Phosphated
/glossary/phosphatedSurface treated with phosphate coating for corrosion resistance and lubricant retention.
A phosphated tube is steel whose surface has been converted to a zinc or manganese phosphate layer, then usually oiled. Kanpur hydraulic shops in Dada Nagar use it for storage rust control, paint adhesion, and drawing lubricant retention. IS 3618:1966 (reaffirmed 2021) covers five classes of phosphate treatment on iron and steel. It is not hot-dip zinc under IS 4736. Quotes from RP Sales are ex-warehouse Kanpur, GST 18% extra as the budgeting basis; the tax invoice applies the rate in force.
In detail
Phosphating is a chemical surface treatment that converts steel surface into an insoluble phosphate coating (typically manganese or zinc phosphate). The crystalline coating provides corrosion protection during storage, improves paint adhesion, and retains lubricating oil for subsequent forming operations. Phosphated tubes are specified for hydraulic systems (oil retention), automotive components, and as a pretreatment before painting or powder coating. The process follows cleaning and acid pickling in the surface preparation sequence.
What phosphating is, and what it is not
Phosphating is a conversion coating. The steel surface reacts with a phosphate solution and grows an insoluble crystalline layer, typically zinc phosphate or manganese phosphate. The layer is then oiled for storage. It is not a metal overlay poured onto the tube. It is not paint, not electro-zinc, and not hot-dip galvanising.
IS 3618:1966, Specification for Phosphate Treatment of Iron and Steel for Protection Against Corrosion, was reaffirmed in 2021 (BIS preview, retrieved 2026-09-03). Scope 1.1 covers processing of iron and steel to produce coatings consisting essentially of metal phosphates intended for use with organic coatings against corrosion. It also covers methods of testing finished coatings. It covers five classes of phosphating processes, un-accelerated and accelerated, designated by composition and coating weight.
Scope 1.1.1 says the standard is for general industrial purposes. It does not cover treatment of ferrous surfaces for deep drawing. It does not deal with phosphating on zinc, cadmium, or aluminium for passivation. Scope 1.2 warns that certain phosphating processes, particularly heavy classes, are likely to alter dimensions, so the designer must allow for that or pick a lighter class.
Phosphated is not galvanised GI pipe. Where IS 4736 applies to the specified steel tube, the amended requirement is a minimum average zinc-coating mass of 360 g/m². Phosphate does not meet that hot-dip zinc requirement. Phosphated is also not chrome plate and not a finished honed or skived seal bore.
Buying phosphated precision tube in Kanpur
Dada Nagar industrial area, about 8 km from the Bartan Bazar yard, is the local hydraulic-cylinder and pump-OEM cluster. Those shops take DIN 2391 ST52 / EN 10305-1 E355 cold-drawn tube, often phosphated and oiled on the OD, with the bore capped. ST45 / E235 tube in BK condition goes into shock absorbers, steering columns, and axles.
RP Sales quotes phosphated precision tube ex-warehouse Kanpur, GST 18% extra. Daily dispatch runs across Uttar Pradesh, Bihar, Madhya Pradesh, Delhi NCR, and Uttarakhand. Intra-Kanpur delivery to Dada Nagar, Panki, and Jajmau (about 10 km) leaves from the Bartan Bazar yard. Ends are capped. Tubes sit in lined cradles so the bore does not rust in transit.
Indicative DIN 2391 bands on the precision tube price list are ₹95-150 per kg by size, ex-warehouse Kanpur, as of 24 May 2026. A 50.0 mm OD × 4.0 mm wall row is ₹100-125 per kg. Those bands are for the tube, not a separate phosphate surcharge. Ask the quotation to state phosphate plus oil as a finish line.
See phosphated vs galvanised precision tubes, precision tubes for hydraulic applications, and the Kanpur steel market. PWD and JJM tenders almost never want phosphated precision tube; they want IS 1239 / IS 3589 water pipe. Keep those RFQs on the ERW pages.
Who phosphated tube is for, and who should not use it
Use phosphated plus oil when the tube needs short-term rust control, paint or powder-coat adhesion, or lubricant retention for a later draw, with almost no OD growth. That is the Dada Nagar hydraulic-cylinder cluster and indoor mechanical stock that will be painted after fabrication. Write the finish as a second line: zinc phosphate plus oil, or manganese phosphate plus oil, IS 3618 class if the drawing names one, ID uncoated, ends capped.
