Secondary-loop internal corrosion control in data-center liquid cooling refers to the engineering measures that keep compatibility, electrochemical corrosion and deposition under control between the coolant (PG25/EG25 water-based fluid or dielectric fluorinated fluid) and the wetted metal surfaces (copper cold plates, aluminium brackets, stainless-steel piping, brazed solder joints, cast-iron pump bodies). The goal is to hold the closed secondary loop's water quality, inhibitor concentration and dissimilar-metal isolation inside an allowed window so that pitting, galvanic attack, biofilm (MIC) and particulate blockage do not raise cold-plate thermal resistance or cause leaks.
TL;DR — Design basis is GB/T 48023-2026 plus OCP secondary-side rules; the practical levers are coolant chemistry, water quality and metal isolation.
Key Figures and Thresholds
- GB/T 48023-2026 (data-center cold-plate liquid cooling) was issued 2026-07-30, effective 2027-02-01; coolant limits sit in normative Annex A.
- A typical secondary loop has 5 wetted metals: copper (T2/C11000), aluminium (6063/3003), stainless (304/316L), solder (Sn-Ag-Cu or Ag-free Sn-Bi/Cu-P), and ductile-iron or engineering-plastic pump/expand tank.
- PG25 = 25% propylene glycol + 75% deionised water; DI water conductivity ≤ 5 μS/cm, makeup ≤ 600 μS/cm, circulating ≤ 2000 μS/cm at pH 6.8–8.5.
- Static weight-loss limits per ASTM D1384-24 / SAE J1034 / ASTM D3306 (88 °C, 336 h): solder ≤ 30 mg/yr, aluminium ≤ 30 mg/yr, copper ≤ 90 mg/yr, cast iron ≤ 90 mg/yr, brass ≤ 100 mg/yr, steel ≤ 120 mg/yr.
- Cu–Al open-circuit potential is about 0.60–0.75 V vs SCE in 3.5% NaCl; with a large cathode (Cu) to small anode (Al) area ratio, localised pitting rate can be amplified 3–10×.
- Total viable count ≤ 1×10⁵ CFU/mL; treat at 1×10⁴ CFU/mL with an isothiazolinone (IT/CMIT) 100–200 ppm shock, then strip biofilm.
- Cold-plate micro-channel hydraulic diameter is 0.5–2.0 mm; keep NAS 1638 ≤ class 7 (about ISO 4406 15/13/10) — particles above 300 μm sharply raise blockage risk.
- Freeze/boil points for PG25: about −12 °C / about 104 °C.
Standards and Test Methods
GB/T 48023-2026 and the OCP baseline
GB/T 48023-2026 is the first national recommended standard to systematically cover the CDU, secondary piping, cold plate and quick-connects of cold-plate liquid cooling; normative Annex A lists secondary-side coolant requirements and test methods. The earlier industry baseline came from OCP Disaggregated Rack Cooling (DRC) and Side-Stand Liquid Cooling (SSLC), each specifying five control categories for secondary coolant: pH, conductivity, corrosion weight-loss, microbiology and particulates.
Material-compatibility testing
Coolant compatibility with metals follows ASTM D1384-24 (static glassware weight-loss, 88 °C, 168 h and 336 h coupons), judged against ASTM D3306 / SAE J1034 per-metal limits. Non-metals (EPDM/NBR/POM/PPS) are assessed by ASTM D471 volume- and hardness-change; thermal and oxidative stability by ASTM D5525 / D6580; foaming by GB/T 12579; antibacterial efficacy by ASTM E2180 / ISO 22196 with GB 4789.2 plate counts.
Electrochemistry and field monitoring
Galvanic pairs Cu–Al, Cu–stainless and steel–stainless are ranked with ASTM G71 (galvanic series) and assessed for pitting potential with ASTM G59 (potentiodynamic polarisation). Field monitoring uses conductivity (online), pH (weekly), ORP (daily), microbial count (monthly) and corrosion coupons (quarterly). Fit a 0.2 μm membrane filter and ion-exchange polishing resin on the CDU side to hold DI conductivity ≤ 5 μS/cm long term.
