Why Chrome Rods Rust or Pit, and How to Prevent It

Where does rust start on a chrome rod?

Rust starts wherever bare steel meets moisture, and on a plated rod that means the points where chrome is missing, damaged, or was never applied.

A hard chrome layer is a barrier over ground and polished steel. It is hard — chrome hardness can reach HV800+ — but it is also thin, measured in tens of micrometers, so the protection depends on the layer staying continuous over the working surface. On a delivered bar, the usual places where the barrier is absent or interrupted are:

• Ends left as sawn. The cut face is unplated steel.
• Features machined after delivery: turned journals, milled flats, drilled holes, threads and keyways. These cut through the plate by definition.
• Mechanical damage from handling: bar-to-bar rubbing, steel straps, slings, chuck jaws and tools used at installation.
• Edges and transitions, where a deposit is hardest to keep uniform.

Pitting is what follows when moisture and chlorides reach one of these points and stay there. The pit works into the base steel under the surrounding plate, so the first visible sign is often a small brown stain or a roughened spot rather than an open hole. By the time a pit is visible on the chrome, the steel underneath has already been attacked, which is why prevention is cheaper than rework.

How thick should the chrome be to prevent rust?

There is no single thickness that prevents rust in every environment. What matters is that the layer is thick enough for the service conditions and is written as a number that can be measured. Yixin Precision Machinery (Changzhou) supplies hard chrome at 10–50 μm per side, chosen per order. Plating is done by long-term partner plants to Yixin's specification, and Yixin re-checks chrome thickness, diameter and surface when the bars come back.

Three points are worth getting right on the drawing:

• Per side, not total. Stating the figure per side removes any ambiguity at inspection.
• Measured on the finished part. Chrome adds to the diameter, so chrome thickness and the final diameter tolerance have to be specified together; otherwise grinding and plating work against each other.
• Finish as well as thickness. Yixin supplies chrome at Ra ≤ 0.4 μm, and a finish callout gives the inspector a second, independent check on the same surface.

Where the duty includes washdown, condensation, humid storage or chloride exposure, specify toward the thicker end of the range. Thicker chrome layers carry a lead time quoted per order, so raise the requirement when the drawing is released rather than after the first lot.

Salt-spray performance is not part of the standard specification. Where a rating is required, we specify a thicker chrome layer and can have the order tested by a third-party laboratory.

Does the base grade or heat treatment change the risk?

The chrome, not the base grade, seals the surface, so grade selection is mainly about strength and machinability. What the base condition changes is how easily the surface is damaged and how much bare steel you create when you machine the bar. The grades are named to their standards: Grade 45 to GB/T 699-2015; 40Cr and 42CrMo to GB/T 3077-2015.

Three examples from the Yixin range show the difference:

• 45 Steel Chrome-Plated Soft Shaft — not induction hardened, base ≈ HRC17. It can still be turned, milled or drilled after supply, which suits rods whose ends you plan to machine, but the unhardened surface marks more readily in handling.
• 45 Steel Induction-Hardened Chrome Shaft — induction-hardened case ≥ HRC55, ≈ 2 mm deep, then hard chrome. Choose it where the working surface must resist wear as well as corrosion.
• 42CrMo Q&T Chrome-Plated Bar — 42CrMo quenched and tempered to HRC30–38 and then hard chrome plated, made to order in Ø8–100 mm, for heavy or shock loads (comparable, for reference only: 42CrMo4, SCM440; chemistry limits differ between standards).

Whichever grade you choose, the corrosion rule does not change: every operation after plating is a chance to break the barrier. If the design requires machining after delivery, say so on the enquiry and plan to protect the freshly cut surfaces.

How should chrome rods be packed, handled and stored?

Protection after plating matters as much as the layer itself. A correctly plated bar that is strapped, dragged or stored wet arrives with the barrier already broken.

Yixin packs export bars wrapped in bubble film and packed in wooden cases. No steel straps are used on finished bars, because they mark the surface. That covers the journey; most damage happens after the case is opened.

Practical rules for the receiving site:

• Keep bars in the closed case, under cover and off the ground until needed; unpack at installation rather than at goods-in.
• Store horizontally and supported, so bars neither bow nor rub against one another.
• Lift with soft slings rather than chains, and never drag one bar across another.
• Keep cases away from chlorides, acids and standing water, including floor washdown.
• Wipe and protect machined features immediately after machining; a turned journal or a cut end is bare steel.
• Where bars will be stored for a long period before installation, describe the storage conditions in the enquiry so that packing and end protection can be discussed before the order is placed.

How do you inspect an incoming lot for corrosion risk?

Inspect the barrier rather than the shine: measure chrome thickness and finish, confirm diameter and straightness, and record the condition of ends and packing at goods-in.

Record any break against the packing condition as well, because that tells you whether the cause is transport or the process. Where independent verification is required, third-party testing is arranged on request, per order.

What to put in your RFQ

• The product and condition by name: 45 Steel Chrome-Plated Soft Shaft, 45 Steel Induction-Hardened Chrome Shaft, 45 Steel Chrome-Plated Linear Shaft, or a chrome-plated Q&T bar in 45 steel, 40Cr or 42CrMo.
• Chrome thickness in μm per side, within 10–50 μm, and whether the service environment calls for the thicker end of the range.
• Surface finish on the chrome, for example Ra ≤ 0.4 μm, and the finish required on any ground areas.
• Diameter and tolerance band, with the final size measured over the chrome.
• Length and end condition: full bars of 4 m, 6 m, 7.5 m or 8.1 m, or CNC cut to length from 200 mm with ends left as sawn.
• Any features you will machine after delivery, so that grade and base condition are chosen for machining rather than for the finished surface alone.
• Whether a salt-spray rating is required, and the standard and duration to be tested.
• Packing, marking and order quantity — minimum 1 t per order, with different sizes combined.

Summary

Rust and pitting on a chrome rod are almost always a break in the barrier rather than a fault of the steel. The exposed points are predictable — sawn ends, features machined after plating, handling marks, and any area where the deposit is thin. Specify chrome thickness per side within the 10–50 μm range together with a finish callout such as Ra ≤ 0.4 μm, choose the base grade for strength and for the machining you plan, and treat packing and storage as part of the corrosion specification. Then inspect thickness, finish, diameter, straightness, ends and packing at goods-in, so that any break is found before the rod goes into service.