A 13Cr tubing string galled on the way into the hole has two outcomes: full breakout at surface and re-run, or a leaking connection at depth. Both are expensive. The first costs a rig day; the second costs a workover. The mechanism is predictable, and it is preventable — but only if the thread compound, running speed, and surface treatment are specified correctly before the job, not diagnosed after the first connection is broken out.

We have supplied L80-13Cr and Super 13Cr tubing strings to gas condensate wells in the Middle East, and galling is consistently the field complaint we track most closely on 13Cr strings. The failure mode is metallurgical, not mechanical — and the solution is in the supply specification, not the running procedure alone.

What Galling Is

Galling is adhesive wear caused by the cold welding and tearing of metal surfaces in sliding contact under high compressive load. It is distinct from abrasive wear (surface material removed by a harder particle) and corrosive wear (material lost through chemical attack). Galling requires:

  1. Intimate metal-to-metal contact — oxide film breakdown
  2. High contact stress — sufficient to fracture the protective film
  3. Sliding motion — relative movement between pin and box during makeup rotation

When all three are present simultaneously, fresh metal is exposed, the two surfaces bond momentarily, and continued rotation tears one or both surfaces. The torn metal accumulates as raised metallic debris on the thread flank — this is the characteristic appearance of galling.

Why 13Cr Is Susceptible

Free tool: Looking up casing OD, wall thickness, weight per metre, ID, or drift diameter? Casing & Tubing Size Lookup →
Spec reference: Casing and tubing collapse, burst, and pipe weight reference data per API 5C3. API 5C3 Spec Tables →

Carbon steel connections are less prone to galling because the passive film on carbon steel (FeO/Fe₂O₃) is comparatively soft and ductile — it deforms rather than shatters under contact stress, providing continuous protection during makeup. Thread compound fills in as the film is compressed.

L80-13Cr (minimum 12% Cr) carries a Cr₂O₃ passive oxide film that is significantly harder and more brittle. Under the contact stress at BTC thread engagement — which can exceed 500 MPa at the first two thread flanks — this oxide layer cracks and flakes rather than deforming. The crack exposes fresh 13Cr metal. Because L80-13Cr has a maximum hardness of HRC 23 (552–655 MPa yield range per API Specification 5CT, 11th Edition), both pin and box surfaces are at similar hardness. There is no softer sacrificial surface. Both sides cold-weld and tear simultaneously.

The additional driver on long tubing strings is heat. Thread friction during rotary makeup on a deep well generates significant local temperature rise at the thread flanks. Elevated temperature destabilizes the passive film further and increases adhesive contact between chromium-bearing surfaces.

The hardness of L80-13Cr — max HRC 23 — is identical to L80 Type 1 (carbon-manganese steel). The galling susceptibility difference is not from hardness but from oxide film chemistry. A buyer who sees L80-1 and L80-13Cr with the same HRC limit and assumes identical connection behaviour is missing the fundamental metallurgical difference.

Connection Types and Their Galling Risk

API BTC (Buttress Thread Coupling)

BTC is the most common connection on 13Cr casing and the one most frequently involved in galling incidents. The buttress profile uses an interference-style engagement that builds contact stress rapidly after pin entry. For carbon steel, this interference fit is a seal mechanism; for 13Cr, the contact stress ramps up before the thread compound film has fully established.

The highest galling risk on BTC is at the first full-contact threads — typically threads 2–5 from the pin end. Galling here is recoverable if caught immediately on the torque curve; galling that develops through multiple threads requires full breakout and re-inspection.

STC (Short Thread Coupling) on Tubing

STC has a less aggressive thread form with lower thread-to-thread interference than BTC. On small-diameter 13Cr tubing (1.900–2-3/8 inch), STC galling is less common than BTC. However, STC is not gas-tight and is inappropriate for gas condensate wells where the tubing must maintain positive seal integrity under thermal cycling.

Premium Connections

Premium connections with metal-to-metal seals introduce a distinct galling risk at the seal zone, which is separate from and often more damaging than galling on the threaded section. The seal zone relies on precision-controlled contact between a tapered pin seal surface and a box seal bore. The contact stress at the metal seal is deliberately higher than on thread flanks — this is what creates the gas-tight seal.

