El piñón está oxidado.

Spanish usually uses estar for a condition or state, especially one that can change. Oxidado describes the current condition of the piñón: it is rusted/oxidized.

So:

• El piñón está oxidado = The pinion/sprocket is rusted.
• Es oxidado would sound unnatural here.

A good rule of thumb is:

• ser = what something is
• estar = how something is

Since rust is treated as a resulting state, estar is the normal choice.

Piñón can have different meanings depending on context. In Spain, common meanings include:

• pinion / sprocket / gear in a mechanical or bicycle context
• pine nut in a food context

So the exact meaning depends on the situation. In El piñón está oxidado, the context strongly suggests the mechanical meaning, because something mechanical can be oxidado.

El means the and is the masculine singular definite article.

It is used because piñón is a masculine singular noun:

• el piñón

This usually means we are talking about a specific pinion/sprocket, not just any one.

Compare:

• El piñón está oxidado = The sprocket is rusted.
• Un piñón está oxidado = A sprocket is rusted.

Because it agrees with piñón, which is masculine singular.

In Spanish, adjectives usually agree in gender and number with the noun they describe.

Examples:

• el piñón oxidado = the rusted sprocket
• la pieza oxidada = the rusted part
• los piñones oxidados = the rusted sprockets
• las piezas oxidadas = the rusted parts

So oxidado has -o because piñón is masculine singular.

It is historically the past participle of oxidar (to oxidize), but in this sentence it works like an adjective.

So está oxidado means is oxidized / is rusted as a state or condition.

You may also see it in verbal structures:

• Se ha oxidado = It has rusted / It has become oxidized.

But in El piñón está oxidado, the focus is on the result, not on the process.

A rough English approximation is pee-NYON.

Important points:

• ñ sounds like the ny in canyon
• the written accent in ó shows that the stress falls on the last syllable

So:

• pi-ñón
• stress on ñón

Not PEE-nyon, but more like pee-NYON.

They mean different things:

• oxidado = oxidized / rusted
• oxidable = oxidizable / able to oxidize

So:

• El piñón está oxidado = The sprocket is rusted.
• El metal es oxidable = The metal can oxidize.

In your sentence, the object already has that condition, so oxidado is the correct word.

Yes, but it means something slightly different.

• El piñón está oxidado focuses on the current state: it is rusted.
• El piñón se ha oxidado focuses on the process/result: it has rusted.

Both are correct, but they are used in different ways:

• if you are describing what you see now, está oxidado is very natural
• if you want to say that it became rusted, se ha oxidado is a good option

Yes, but El piñón está oxidado is the most neutral and natural order.

Spanish sometimes changes word order for emphasis:

• Está oxidado el piñón can sound more emphatic or context-driven, like The sprocket is rusted.

For a learner, the standard order is best:

• subject + verb + adjective

So stick with El piñón está oxidado unless you have a specific reason to emphasize something.

Spanish often uses oxidado where English might say rusted or rusty, depending on context.

• oxidado literally means oxidized
• in everyday use, for metal objects, it often corresponds to rusted or rusty

Spanish does have words like herrumbroso, but oxidado is extremely common and natural for this kind of sentence.

Yes, it matters for both pronunciation and spelling.

Without the accent mark, the stress rules would change, and piñon would not represent the standard spelling of this word.

The accent tells you the stress is on the last syllable:

• pi-ÑÓN

In Spanish, accent marks are part of correct spelling, so you should write piñón, not piñon.