Technical knowledge
Pickling, passivation and iron contamination control
Why free iron on the surface causes rust and pitting, how pickling and passivation differ, when each is required, and how to verify that a surface is clean without damaging it.
Where the iron comes from
- Shared grinding wheels, wire brushes and tools used previously on carbon steel.
- Handling and storage on steel racks, chains or slings.
- Weld spatter and arc strikes from adjacent carbon steel work.
- Shop environment where grinding dust from carbon steel settles.
- Hydrotest water or cleaning cloths carrying iron residue.
Pickling versus passivation
| Treatment | What it does | When it is used |
|---|---|---|
| Pickling | Dissolves a thin layer of metal, removing embedded iron, heat tint and the chromium depleted layer | After welding, hot forming or heat treatment |
| Passivation | Removes free iron and promotes a chromium rich passive film without significant metal removal | After machining, handling and cleaning |
| Mechanical cleaning | Removes deposits without chemical action | Supplementary, with dedicated non ferrous tools |
Good practice
- Prevent contamination first with dedicated tools, storage and handling.
- Clean and degrease before any chemical treatment, since oil blocks the reaction.
- Use citric acid or nitric acid based treatments appropriate to the grade and to the local regulations.
- Rinse with low chloride water and dry thoroughly.
- Verify with a ferroxyl test or a surface iron test where required, and avoid the test on surfaces that will contact the process fluid.
Cautions
Note Never use hydrochloric acid based products on stainless or nickel alloys, and never use carbon steel tools or steel wool at any stage. For titanium, use only approved cleaning methods, because some treatments attack the oxide film rather than restoring it.
Questions about this topic
Is passivation the same as pickling?
No. Pickling removes metal and eliminates heat tint and the depleted layer; passivation removes surface iron and promotes the passive film without meaningful metal removal.
Why does stainless rust after fabrication?
Almost always because of free iron embedded from shared tools, handling or weld spatter, rather than a problem with the alloy itself. Cleaning and passivation remove the iron and restore the surface.
Do nickel alloys need passivation?
They benefit from cleaning and from removal of surface iron, but they do not rely on the same chromium oxide mechanism as stainless steel. Use methods approved for the specific alloy.
Questions about our products
What products does Hangbo Alloy supply?
Four product families, all available in nickel-based alloys, titanium, duplex and special stainless steel: flanges, butt weld pipe fittings, elbows, and seamless tube and pipe. Flanges cover weld neck, long weld neck, slip-on, blind, socket weld, threaded and lap joint patterns. Fittings and elbows cover concentric and eccentric reducers, equal and reducing tees, caps, stub ends, and 45°, 90° and 180° returns in both long and short radius, plus 3D and 5D bends for piggable lines. Seamless tube and pipe is supplied cold drawn or hot finished from 6 mm to 630 mm outside diameter.
Which alloy grades are available for these products?
More than 90 grades are published in our grade library. The most requested are Inconel 625, 718, 600 and X-750, Hastelloy C-276, C-22 and B-2, Monel 400 and K-500, Incoloy 800H, 825 and 925, duplex 2205 and super duplex 2507, 316L, 904L and 254SMO stainless steel, and titanium Grade 2 and Grade 5. Nickel-based grades can also be supplied with VIM plus ESR double melting where cleanliness or fatigue life is critical.
What sizes, wall thicknesses and pressure classes can you supply?
Flanges run from NPS ½ to NPS 24 to ASME B16.5 and from NPS 26 to NPS 60 to ASME B16.47 Series A or B, in pressure classes 150 through 2500. Butt weld fittings and elbows run from NPS ½ to NPS 48 (DN 15 to DN 1200) in wall thickness Schedule 5S through XXS. Seamless tube and pipe covers 6 mm to 630 mm outside diameter, in random lengths of about 5 to 7 metres or fixed lengths up to 12 metres. Non-standard and intermediate sizes are quoted on request.
Which material and dimensional standards do you work to?
Forged flanges to ASTM B564, B381 and A182 with dimensions to ASME B16.5 and ASME B16.47. Butt weld fittings and elbows to ASTM B366 and A403 with dimensions to ASME B16.9 and MSS SP-43. Seamless tube and pipe to ASTM B444, B423, B622, B163, B167, B861, A312, A213 and A790. Marking follows MSS SP-25 and the applicable product standard. If your project is governed by EN, DIN, JIS or a company specification, send the document and we will confirm equivalence before quoting.
Is there a minimum order quantity?
There is no minimum for standard sizes and grades held in stock, and prototypes, spares and small trial batches are accepted. Made-to-order melts and special forgings carry a minimum weight that depends on the alloy and the route, typically from about 500 kg for special grades.
What inspection and certification is issued with the goods?
Every shipment carries an EN 10204 3.1 mill test certificate quoting the actual chemical and mechanical results for the specific heat lot, together with dimensional records and heat treatment data where applicable. Testing is carried out in house by optical emission spectrometry, tensile, impact and hardness testing, plus ultrasonic and eddy current examination, with hydrostatic pressure and intergranular corrosion testing to ASTM G28 or A262 Practice E where the specification requires it. Third party inspection by SGS or another recognised agency can be arranged, and their report is issued alongside the shipment.
What are typical lead times?
Standard stock items ship within days. Common production grades such as Inconel 625 or duplex 2205 are typically a few weeks. Special chemistries, larger forgings and items that require third party inspection take longer and are quoted individually against the requirement.
How are the products packed for export?
Flanges and fittings receive a rust preventive coating, are individually wrapped and are packed in plywood cases secured with steel banding. Tube ends are capped and bundles are strapped. Packing lists quote heat numbers so every case can be cross-checked against the mill certificate on arrival. We quote EXW, FOB, CFR, CIF and DAP, and mixed-alloy orders can be consolidated into one shipment with a single documentation package.
How do I request a quotation or a sample?
Send the grade or UNS number, the dimensions including schedule or wall thickness, the governing material and dimensional standards, quantity, any heat treatment or NDT scope and the certificate type you need; a drawing is ideal for special flanges and fittings. Email [email protected] or message us on WhatsApp +86 15618279078 and we normally issue a formal quotation within 24 hours on working days. Samples and previous test reports can be arranged before the first order.
Talk to an engineer
Share your duty conditions and we will shortlist candidate alloys within one working day.