or a percentage of the substrate exposed with remaining coatings tightly adhered.
• Partial or full removal of the topcoat, prior to application of a new topcoat
• Roughening of existing coatings for subsequent overcoating.
• Coating repair, including inspection of remaining coatings with a dull putty knife and subsequent feathering into adjacent sound coating.

Section 6 addresses post-PLA inspections to be performed prior to coating application, which include:

• Inspection of any retained coatings for evidence of defects such as cracking, blistering, chalking, delamination, and for surface characteristics such as uniform roughening.
• Surface cleanliness (where the coating is removed) and adequate roughening and adhesion of any remaining coatings.
• Removal of dust and surface residue.
• Management of the waste stream.
• Removal of surface imperfections (sharp edges, spatter, weld slag, laminations. etc.), and subsequent re-roughening.

AMPP Guide 21611 Highlights

AMPP Guide 21611 contains considerable text and eight tables designed to help a specifier decide whether PLA is a viable alternative for a given project. Three tables (Tables, 3, 4, and 5) are particularly useful. Table 3 compares PLA to ultra-high pressure water jetting, abrasive blast cleaning, power tool cleaning, and chemical stripping across 17 different attributes that span subjects ranging from productivity to surface quality, safety, waste generation, and others. Table 4 lists various substrates and whether they are compatible with PLA, and Table 5 lists 24 materials and whether they are suitable for removal by PLA.

Summary

Surface preparation of steel can be performed using traditional methods such as hand and power tool cleaning, wet and dry abrasive blast cleaning, water jetting and chemical stripping, or using non-traditional methods that incorporate abrasives like dry ice, ice, sodium bicarbonate, pliant media (sponge), and others. Pulsed Laser Ablation or PLA is another option for non-traditional methods. It incorporates a high energy laser to remove coatings and surface contaminants or prepare new steel for coating. Recently published AMPP Guide 21611 helps specifiers decide whether PLA is an appropriate method to specify, and Standard Practice SP21511-1 helps determine whether the prepared surfaces conform to the requirements.

[1] Pulsed Laser Ablation Technical Guide for Ferrous Metal Substrates

[2] Laser Ablation for Surface Preparation of Ferrous Metals, Pulsed Laser

[3] Standard Practice for Indicating Oil or Water in Compressed Air