For 13Cr premium connections, the seal area is the most vulnerable location. Galling at the seal zone destroys the gas-tight function. The damage is not visible externally during makeup and cannot be confirmed by a short makeout test at ambient temperature. It typically manifests as a leak path under thermal cycling or pressure reversal downhole.

What we see when customers report 13Cr connection issues: The most common sequence is: (1) the customer ran 13Cr tubing on site using the same thread compound and running procedure as their previous carbon steel string, (2) the first few stands went on normally, (3) a high-torque event was observed partway down the string, (4) they continued to final torque, (5) the well leaked at a joint on initial production. When we review the running report, the high-torque event was the first galling indicator — a sign the oxide film had broken and galling had initiated. The correct response to any anomalous torque event on 13Cr is to stop and break out that connection, not continue to final torque.

Diagnosis — Identifying Galling in the Field

Visual inspection after breakout

Pull the connection apart and inspect both pin and box thread surfaces under good lighting. Galling appears as:

  • Bright metallic patches, 3–15 mm long, with a roughened surface that reflects light differently from the machined thread flank
  • Raised edges around the bright patch — metal has transferred from one surface and adheres to the thread flank
  • Fine seizing lines running along the thread helix — these indicate the connection was rotated after initial galling, extending the damage path
  • Polished zones on opposing thread surfaces that correspond to the bright patches — metal has been removed from one side and transferred to the other

Light galling (small isolated patches, no metal transfer across multiple threads) may be addressable at the connection shop. Severe galling (multiple threads affected, raised metal debris, damaged thread form, or any galling at the seal zone) requires scrapping the connection.

Torque curve interpretation

The torque-turn monitoring system, if used, provides an early warning of galling before the connection is fully made up. On a clean 13Cr makeup:

  • Torque increases smoothly through thread engagement
  • A steady shoulder develops as the pin seats
  • Final torque is within ±10% of the reference makeup torque

Galling produces:

  • Irregular torque spikes during rotation — the cold-welded patches break and reform, causing step-changes in torque
  • Premature torque shoulder — the torque rises to or beyond final torque before the connection reaches the correct makeup position
  • Torque drop after spike — metal tears off the surface, momentarily reducing friction before a new contact zone develops

A torque curve that deviates significantly from the reference graph is cause for immediate stop and breakout, regardless of final torque value.

Thread Compound Selection for 13Cr

The thread compound specification for 13Cr is the single most important preventive measure — more impactful than running speed or surface treatment alone.

Do not use:

  • Standard API Modified thread compound (typically zinc-based or lead-based metallic pigments) — zinc pigments can cause liquid metal embrittlement at HPHT temperatures on 13Cr; lead compounds are restricted in most export markets
  • Any compound with copper, aluminium, or zinc flake pigments — these are all higher-risk with chromium-bearing alloys
  • General-purpose pipe dope marketed for carbon steel without a specific 13Cr or CRA designation

Use instead:

  • Teflon (PTFE)-based or polymer-based thread compounds specifically qualified for CRA alloys
  • Products such as Jet-Lube 550-T, Bestolife 2000, or equivalent — check with the connection manufacturer for their qualification-tested product list
  • For sour service wells: verify the selected compound is not restricted under NACE MR0175 / ISO 15156-2. Some organometallic compounds are excluded from H₂S environments. This is a separate constraint from anti-galling performance.

Sour service and thread compound: In wells where H₂S is present, the thread compound must be qualified both for CRA anti-galling performance and for compatibility with the H₂S environment. A compound that is excellent for 13Cr anti-galling may contain organometallic components that are prohibited in H₂S service. Verify against the NACE MR0175 / ISO 15156 requirements before specifying the compound. The connection manufacturer can advise which products are on their qualified list for each service condition.

Surface Treatments That Reduce Galling Risk

Phosphate coating (Parkerizing)

Phosphate conversion coating — Parkerizing — applied to the 13Cr pin end is the most widely used and proven anti-galling surface treatment for OCTG connections. The phosphate layer converts the surface oxide to iron/zinc phosphate crystals that retain lubricant and provide a sacrificial surface during initial thread engagement. The coating does not change the dimensions of the connection or the calibrated thread form.