On hydraulic barrels, phosphate the OD if the drawing allows it. Keep the finished H8/H9 bore oil-clean and capped. Phosphate residue inside a seal bore is a reject. If the ID will be skived or honed after delivery, say so on the RFQ so the mill does not coat the bore.
Do not use phosphated tube as outdoor GI. Wet, buried, or fire-riser duty needs IS 4736 zinc, usually heavy class. Do not use it as a substitute for stainless in Jajmau ETP chemical lines. Do not hot-dip galvanise a finished honed or skived bore.
IS 3618 scope 1.2 is the dimensional warning: heavy phosphate classes can move fits. Precision OD, threads, and H8 bores should stay on a light conversion plus oil, or be masked. Threads cut after a heavy phosphate class may need chasing. Gauge them after coating.
Phosphated versus galvanised on precision tube
The comparison below separates phosphate conversion from hot-dip zinc so a Kanpur buyer can see why the two finishes are not interchangeable. The IS 4736 line reflects the amended 360 g/m² minimum average zinc-coating mass only when that standard applies to the specified tube; the underlying tube standard must still be stated on the RFQ.
| Attribute | Phosphated + oiled | Galvanised / zinc (IS 4736) |
|---|---|---|
| Protection | Storage and light indoor corrosion | Outdoor / wet zinc protection |
| Dimensional impact | Low; IS 3618 still warns on heavy classes | Higher; amended IS 4736 minimum average zinc mass is 360 g/m² where applicable |
| Hydraulic bore | OD only; keep ID capped and oil-clean | Unsuitable on a finished H8/H9 bore |
| Governing coating spec | IS 3618:1966 (reaffirmed 2021), five classes | IS 4736 coating-mass class, cited beside the tube spec |
| Typical Kanpur buyer | Dada Nagar hydraulic-cylinder shops | PWD / JJM / fire GI, not precision barrels |
Wall thickness and pressure capacity are steel dimensions, not coating. Zinc or phosphate does not make a thin wall acceptable. Write the tube standard (DIN 2391 / EN 10305) and the coating standard as two lines on the PO.
How to specify phosphated tube on an RFQ
A coated precision-tube order has two specifications: the tube and the coating. Incomplete RFQs come back as "phosphate?" on a black tube quote, and the Dada Nagar inspector then rejects a rusty bore.
- Tube: DIN 2391 or EN 10305-1, grade ST52 / E355 or ST35 / E235, delivery BK, BK+S, or NBK.
- Size: OD × ID or OD × wall in mm, tolerance class, straightness, length, quantity.
- Bore: as-drawn, honed H8/H9, or skived / SRB. State if ID coating is prohibited.
- Coating: zinc phosphate or manganese phosphate, plus oil. Cite IS 3618 if the drawing names it. Do not write "coated" alone.
- Masking: threads, bearing seats, and finished ID. Gauge threads after coating.
- Documents: EN 10204 3.1 MTC for the tube; supplier declaration or coating certificate for phosphate. Heat number on tag and challan.
- Packing: oiled OD, capped ends, lined cradles. Delivery ex-warehouse Kanpur. GST 18% extra.
Worked mass example for a phosphated 50 × 4 mm tube
Steel mass does not include the phosphate film. Use the site formula once: W = (OD − t) × t × 0.02466 kg/m. For the 50.0 mm OD × 4.0 mm wall row on the precision price list, W = (50.0 − 4.0) × 4.0 × 0.02466 = 46 × 4 × 0.02466 = 4.54 kg/m. A 6 m length is about 27.2 kg.
At ₹100-125 per kg DIN 2391 (as of 24 May 2026, ex-warehouse Kanpur, GST 18% extra), that 6 m tube is a steel-mass budget of 27.2 × 100 = ₹2,720 to 27.2 × 125 = ₹3,400 before GST and freight. Phosphate plus oil is a finish, not a second kg/m line. If the mill weighs coated tube, the extra mass is small; do not invent a coating g/m² for phosphate when IS 3618 class weights were not on the public preview page.