Coolant and Water Quality Limits
How to read the limits
Conductivity, pH and inhibitor reserve alkalinity are the three fast-moving indicators. A rising conductivity driven by Cl⁻/SO₄²⁻ points to ingress contamination, whereas rising dissolved Fe/Cu/Al points to active corrosion; the corrective action differs, so measure both alongside ORP.
| Parameter | Value / Unit | Source |
|---|---|---|
| DI water conductivity | ≤ 5 μS/cm | SGS data-center liquid-cooling guide, 2026-08 |
| Makeup water conductivity | ≤ 600 μS/cm | GB 50050 / project data |
| Circulating water band | ≤ 2000 μS/cm at pH 6.8–8.5 | SGS loop-water management |
| Ca/Mg/Cl dissolved ions | ≤ 300 mg/L | Project water spec |
| Turbidity / suspended solids | ≤ 10 NTU / ≤ 20 mg/L | Project water spec |
| Total viable count | ≤ 1×10⁵ CFU/mL (treat at 1×10⁴) | GB 4789.2 / field |
| ASTM D1384 solder / aluminium | ≤ 30 mg/yr | SAE J1034 / ASTM D3306 |
| ASTM D1384 copper / cast iron | ≤ 90 mg/yr | SAE J1034 / ASTM D3306 |
| ASTM D1384 brass / steel | ≤ 100 / 120 mg/yr | SAE J1034 / ASTM D3306 |
| Cu–Al open-circuit potential | 0.60–0.75 V vs SCE (3.5% NaCl) | ASTM G71 galvanic series |
| PG25 freeze / boil | ≈ −12 °C / ≈ 104 °C | Coolant handbook |
| Micro-channel hydraulic diameter | 0.5–2.0 mm (mostly 0.8–1.2 mm) | OCP DRC / SSLC |
| Particulate NAS 1638 / ISO 4406 | NAS ≤ 7 / ISO ≈ 15/13/10 | Field acceptance |
Galvanic Corrosion and Area Effects
Choosing the galvanic pair
The pair that decides service life is copper cold plate against aluminium bracket / quick-connect shell, and copper against stainless piping. Copper (corrosion potential ≈ +0.32 V vs SCE) and aluminium (≈ −0.35 V) differ by about 0.67 V; with a continuous liquid path and no isolation, milliampere-level galvanic current flows even at 200–2000 μS/cm conductivity.
The area-ratio rule
A large cathode with a small anode is the worst ratio. Copper cold plate (cathode) against an aluminium quick-connect shell (anode, cathode:anode up to 15:1–30:1) can pit and perforate within 12–24 months. Three fixes: isolate Cu–Al with nylon/PEEK/EPDM washers; hard-anodise aluminium (20–25 μm) or coat with insulating epoxy powder; or move the shell to nickel-plated copper or 316L to match cold-plate potential.
Inhibitor Systems
- Silicate type: friendly to aluminium and cast iron, films fast, but weak on copper and prone to scale in hot hard water; typical dose SiO₂ 800–1500 ppm.
- Organic-acid type (OAT): decanoate / 2-ethylhexanoate / suberate, thin film with little thermal penalty, compatible with Cu-Al-steel — the preferred route for cold plates.
- Hybrid OAT (carboxylate + molybdate): adds Na₂MoO₄ 500–800 ppm for cast-iron pitting margin in CDU pump circuits.
- Always pair the backbone with a copper pasivator such as benzotriazole (BTA) plus antifoam and a shock biocide.
Water-quality management rhythm
Inspect the CDU weekly; run a full lab panel monthly (pH, conductivity, turbidity, Cl⁻, SO₄²⁻, hardness, microbial count, reserve alkalinity); hang quarterly coupons (Cu, Al, steel, solder ×2 each); every six months check cold-plate ΔP and branch-flow deviation. Biocide is shock-dosed: IT/CMIT 100–200 ppm held 12–24 h, then a full change-out or activated-carbon/resin stripping. Nitrogen-blanket the expansion tank to 0.02–0.05 MPa(g) to keep dissolved oxygen below 3 mg/L.
Coating and Surface-Treatment Fit
Where coatings help — and where they do not
Do not rely on a coating on the wetted coolant surface: copper micro-channels are maintained by passivation and inhibitor, and any channel coating raises thermal resistance and, at pinholes, forms a large-cathode/small-anode cell. Real coating value lies on non-wetted parts — rack frame, aluminium brackets, external piping, expansion-tank shell and CDU enclosure — specified to ISO 12944 indoor low-corrosivity C1–C2.