Phosphate coating is typically applied at the mill or connection shop before thread protectors are fitted. It is a routine step for premium 13Cr connections and should be requested explicitly for BTC connections as well. Specify "phosphate-coated pin" or "phosphatized thread surface" in the connection finishing requirements on the PO.

Copper plating and nickel plating

Electroplated copper or nickel on 13Cr threads provides excellent anti-galling protection for non-sour, moderate-temperature service. In sour wells or at HPHT temperatures, plating compatibility with NACE requirements must be verified. Copper plating is typically excluded from H₂S service.

Non-galling surface hardening (Nitriding, CrN coating)

For premium connections on Super 13Cr and higher-alloy CRA tubing, physical vapour deposition (PVD) hard coatings (CrN, TiN) provide superior anti-galling protection and maintain dimensional stability. These are more expensive than phosphate coating and are typically reserved for HPHT or aggressive-environment applications.

Running Procedure to Minimise Galling

Speed and lubrication are the two controllable variables during makeup.

Running speed:

  • Carbon steel BTC: 25–35 RPM is typical
  • 13Cr BTC: 10–20 RPM maximum
  • 13Cr premium connections: 10–15 RPM maximum
  • Any 13Cr connection in a deviated well (>30° inclination): 5–10 RPM, with continuous torque monitoring

Lubrication:

  • Apply the qualified 13Cr compound to both pin and box threads, not just the pin
  • Apply to the shoulder area for connections with a torque shoulder
  • Do not use petroleum-based greases as a substitute for the specified compound — these are insufficient lubricants for high-contact-stress thread engagement

Make-up verification:

  • Monitor torque and turns from pin entry through final makeup
  • Any torque event that exceeds the reference curve by more than 15% warrants a stop
  • Do not back up a connection that has started to shoulder — full breakout and re-inspection is required

When NOT to Use 13Cr Connections (As Specified for Carbon Steel)

13Cr connections used without 13Cr-specific precautions will experience elevated galling risk. Do not proceed with standard carbon steel connection procedures in these conditions:

  • Using standard API Modified thread compound without CRA qualification — the most common preventable cause of 13Cr galling in the field
  • Running at carbon steel makeup speed — 25–35 RPM is too fast for 13Cr; reduce to ≤20 RPM
  • Specifying phosphate-uncoated pin — the default mill supply may not include phosphate coating; request it explicitly
  • Re-running a 13Cr connection that showed a torque anomaly without breakout and re-inspection — the damage from the first galling event propagates rapidly on the re-run
  • Using BTC on 13Cr tubing in a high-angle well (>45° inclination) — bending loads at thread flanks in deviated wells significantly increase contact stress; specify premium connections from the well design stage, not as a remediation after galling has occurred

Purchase Order Guidance

Connection specification for 13Cr

A complete 13Cr connection specification on the PO should include:

  1. Grade and type: "L80-13Cr per API Specification 5CT, 11th Edition"
  2. Connection: BTC per API 5B, or connection series name for premium (e.g., ZC LC-57)
  3. Surface treatment: "phosphate-coated pin" (not optional for 13Cr)
  4. Thread compound: name the specific product — do not write "suitable thread compound"
  5. MTC: EN 10204 3.1 minimum; 3.2 (third-party witnessed) for gas wells and sour service
  6. Thread inspection: gauge all threads per API 5B; 100% inspection on premium connections

Procurement trap

Wrong: "L80-13Cr casing, BTC connection, API Modified thread compound."

What the mill ships: Compliant L80-13Cr BTC casing with standard zinc-based API Modified compound — exactly what was ordered. The mill has no obligation to flag the compound incompatibility.

Right: "L80-13Cr casing, BTC connection, phosphate-coated pin, Jet-Lube 550-T or equivalent CRA-qualified non-metallic thread compound."

The connection manufacturer's recommendation takes precedence over a generic product name. If you are specifying ZC premium connections on 13Cr, the qualification documentation specifies the approved compound. Anything outside that list requires a formal substitution review.