Compare with GI only on a separate RFQ. Where IS 4736 applies, specify its amended minimum average zinc-coating mass of 360 g/m² alongside the underlying tube standard. Hot-dip zinc changes finished dimensions, which is why Dada Nagar cylinder barrels stay phosphated and oiled rather than being galvanised after bore finishing.
Common mistakes with phosphated tube
In practice, buyers write "GI precision tube" and receive hot-dip zinc on a tight OD. Threads and H8 fits then fail the gauge. Write phosphate plus oil, or write IS 4736 class if zinc is really required.
A second mistake is phosphating a finished skived or honed bore. Seal life drops as soon as crystals or sludge sit on the ID. Cap the bore. State "ID uncoated" on the PO.
A third mistake is treating phosphate as permanent outdoor protection. The precision cluster is explicit: phosphating with oil is storage and light protection, not a buried-water system. Outdoor hydrants stay on GI.
A fourth mistake is skipping the MTC because "it is only a coating." The tube still needs grade, heat, and delivery condition on EN 10204 3.1. Heat numbers on the tag must match the challan.
Standards for phosphated tube
IS 3618:1966 (reaffirmed 2021) is the Indian phosphate-treatment specification fetched from the BIS e-sale preview on 2026-09-03. Use it when the drawing wants a named phosphate class and a test method. The public preview states five classes and the dimensional warning in 1.2; it does not print class-by-class coating masses on the preview page, so those masses are not quoted here.
The tube underneath remains DIN 2391-1:1994 or EN 10305-1:2016, grade E235 / E355, delivery BK / BK+S / NBK. EN 10204:2004 covers the inspection document. ISO 286-1:2010 covers H8/H9 if the bore is finished. ISO 21920-2:2021 is the current ISO profile-parameter standard that revises ISO 4287:1997; Kanpur RFQs on this site still write Ra per ISO 4287.
Hot-dip zinc, when it is actually required, is IS 4736, not IS 3618. Cite both the tube standard and the coating standard. Do not point at the BIS homepage as a clause source.
Where you'll meet this in procurement
You will meet Phosphated in manufacturing notes, surface-finish call-outs, and precision-tube RFQs. Confirm that Phosphated is required for the duty (tolerance, bore finish, or coating), and that the quote states the same process the inspector will verify on receipt.
Related terms
Related resources
- Product catalogue
- Phosphated vs galvanised precision tubes
- Precision tubes, hydraulic applications
- Precision tube price list
- DIN 2391 ST52 hydraulic tube
- Hydraulic cylinder industry
- Steel pipes in Kanpur
- GI pipes in Kanpur
- Kanpur steel market
- IS 4736 zinc coatings
- Request a quotation (RFQ)
- Back to full glossary
Key points
- Phosphating is a thin conversion coating (zinc or manganese phosphate), usually oiled, not hot-dip GI.
- IS 3618:1966 (reaffirmed 2021) covers five classes on iron and steel and warns that heavy classes can change dimensions.
- Dada Nagar cylinder shops phosphate the OD and keep the H8/H9 bore capped and oil-clean.
- Where IS 4736 applies, its amended minimum average zinc-coating mass is 360 g/m²; hot-dip zinc remains a different finish and product requirement.
- Quotes are ex-warehouse Kanpur, GST 18% extra. Steel mass uses (OD − t) × t × 0.02466 kg/m.
Frequently asked questions
Sources
- IS 3618:1966 (reaffirmed 2021). Specification for Phosphate Treatment of Iron and Steel for Protection Against Corrosion, scope 1.1, 1.1.1, 1.2 (BIS e-sale preview) (Bureau of Indian Standards)
- IS 4736:1986. Hot-dip zinc coatings on steel tubes: coating-mass classes used on this site for GI contrast (Bureau of Indian Standards)
- DIN 2391-1:1994. Seamless precision steel tubes. Technical delivery conditions (Deutsches Institut für Normung)
- EN 10305-1:2016. Steel tubes for precision applications. Seamless cold-drawn tubes (European Committee for Standardization (CEN))
- EN 10204:2004. Metallic products. Types of inspection documents (European Committee for Standardization (CEN))