Insulating the aluminium bracket
For aluminium brackets and quick-connect shells use hard anodising 20–25 μm with seal plus EPDM/PTFE insulating washers, or chrome-free conversion pretreatment plus 60–80 μm epoxy powder with coating volume resistivity ≥ 1×10¹² Ω·cm. The Cu cold-plate to Al-bracket interface must be electrically broken — mechanical fastening only.
Failure Modes and Criteria
Diagnostic logic
Read symptoms by metal and location: green-blue deposits at channel inlets signal copper pitting from Cl⁻; white flocculent residue at a Cu–Al joint signals galvanic attack of the aluminium; grey-black around a braze seam signals crevice attack under low pH; slimy film with rust blooms signals MIC; rising ΔP signals particulates; red rust and yellow water signal oxygen attack on steel.
| Failure | Visible sign | Threshold | Remedy |
|---|---|---|---|
| Copper pitting | channel-inlet pits, blue-green deposit | Cl⁻ > 50 mg/L or ORP > +400 mV | change fluid, carbon + resin polish, keep Cl⁻ < 20 mg/L |
| Aluminium galvanic attack | white residue at contact, local pits | Cu–Al potential > 0.5 V | add EPDM/PEEK washer, anodise 20–25 μm |
| Solder crevice attack | grey-black at quick-connect seam | pH 100 mg/L | raise pH to 8.0–8.5, switch to Ag-free Sn-Bi solder |
| Microbiologically influenced corrosion | slimy film, localised rust | count > 1×10⁴ CFU/mL | IT/CMIT 100–200 ppm shock + change-out |
| Particulate blockage | ΔP rise > 15% / flow dev > 10% | > 300 μm and NAS > 8 | add 0.2 μm filter + backwash |
| Oxygen attack | red rust on pump, yellow water | DO > 3 mg/L | N₂ blanket 0.02–0.05 MPa(g) |
Operation and Monitoring
Baseline instrumentation
Minimum viable monitoring: online conductivity and temperature, a weekly handheld pH/ORP, a monthly microbial dip-slide, and quarterly corrosion coupons compared to the ASTM D1384 weight-loss limits.
Change-out interval
Three to five years is typical if reserve alkalinity stays ≥ 6 mL and count stays below 1×10³ CFU/mL; halve the interval if reserve alkalinity drops below 3 mL or stabilised pH falls under 7.5.
FAQ
Can I just pour automotive antifreeze into a data-center loop?
No. Automotive coolant carries a high silicate/phosphate package (SiO₂ 2000–3000 ppm) that plugs micro-channels, and OAT versions include molybdate/borate that discolours copper long-term and lack benzotriazole for copper. Use a dedicated secondary coolant: PG25/EG25 plus carboxylate, BTA, antifoam and biocide.
Is high conductivity contamination or corrosion?
Judge with ORP and metal-ion content. Rising Cl⁻/SO₄²⁻ means ingress contamination (water, handling residue, cleaning agents); rising dissolved Fe/Cu/Al means corrosion dissolution. Contamination → fluid change + resin; corrosion → pH trim + top-up inhibitor + coupon recheck.
Can a copper cold plate connect straight to stainless piping?
Yes, if no aluminium is in the loop. Cu–stainless potential is about 0.2 V in 3.5% NaCl — a moderate pair usually controlled by BTA + carboxylate. If aluminium is added to form a Cu-Al-SS triangle, aluminium becomes the sacrificial pole and must be isolated.
Pure deionised water without inhibitor — is that safe?
No. Low conductivity only delays scale and current paths; DI water is a hungry solvent that keeps dissolving the copper/aluminium passive film, and within weeks dissolved metal returns conductivity to 200–500 μS/cm and accelerates galvanic attack. PG25/EG25 + carboxylate + BTA is the minimum formulation.
How often to fully change the coolant?
Every three to five years provided reserve alkalinity ≥ 6 mL and count < 1×10³ CFU/mL. Shorten to 18–24 months with resin polishing and pre-filming if reserve alkalinity falls below 3 mL or pH stabilises under 7.5.
Last updated: 2026-10-08 | Prepared by Kexin New Materials (Guangdong) Co., Ltd. For B2B technical reference only; trade terms EXW / FOB — no retail, DDP or overseas-warehouse quotation.