For 13Cr properties and service limits, see the API 5CT specification tables → and the Sour Service Grade Selector →

For premium connection makeup torque guidance, see the Premium Connection Make-Up Torque and Running Guide →

Frequently Asked Questions

Why do 13Cr connections gall more easily than carbon steel connections?

13Cr martensitic stainless steel carries a Cr₂O₃ passive oxide film that is harder and more brittle than the FeO/Fe₂O₃ film on carbon steel. Under thread makeup contact stress, this oxide layer fractures rather than deforming plastically. When the protective film ruptures, bare metal contacts bare metal at the thread flank — both surfaces are similar in hardness (max HRC 23 for L80-13Cr), so neither acts as a sacrificial surface. Cold welding of the fresh metal surfaces initiates the galling failure.

What does galled thread look like in the field?

Galling produces bright, torn metallic patches on the thread flanks and crest surfaces, typically 3–12 mm in length, with raised edges where metal has transferred from one surface to the other. On BTC connections the galling is most visible at the first two engaged threads from the pin face. On premium connections, galling at the metal-to-metal seal area appears as a circumferential scratch or gouge rather than discrete patches, and the torque curve during makeup shows an abrupt early shoulder rather than a smooth progression.

Which thread compound should be used with 13Cr?

Use a non-metallic or CRA-compatible thread compound formulated without lead, zinc, or copper pigments. Products commonly specified for 13Cr include Jet-Lube 550-T, Bestolife 2000, and similar Teflon or PTFE-based compounds. Standard API Modified thread compound (zinc-based or lead-based formulations) is not recommended for 13Cr because zinc can cause liquid metal embrittlement at elevated HPHT temperatures, and lead compounds are restricted in most export markets.

Can a galled 13Cr connection be repaired and reused?

A lightly galled connection — small pickup marks, no metal transfer across thread flanks — can sometimes be cleaned and re-inspected at the connection shop. The thread flanks must be stoned to remove raised metal, a new Drift gauge pass and thread profile gauge inspection must be completed, and the surface treatment (typically phosphate coating) must be reapplied. A heavily galled connection with torn metal, deformed thread form, or galling at the metal-to-metal seal zone cannot be repaired and must be scrapped.

At what rotary speed should 13Cr connections be made up?

13Cr casing and tubing connections should be made up at 10–20 RPM, significantly slower than the 25–35 RPM typical for carbon steel casing. The reduced speed limits frictional heat generation at thread flanks, which is the primary driver of oxide film breakdown and subsequent galling. For premium 13Cr connections, reduce to 10–15 RPM and monitor the torque turn curve from pin entry through final makeup.

Is galling a problem with API BTC connections on 13Cr?

Yes, and BTC on 13Cr is higher risk than BTC on carbon steel for two reasons: the oxide film behavior described above, and the BTC interference fit. BTC uses an interference-style thread engagement that generates high flank contact stress at the final makeup position — exactly the condition that breaks down the passive film on 13Cr. For tubing sizes (1.900–4-1/2 inch), STC connections are somewhat less prone to galling than BTC because of lower thread interference, but premium connections with controlled-surface metal-to-metal seals and anti-galling treatment are the preferred solution for 13Cr gas tubing strings.

What surface treatment prevents galling on 13Cr connections?

Phosphate coating (phosphatizing or Parkerizing) applied to the pin end of 13Cr connections is the most effective field-proven anti-galling surface treatment. The phosphate layer acts as a conversion coating that retains thread compound lubricant and provides a sacrificial surface during the initial thread engagement. It does not change the dimensional properties of the connection. Request this treatment explicitly on the PO for 13Cr connections: specify 'phosphate-coated pin' or 'Parkerized thread surface' in the connection finishing requirements.

Does galling affect the pressure integrity of a connection?

It depends on where the galling occurs and its severity. Galling on thread flanks, if light, may not affect the leak path of a BTC connection that relies on thread compound for sealing. Galling at the metal-to-metal seal zone of a premium connection destroys the sealing mechanism — the seal depends on precise metal-to-metal contact, and any material pickup or tearing in the seal area creates a leak path that cannot be reliably detected by a short-duration makeup test.