Shot peening is one of the most tightly “controlled” surface enhancement processes on paper—intensity, coverage, media, equipment checks. Yet many of the problems that burn time, create escapes, or show up as corrosion findings do not come from the core peening parameters. They come from what the specifications do not clearly state (or do not clearly require on the drawing).

Most specifications correctly define how intensity is derived and verified (Almen strips, saturation curves, etc.). SAE J443 explicitly addresses the determination and verification of peening intensity.

The gap lies in translating this general method to an actual part:

• Where should intensity be verified on a complex geometry?

• Which surfaces are considered “representative”?

• What happens when a drawing shows multiple peen zones but provides no verification scheme?

In practice, this often becomes tribal knowledge. Different shops select different “representative” locations. Even within a single shop, programmers may choose convenient locations that do not reflect worst-case stand-off distance, impact angle, shadowing, or nozzle accessibility. The result is predictable: verification passes, yet critical areas are still under-peened or over-peened.

As a supplier of shot peening services, this issue has been a source of frustration for decades. I have spoken at numerous SAE and Nadcap meetings over the years on this very topic. When quoting a job without being informed of the required intensity verification locations, suppliers are forced to make educated assumptions. Each verification location carries a cost.

For example, if one supplier assumes five intensity verification locations are required to qualify a part, while another assumes ten, the supplier quoting five locations will likely be awarded the job due to lower cost. From an engineering standpoint, however, do five locations adequately represent the part?

What if more than one supplier is processing the same part—should the number and position of verification locations not be standardized? And what if field data later show that parts from one supplier exhibit substantially lower fatigue life than those from another? Would it not be valuable to compare equivalent, matching verification data if shot peening were suspected to be the root cause?

• Require the drawing (or a controlled peening map) to define:

◦ peen zones (Zone A, B, C, etc.)

◦ target intensity for each zone

◦ verification location(s) for each zone (or the agreed representative coupon or fixture)

any “worst-case” access constraints that must be represented

Add language such as:

• “Verification shall be performed at a location that matches the part surface condition for stand-off distance, impingement angle, and access constraints for each defined peen zone.”

If mini-strips, subscale holders, sacrificial coupons, or dedicated fixtures are acceptable, state this explicitly and require correlation and validation.

Many specifications mention cleaning, but the requirements are frequently too generic to prevent:

• embedded contamination

• corrosion initiation under residues

• coating adhesion issues after peening

• variability between suppliers (“our standard wash”)

AMS 2430 sample language also notes that post-peening cleaning instructions may be required, including the removal of iron contamination where applicable.

This is not “wrong,” but in the reality of modern manufacturing, it is often incomplete without:

• the cleaning method class (aqueous alkaline, nitric acid, solvent wipe, ultrasonic, etc.)

• compatibility constraints (alloy, heat-treat condition, coatings)

• acceptance criteria (residue limits, water-break testing, ionic contamination limits, etc.)

• timing requirements (how soon after peening preservation must be performed)

Examples of requirements that offer little actual guidance:

• “Pre-peen cleaning shall remove oils and coolants and leave surfaces free of residues that inhibit coverage or promote corrosion.”

• “Post-peen cleaning shall remove loose media, dust, and masking residues without attacking the base material or coatings.”

Even simple acceptance criteria reduce disputes:

• visual inspection plus wipe testing for residue

• a water-break-free requirement (where compatible)

• “No adhesive residue permitted” (directly tied to masking controls, as addressed below)

Add a maximum time-to-protect requirement for corrosion-sensitive parts:

• “Parts shall be dried and preserved within X hours of final rinse or cleaning.”

This is the issue that quietly produces some of the most severe field findings.

Specifications commonly state which areas shall be masked, but they often do not specify:

• what types of masking materials are permitted to contact the part

• whether adhesive contact is allowed on bare metal or critical surfaces

• how long masking materials may remain in place

• how residue removal is performed and verified

Why it matters: Adhesives and tape systems can trap moisture and chemicals, creating crevice conditions. Crevice corrosion is a well-documented mechanism for alloys that rely on passive films (such as stainless steels and aluminum alloys), particularly when an oxygen-depleted crevice forms beneath a poorly bonded “seal.”

Separately, industry safety case studies have shown corrosion and cracking initiating beneath adhesive tape when chlorides are present—a classic failure mode for stainless steels at elevated temperatures.

Even when the adhesive itself is not inherently “corrosive,” the combination of trapped moisture, chloride-bearing contaminants, cleaning chemical residues, and time can be sufficient to initiate an attack.

A binary rule is required for each surface type:

• “Adhesive-backed masking is prohibited on all bare ferrous surfaces, shot-peened critical surfaces, and corrosion-sensitive alloys unless specifically approved.”

• or “Adhesive contact is permitted only if the tape meets defined requirements, and residue removal and verification are performed.”

Add requirements such as:

• low ionic content, low halogen, and low sulfur levels (especially for stainless steels and high-strength steels)

• temperature compatibility (no adhesive bake-on)

• clean removability (no residue) and a validated removal method

a maximum allowable dwell time on the part

Identify preferred methods such as:

• metal or polymer shields or boots

• mechanical clamps or fixtures

• approved peelable stop-off coatings, where compatible

“After mask removal and cleaning, surfaces shall be free of visible residue and shall pass the specified cleanliness verification method.”

Considerable thought must be given to this by the primes. If an aerospace prime specifies a particular brand of tape—something that is already occurring on a limited scale—it would create significant challenges across the shot peening industry.

Our company processes parts for dozens of primes, and it is easy to see how this would quickly become a logistical nightmare if unique tape were required for each customer. Suppliers must be able to deliver compliant parts while keeping processes as lean and efficient as possible.

Suggested drawing note (intensity verification flow-down)

SHOT PEENING: Peen per [SPEC]. Zones and intensities as shown. Intensity verification shall be performed at the verification locations identified on this drawing or peen map for each zone. Any alternate verification method or alternate location shall require engineering approval.

Suggested drawing note (cleaning)

CLEANING: Pre- and post-peen cleaning shall be performed per [SPEC/PROCESS] using a method compatible with the material and coatings. The post-peen condition shall be free of media dust, loose debris, and masking residue. Preserve within [X] hours after final cleaning and drying.

Suggested drawing note (masking / adhesive control)

MASKING: Mask as required to protect non-peened areas. Tape shall be low-residue and controlled for ionic and halogen contamination. Remove masking and clean per an approved procedure, and verify that no residue remains.

If shot peening is expected to be repeatable across suppliers and over time, the “edges” cannot be left undefined. Intensity verification, verification location flow-down, cleaning definition, and masking and adhesive controls are where much of the real risk resides—particularly corrosion risk that only appears after the peening certification package looks flawless.

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