Pre-filming agents (also called prefilming agents, pretreatment filming agents, or passivation agents) are specialized chemicals used primarily in industrial circulating cooling water systems, boilers, and heat exchangers. Their main purpose is to rapidly form a thin, uniform, protective film (often a passive oxide or inhibitor layer) on metal surfaces—especially carbon steel, copper alloys, and other materials—right after cleaning or during system startup/commissioning. This film helps prevent initial corrosion, under-deposit corrosion, and electrochemical attack while the system transitions to ongoing treatment with corrosion inhibitors, scale inhibitors, and biocides.
They are typically applied in a dedicated pretreatment phase (often after chemical cleaning) at elevated concentrations for a short period (hours to days), then the system switches to maintenance dosing. This is common in power plants, petrochemical facilities, refineries, HVAC, and manufacturing cooling systems.
Common Chemical Types and Products in the Pre-Filming Agent Category
Pre-filming agents are usually blended formulations rather than single pure chemicals. They combine several functional components for fast film formation, cleaning of residual rust/scale, dispersion, and compatibility with system metals. Key active ingredients include:
- Phosphate-based systems (most traditional and widely referenced):
- Polyphosphates or orthophosphates (e.g., sodium hexametaphosphate or similar) — Primary film-formers that promote protective oxide layers and remove corrosion products.
- Zinc salts (e.g., zinc sulfate or zinc chloride) — Often paired with phosphates at 30–60 ppm zinc and 300–600 ppm phosphate for rapid passivation on steel.
- Effective for carbon steel; helps form a stable film quickly.
- Phosphonate and polymer blends (low-phosphorus or phosphorus-free modern options):
- 2-Phosphonobutane-1,2,4-tricarboxylic acid (PBTC)
- Hydroxyphosphonoacetic acid (HPAA)
- 1-Hydroxyethane-1,1-diphosphonic acid (HEDP)
- Acrylic acid copolymers or terpolymers (e.g., acrylic hydroxypropyl acrylate terpolymer)
- Hydrolyzed polymaleic anhydride (HPMA)
- These provide scale inhibition alongside film-forming and are used in low-P formulations to meet environmental regulations.
- For copper alloys and multi-metal systems:
- Azoles such as benzotriazole (BZT), tolyltriazole (TTA), or methyl benzotriazole — Form protective films on copper surfaces to prevent corrosion and dezincification.
- Other supporting components:
- Complexing agents / chelators (e.g., sodium gluconate) — Help remove oxides and improve film uniformity.
- Dispersants and surfactants — Aid in cleaning residual deposits and ensure even film distribution.
- Zinc salts or molybdates in some blends for enhanced passivation.
Commercial examples or product types often include:
- Proprietary blends like JM-830 (high-efficiency pre-film agent), SGR-0902W (near-zero discharge type), or similar formulations from water treatment suppliers.
- Cleaning + pre-filming combination agents (sometimes called “cleaning pre-filming agents”) that perform both deposit removal and film formation in one step.
- Phosphorus-free or low-phosphorus variants designed for environmental compliance and wastewater reuse systems.
In boiler and steam systems, a related but distinct category is film-forming amines (FFAs) or filming amines (e.g., octadecylamine, ethoxylated soya amine). These create a hydrophobic monomolecular film to repel water and corrosives, but they are more commonly used for ongoing condensate line protection rather than initial pre-filming in open cooling systems.
Typical Application and Benefits
- Dosage: Shock or short-term high-dose application (e.g., circulated for 24–48 hours post-cleaning).
- Benefits: Fast film formation (hours instead of weeks), uniform dense protective layer, reduced initial corrosion rates, compatibility with various water qualities (including those with copper/iron ions), and improved long-term system performance when followed by standard treatment programs.
- Context in water treatment programs: These agents fit into a broader suite alongside scale & corrosion inhibitors, biofilm/sludge dispersants, biocides, and stripping/cleaning agents (the latter being more aggressive for removing existing slime or deposits, while pre-filming agents focus on protective layer creation after cleaning).
Pre-filming is especially recommended immediately after system cleaning to passivate fresh metal surfaces and avoid flash corrosion.